Article

Network strain filter: A new tool for monitoring and detecting transient deformation signals in GPS arrays

December 2010
Journal of Geophysical Research Atmospheres 115(B12)

DOI:10.1029/2010JB007442

Authors:

Woods Hole Oceanographic Institution

We have developed a tool to detect transient deformation signals from large-scale (principally GPS) geodetic arrays, referred to as a Network Strain Filter (NSF). The strategy is to extract spatially and temporally coherent signals by analyzing data from entire geodetic networks simultaneously. The NSF models GPS displacement time series as a sum of contributions from secular motion, transient displacements, site-specific local benchmark motion, reference frame errors, and white noise. Transient displacements are represented by a spatial wavelet basis with temporally varying coefficients that are estimated with a Kalman filter. A temporal smoothing parameter is also estimated online by the filter. The problem is regularized in the spatial domain by minimizing a smoothing (Laplacian) norm of the transient strain rate field. To test the performance of the NSF, we carried out numerical tests using the Southern California Integrated GPS Network station distribution and a 3 year long synthetic transient in a 6 year time series. We demonstrate that the NSF can identify the transient signal, even when the colored noise amplitude is comparable to that of transient signal. Application of the method to actual GPS data from the Japanese GPS network (GEONET) on the Boso Peninsula also shows that the NSF can detect transient motions resulting from aseismic fault slip.

Transient Ground Deformation in Tectonically Active Regions and Implications for the Mechanical Behavior of the Crust and Upper Mantle

Thesis

Jan 2017

Trever Hines

Modern-day geodetic tools, such as global navigation satellite systems (GNSS), detect tectonic ground deformation to within millimeter accuracy. In the past decade, our ability to resolve ground deformation in the Western United States has greatly improved with the Plate Boundary Observatory (PBO) project, which consists of about 1100 continuously operating GNSS stations. With this unprecedented quality and quantity of data, we can observe the subtle signal from transient tectonic processes. For example, we can detect ground deformation in the days to years following large earthquakes, which is caused by aseismic afterslip and/or ductile deformation in the upper mantle. The PBO has also allowed us to resolve transient deformation associated with slow slip events on the Cascadia subduction zone. In this dissertation, I discuss techniques for detecting transient deformation in geodetic data, and I analyze this transient deformation to better understand the mechanical behavior of the crust and upper mantle. Most studies of ground deformation throughout the earthquake cycle (i.e., interseismic deformation) indicate that the lower crust is orders of magnitude stronger (more viscous) than the upper mantle. In Chapter 2, I demonstrate that the methods used in these studies are biased towards inferring a more viscous lower crust and less viscous upper mantle. I conclude that these interseismic studies do not necessarily rule out the possibility that the lower crust can deform ductilely on earthquake cycle timescales. In Chapters 3 and 4, I introduce a method for discerning the physical mechanisms driving postseismic deformation, where I consider candidate mechanisms to be afterslip and viscous relaxation in the crust and upper mantle. I apply this method to postseismic deformation following the 2010 El Mayor-Cucapah earthquake in Baja California. I find that a Burgers rheology upper mantle is necessary to describe far-field deformation in the three years following the earthquake. In general, upper mantle viscosities inferred from interseismic deformation are larger than those estimated from postseismic deformation, which occurs over a much shorter timescale. By describing the upper mantle with a Burgers rheology, which contains a transient and steady-state phase of deformation, I am able to reconcile these conflicting studies. Chapters 5, 6, and 7 are on detecting transient geophysical signal in geodetic data. Before geophysical signal can be identified, the noise in geodetic data must be quantified. In Chapter 5, I point out a bias in the commonly used method for characterizing noise in geodetic data, and I propose an alternative unbiased method. In Chapter 6, I demonstrate that transient geophysical signal can be robustly detected by using a machine learning technique known as Gaussian process regression. Finally, in Chapter 7 I assess the utility of borehole strain meters (BSMs) for detecting transient deformation. I find that BSM data, which records strains over an 8.7 centimeter baseline, are difficult to reconcile with regional strains derived from GNSS data.

Monitoring of the spatio-temporal change in the interplate coupling at northeastern Japan subduction zone based on the spatial gradients of surface velocity field

Article

Full-text available

Apr 2018

Takeshi Iinuma

A monitoring method to grasp the spatio-temporal change in the interplate coupling in a subduction zone based on the spatial gradients of surface displacement rate fields is proposed. I estimated the spatio-temporal change in the interplate coupling along the plate boundary in northeastern (NE) Japan by applying the proposed method to the surface displacement rates based on global positioning system observations. The gradient of the surface velocities is calculated in each swath configured along the direction normal to the Japan Trench for time windows such as 0.5, 1, 2, 3 and 5 yr being shifted by one week during the period of 1997-2016. The gradient of the horizontal velocities is negative and has a large magnitude when the interplate coupling at the shallow part (less than approximately 50 km in depth) beneath the profile is strong, and the sign of the gradient of the vertical velocity is sensitive to the existence of the coupling at the deep part (greater than approximately 50 kmin depth). The trench-parallel variation of the spatial gradients of a displacement rate field clearly corresponds to the trenchparallel variation of the amplitude of the interplate coupling on the plate interface, as well as the rupture areas of previous interplate earthquakes. Temporal changes in the trench-parallel variation of the spatial gradient of the displacement rate correspond to the strengthening or weakening of the interplate coupling. We can monitor the temporal change in the interplate coupling state by calculating the spatial gradients of the surface displacement rate field to some extent without performing inversion analyses with applying certain constraint conditions that sometimes cause over- and/or underestimation at areas of limited spatial resolution far from the observation network. The results of the calculation confirm known interplate events in the NE Japan subduction zone, such as the post-seismic slip of the 2003 M8.0 Tokachi-oki and 2005 M7.2 Miyagi-oki earthquakes and the recovery of the interplate coupling around the rupture area of the 1994 M7.6 Sanriku-Haruka-oki earthquake. The results also indicate the semi-periodic occurrence of slow slip events and the expansion of the area of slow slip events before the 2011 Tohoku-oki earthquake (M9.0) approaching the hypocentre of the Tohoku-oki earthquake. The Author(s) 2017. Published by Oxford University Press. All rights reserved.

Seismic and aseismic fault slip before and during the 2011 off the Pacific coast of Tohoku Earthquake

Article

Jul 2011
EARTH PLANETS SPACE

The March 11, 2011, off the Pacific coast of Tohoku Earthquake is the first great subduction zone earthquake to be recorded by a dense, modern network of geodetic quality Global Positioning System (GPS) receivers, and hence presents an unprecedented opportunity to study the evolution of fault slip during the time periods immediately before, during, and after a truly great earthquake. We utilized sub-daily GPS data to constrain the slip distribution in the March 9 Mw 7.3 foreshock, the 50-hour time period between the foreshock, and the coseismic slip during the Mw 9.0 mainshock. We find that the foreshock ruptured downdip from its hypocenter, that there was considerable (Mw ˜ 7.3) slip during the 50 hours between the foreshock and the mainshock, and that the peak mainshock slip was about 35 m. The mainshock's epicenter may have been triggered by afterslip that followed the foreshock. Additionally, the mainshock slip distribution was centered further updip than the peak interseismic coupling estimated from GPS data in the ten years before the earthquake.

Nonlinear dynamics of volcanic systems from geodetic data

Thesis

Full-text available

Feb 2018

Damian Walwer

We study the use of the "multichannel singular spectrum analysis" on GPS time series. This method allows to simultaneously analyze a set of time series in order to extract from it common modes of variability without using any a priori on the temporal or the spatial structure of geophysical fields. The extracted modes correspond either to nonlinear trends, oscillations or noise. The method is applied on a set of GPS time series recorded at Akutan, a volcano located in Aleutian arc in Alaska. Two types of signals are extracted from it. The first one corresponds to seasonal deformations and the other represents two successive cycles of inflation and subsidence of Akutan volcano. The inflations are fast and short and are followed by deflations that are slower and longer. In the second part we take benefit of the M-SSA to analyze GPS time series recorded at several volcanoes. Okmok and Shishaldin in Alaska and Piton de la Fournaise in La Réunion possess a part of their deformation history that is similar to Akutan volcano. The cyclic nature of the observed deformations leads us to make an analogy between the oscillatory regime of a simple nonlinear oscillator and the deformation cycles of these volcanoes. Geochemical, petrological and geophysical data available for Okmok and Piton de la Fournaise combined with the constraint on the qualitative dynamics bring by the nonlinear oscillator allow to propose a physical model. Two shallow reservoirs are connected by a cylindrical conduit in which the magma have a viscosity that depends on the temperature. Such system behaves like the nonlinear oscillator mentioned above. When the temperature gradient inside theconduit is large enough and the flux of magma entering the shallow system is bounded by values that are determined analytically anonlinear oscillatory regime arises.

Single-station automated detection of transient deformation in GPS time series with the relative strength index: A case study of Cascadian slow slip: AUTOMATED TRANSIENT DETECTION

Article

Full-text available

Nov 2016

The discovery of slow-slip events over the past decades has changed our understanding of tectonic hazards and the earthquake cycle. Proper geodetic characterization of slow-slip events is necessary for studies of regional interseismic, coseismic and postseismic deformation, and miscalculations can affect our understanding of the regional stress field and tectonic hazard. Because of the proliferation of GPS data over the last two decades, an automated algorithm is required to analyze the signals and model the deformation on a station by station basis. Using the relative strength index (RSI), a financial momentum oscillator, we test the ability to detect events of various sizes and durations. We first determine the statistics of the RSI under different noise conditions and then use this information as the basis for the automated transient detection algorithm by testing different synthetic signals. We then apply the technique to daily GPS displacement time series from 213 stations along the Cascadia subduction zone to form a record of transient deformation between 2005 and 2016. Our estimates of the spatial extent, duration and propagation of major Episodic Tremor and Slip (ETS) events are consistent with previous studies. We use the automated detections to remodel the displacement time series and obtain transient deformation rates over the past decade and discuss the tectonic implications. Finally, we analyze the correlation between transient detections and tremor showing good agreement between the two at slab depths commonly associated with ETS events.

Detecting Slow Slip Signals in Southwest Japan Based on Machine Learning Trained by Real GNSS Time Series

Article

Full-text available

Feb 2025

With the increase in Global Navigation Satellite System (GNSS) observations, the requirement for objective and automated detection of slow slip event (SSE) signals hidden in displacement time series is increasing. However, machine learning for GNSS time series has rarely been attempted. Especially, the physical meanings of the spatio‐temporal noise variations and their effects on the detection performance have been not so deeply discussed. In this study, we conducted a single‐site SSE detection based on machine learning trained by real GNSS observations of southwest Japan to directly consider the complicated spatiotemporal characteristics of observational noise. Based on a catalog of 284 short‐term SSEs, approximately 26,000 time series containing SSE signals or noises were extracted as training data. The signal data predominantly had an amplitude of 1.5–2.0 mm. The model architecture following the Generalized Phase Detection, which was originally proposed for seismic wave detection, was then adopted. We obtained an accuracy of 75% for the test data. As expected, the detectability were mainly controlled by the signal amplitude, and false positive appears to be caused primarily by the temporally correlated noise that resemble the onset or termination of the SSE signal. We examined the correlation between detection performance and noise properties at each site, such as standard deviation and slope of power spectrum. The analysis of this study is expected to facilitate a straightforward evaluation of the influence of noise characteristics on the detection performance, and clarify the crucial topics to improve detection precision.

Detecting Transient Uplift at the Active Volcano Ol Doinyo Lengai in Tanzania With the TZVOLCANO Network

Article

Full-text available

Jul 2024
GEOPHYS RES LETT

Plain Language Summary Ol Doinyo Lengai is an active stratovolcano in the youthful Natron Rift characterized by alternating periods of calm lava flows and explosive eruptions. Volcanic hazards and risk are major issues in the Natron Rift largely because of nearby communities, tourism, and air traffic. Volcanic eruptions are often preceded by magma intruding into a shallow reservoir that causes observable surface uplift. The uplift phase normally occurs before an eruption and is regarded as a key precursor for eruptive processes. These precursory uplift signals can be quite small. In this study we use high precision terrestrial data to monitor surface motions and use numerical approaches to detect potential volcanic signals due to magma reservoir changes. We detect rapid uplift from March 2022 to December 2022 and continued steady uplift through August 2023. Modeling suggests the uplift is from an active shallow magmatic source. This work demonstrates the ability to detect temporal changes in surface uplift at an active volcano.

Detection of slow slip events using wavelet analysis of GNSS recordings

Preprint

Full-text available

Dec 2022

In many places, tectonic tremor is observed in relation to slow slip and can be used as a proxy to study slow slip events of moderate magnitude where surface deformation is hidden in Global Navigation Satellite System (GNSS) noise. However, when no clear relationship between tremor and slow slip occurrence is observed, these methods cannot be applied, and we need other methods to be able to better detect and quantify slow slip. Wavelets methods such as the Discrete Wavelet Transform (DWT) and the Maximal Overlap Discrete Wavelet Transform (MODWT) are mathematical tools for analyzing time series simultaneously in the time and the frequency domain by observing how weighted differences of a time series vary from one period to the next. We use wavelet methods to analyze GNSS time series of slow slip events in Cascadia. We use detrended GNSS data, apply the MODWT transform and stack the wavelet details over several nearby GNSS stations. As an independent check on the timing of slow slip events, we also compute the cumulative number of tremor in the vicinity of the GNSS stations, detrend this signal, and apply the MODWT transform. In both time series, we can then see simultaneous waveforms whose timing corresponds to the timing of slow slip events. We assume that there is a slow slip event whenever there is a positive peak followed by a negative peak in the wavelet signal. We verify that there is a good agreement between slow slip events detected with only GNSS data, and slow slip events detected with only tremor data. The wavelet-based detection method effectively detects events of magnitude higher than 6 as determined by independent event catalogs. As a demonstration of using the wavelet analysis in a region without significant tremor, we also analyze GNSS data from New Zealand and detect slow slip events that are spatially and temporally close to those detected previously by other studies.

Revealing transient strain in geodetic data with Gaussian process regression

Article

Dec 2017

Transient strain derived from global navigation satellite system (GNSS) data can be used to detect and understand geophysical processes such as slow slip events and post-seismic deformation. Here we propose using Gaussian process regression (GPR) as a tool for estimating transient strain from GNSS data. GPR is a non-parametric, Bayesian method for interpolating scattered data. In our approach, we assume a stochastic prior model for transient displacements. The prior describes how much we expect transient displacements to covary spatially and temporally. A posterior estimate of transient strain is obtained by differentiating the posterior transient displacements, which are formed by conditioning the prior with the GNSS data. As a demonstration, we use GPR to detect transient strain resulting from slow slip events in the Pacific Northwest. Maximum likelihood methods are used to constrain a prior model for transient displacements in this region. The temporal covariance of our prior model is described by a compact Wendland covariance function, which significantly reduces the computational burden that can be associated with GPR. Our results reveal the spatial and temporal evolution of strain from slow slip events. We verify that the transient strain estimated with GPR is in fact geophysical signal by comparing it to the seismic tremor that is associated with Pacific Northwest slow slip events.

Postseismic Process Inversion Using Full Time Series of Surface Deformation: Full Time‐Series Inversion (FTI) Theory and Its Application to the 2017 Sarpol‐e Zahab Earthquake

Article

Full-text available

Sep 2022

Postseismic processes provide important opportunities to probe into and investigate the frictional, viscous, and porous properties of the seismogenic fault and the surrounding Earth media. To accommodate the temporal and spatial resolutions and long‐term baseline stability of different deformation data, we develop a full time‐series inversion (FTI) technique, which jointly inverts for afterslip patterns using full time series of Global Navigation Satellite System, SAR, and strainmeter data. The FTI linearizes the inversion problem with a prescribed source evolution function to achieve efficient inversion. We conduct synthetic tests to validate the spatial and temporal resolution of the FTI algorithm. FTI outperforms static inversion techniques in terms of inversion stability under high noise level. We apply different parameterization strategies to evaluate its resolution for slip evolution parameters. The tests show that FTI can discriminate spatially separated afterslip with distinct evolution functions. Finally, we apply FTI to investigate the afterslip process following the 2017 Mw 7.3 Sarpol‐e Zahab earthquake that occurred along the Iran‐Iraq border in northwestern Zagros using Synthetic Aperture Radar Interferometry time series derived from the Sentinel‐1 observations 1 year after the mainshock. Similar to the synthetic tests, the algorithm is capable to discriminate afterslip with different evolution functions in the up‐ and downdip portions of the coseismic rupture zone. By comparing with the stress‐driven afterslip model simulated using rate‐strengthening frictional law, we demonstrate the stability of FTI in resolving the afterslip process. We emphasize the importance of incorporating early postseismic observations for deciphering afterslip evolution and frictional parameters.

Automatic Detection of Slow Slip Events Using the PICCA: Application to Chilean GNSS Data

Article

Full-text available

Dec 2021

The detection of transient events related to slow earthquakes in GNSS positional time series is key to understanding seismogenic processes in subduction zones. Here, we present a novel Principal and Independent Components Correlation Analysis (PICCA) method that allows for the temporal and spatial detection of transient signals. The PICCA is based on an optimal combination of the principal (PCA) and independent component analysis (ICA) of positional time series of a GNSS network. We assume that the transient signal is mostly contained in one of the principal or independent components. To detect the transient, we applied a method where correlations between sliding windows of each PCA/ICA component and each time series are calculated, obtaining the stations affected by the slow slip event and the onset time from the resulting correlation peaks. We first tested and calibrated the method using synthetic signals from slow earthquakes of different magnitudes and durations and modelled their effect in the network of GNSS stations in Chile. Then, we analyzed three transient events related to slow earthquakes recorded in Chile, in the areas of Iquique, Copiapó, and Valparaíso. For synthetic data, a 150 days event was detected using the PCA-based method, while a 3 days event was detected using the ICA-based method. For the real data, a long-term transient was detected by PCA, while a 16 days transient was detected by ICA. It is concluded that simultaneous use of both signal separation methods (PICCA) is more effective when searching for transient events. The PCA method is more useful for long-term events, while the ICA method is better suited to recognize events of short duration. PICCA is a promising tool to detect transients of different characteristics in GNSS time series, which will be used in a next stage to generate a catalog of SSEs in Chile.

Transient Variations in Glacial Mass Near Upernavik Isstrøm (West Greenland) Detected by the Combined Use of GPS and GRACE Data: Greenland Upernavik Transient Changes

Article

Nov 2017

The continuously operating Global Positioning System (GPS) sites mounted on bedrock around the coast of Greenland provide important geodetic datasets to quantify the solid Earth's response to historical and present-day ice mass variations. The presence of colored noise and irregular seasonal signals makes it difficult to detect transient changes in GPS time series. Here we apply the Multichannel Singular Spectral Analysis to the combination of GPS data and Gravity Recovery and Climate Experiment (GRACE) data so that we can identify and fully utilize the spatial correlations from these two independent datasets. Using the GPS and GRACE data near Upernavik Isstrøm in West Greenland as an example, we demonstrate that this method successfully detects two transient signals in ice mass variations during 2008 and 2014. Our forward modeling of loading displacements due to changes in surface mass balance (SMB) and ice dynamics suggests that the transient change starting in mid-2008 was due to the combined contributions from dynamically-induced mass loss and SMB. The transient change starting in mid-2011 was mainly due to ablation Specifically, the ice melted more in 2012 and less in 2013 with little contribution from anomalies in accumulation.

A geodetic matched‐filter search for slow slip with application to the Mexico subduction zone

Article

Full-text available

Aug 2017

Since the discovery of slow slip events, many methods have been successfully applied to model obvious transient events in geodetic time series, such as the widely used network strain filter. Independent seismological observations of tremors or low frequency earthquakes and repeating earthquakes provide evidence of low amplitude slow deformation but do not always coincide with clear occurrences of transient signals in geodetic time series. Here, we aim to extract the signal corresponding to slow slips hidden in the noise of GPS time series, without using information from independent datasets. We first build a library of synthetic slow slip event templates by assembling a source function with Green's functions for a discretized fault. We then correlate the templates with post-processed GPS time series. Once the events have been detected in time, we estimate their duration T and magnitude Mw by modelling a weighted stack of GPS time series. An analysis of synthetic time series shows that this method is able to resolve the correct timing, location, T and Mw of events larger than Mw 6 in the context of the Mexico subduction zone. Applied on a real data set of 29 GPS time series in the Guerrero area from 2005 to 2014, this technique allows us to detect 28 transient events from Mw 6.3 to 7.2 with durations that range from 3 to 39 days. These events have a dominant recurrence time of 40 days and are mainly located at the down dip edges of the Mw> 7.5 SSEs.

GPS station short-term dynamic characteristics of micro displacement before Menyuan M6.4 earthquake

Article

Full-text available

Jun 2016

Continuous observation data from 24 GPS stations are selected in the area (33.0°N–41.0°N, 95.0°E–105.0°E) for this study (the period is from Jan. 1, 2015 to Jan. 20, 2016). Three components, NS, EW and UD, of the daily solutions are filtered by the Hilbert–Huang transform (HHT) with frequency band of 5.787 × 10−7–7.716 × 10−8 Hz (20–150 days in period). And short-term dynamic characteristics of micro displacement before Menyuan M6.4 earthquake are studied by using the temporal dependencies and cross spectrum analysis. The results show that before the earthquake the horizontal undulatory motions are higher than the average level in the series data which indicate the disturbance feature of regional stress before the earthquake. Three GPS stations on Qinghai-Tibet Plateau with their setting perpendicular to the seismogenic fault have consistent movement. The increase of amplitude of the horizontal micro motion observed before the quake is conducive to the earthquake occurrence. However, we could not be sure if the undulatory motion triggered the earthquake. It is quite necessary to build more GPS continuous observation stations and optimize the monitoring network so as to improve the understanding of the short-term dynamic crustal variation before earthquake.

Data-adaptive detection of transient deformation in geodetic networks

Article

Mar 2016

The recent development of dense and continuously operating Global Navigation Satellite System (GNSS) networks worldwide has led to a significant increase in geodetic data sets that sometimes capture transient-deformation signals. It is challenging, however, to extract such transients of geophysical origin from the background noise inherent to GNSS time series and, even more so, to separate them from other signals, such as seasonal redistributions of geophysical fluid mass loads. In addition, because of the very large number of continuously recording GNSS stations now available, it has become impossible to systematically inspect each time series and visually compare them at all neighboring sites. Here we show that Multichannel Singular Spectrum Analysis (M-SSA), a method derived from the analysis of dynamical systems, can be used to extract transient deformations, seasonal oscillations, and background noise present in GNSS time series. M-SSA is a multivariate, nonparametric, statistical method that simultaneously exploits the spatial and temporal correlations of geophysical fields. The method allows for the extraction of common modes of variability, such as trends with nonconstant slopes and oscillations shared across time series, without a priori hypotheses about their spatiotemporal structure or their noise characteristics. We illustrate this method using synthetic examples and show applications to actual GPS data from Alaska to detect seasonal signals and microdeformation at the Akutan active volcano. The geophysically coherent spatiotemporal patterns of uplift and subsidence thus detected are compared to the results of an idealized model of such processes in the presence of a magma chamber source.

A multi-data stream assimilation framework for the assessment of volcanic unrest

Article

Dec 2015
J VOLCANOL GEOTH RES

Active volcanoes pose a constant risk to populations living in their vicinity. Significant effort has been spent to increase monitoring and data collection campaigns to mitigate potential volcano disasters. To utilize these datasets to their fullest extent, a new generation of model-data fusion techniques is required that combine multiple, disparate observations of volcanic activity with cutting-edge modeling techniques to provide efficient assessment of volcanic unrest. The purpose of this paper is to develop a data assimilation framework for volcano applications. Specifically, the Ensemble Kalman Filter (EnKF) is adapted to assimilate GPS and InSAR data into viscoelastic, time-forward, finite element models of an evolving magma system to provide model forecasts and error estimations. Since the goal of this investigation is to provide a methodological framework, our efforts are focused on theoretical development and synthetic tests to illustrate the effectiveness of the EnKF and its applicability in physical volcanology. The synthetic tests provide two critical results: (1) a proof of concept for using the EnKF for multi dataset assimilation in investigations of volcanic activity; and (2) the comparison of spatially limited, but temporally dense, GPS data with temporally limited InSAR observations for evaluating magma chamber dynamics during periods of volcanic unrest. Results indicate that the temporally dense information provided by GPS observations results in faster convergence and more accurate model predictions. However, most importantly, the synthetic tests illustrate that the EnKF is able to swiftly respond to data updates by changing the model forecast trajectory to match incoming observations. The synthetic results demonstrate a great potential for utilizing the EnKF model-data fusion method to assess volcanic unrest and provide model forecasts. The development of these new techniques provides: (1) a framework for future applications of rapid data assimilation and model development during volcanic crises; (2) a method for hind-casting to investigate previous volcanic eruptions, including potential eruption triggering mechanisms and precursors; and (3) an approach for optimizing survey designs for future data collection campaigns at active volcanic systems.

Detecting slow slip signals in southwest Japan based on machine learning trained by real GNSS time series

Preprint

Nov 2024

Seismic effects at GPS stations: Case study of the 24 August 2016 Amatrice earthquake

Article

Jun 2024

The seismic event of the 2016 Amatrice earthquake and its subsequent aftershocks have provided a significant opportunity to study seismic waves and their effects on the Apennine región, a very active seismic area. This research utilizes GPS data from three geodetic networks (ItalPos, NetGEO, and RING) to analyze the seismic waves generated by the earthquake, capturing seismic effects with great precision and resolution. By examining data from various GPS stations within the region, the study demonstrates the efficacy of GPS in providing detailed and accurate representations of ground motion. The study identifies co-seismic displacements and accelerations at monitoring stations by processing and analyzing GPS data, including precise point positioning strategies and wavelet adjustments. Moreover, a least squares adjustment method is employed to optimize the estimation of temporal parameters associated with seismic event detection across the spatial network of stations. The results obtained from GPS data are validated against seismic equipment, affirming their reliability in characterizing seismic events. Furthermore, the study elucidates the propagation of surface waves and path effects over the affected region, contributing to a comprehensive understanding of the earthquake impact. This research underscores the potential of GPS data as a valuable tool for rapid and precise characterization of seismic events, offering insights into ground motion dynamics and facilitating timely response and mitigation efforts.

Detecting slow slip signals in southwest Japan based on machine learning trained by real GNSS time series

Preprint

May 2024

A machine learning approach for slow slip event detection using GNSS time-series

Article

Oct 2023
J S AM EARTH SCI

A two-step inversion for fault frictional properties using a temporally varying afterslip model and its application to the 2019 Ridgecrest earthquake

Article

Full-text available

Jan 2023
EARTH PLANET SC LETT

The stress status and frictional parameters of a fault system control its slip behaviors over earthquake cycles. These parameters generally exhibit large spatial variations in a real fault system, and resolving such variations is of critical importance for realistic assessments of seismic hazard. Slowly evolving after slip reflects the slip response of a fault system to the coseismic stress loading, which is commonly observed following large earthquakes, thus providing information for constraining the fault stress and frictional parameters. In this study, we demonstrate that two independent and determinable frictional properties (i.e., σ (a − b) and R init = log(v init /v0), where σ, (a − b), v init and v0 are the effective normal stress, velocity-dependence frictional parameters, initial slip velocity, and reference velocity, respectively) can be obtained from an afterslip evolution process. We propose a two-step strategy that uses the temporally segmented afterslip models to invert for the independent parameters. After validating the performance of our inversion procedure by synthetic tests, we apply it to the 2019 Ridgecrest earthquakeafterslip process. Our results show that σ (a − b) in the main afterslip area is 0.2∼0.5 MPa. The spatial distribution of this parameter suggests a significant difference of pore pressure at the two ends of thecoseismic rupture. The depth profile of the effective normal stress in the afterslip concentration area reveals that the pore fluid pressure is hydrostatic above 5 km depth and increases to lithostatic from 5 to 20 km, indicating a gradual permeability decrease in this depth range. Our analysis also shows that the afterslip models which assume a uniform slip evolution function cannot be directly used for the estimation of the frictional properties.

Detection of Slow Slip Events Using Wavelet Analysis of GNSS Recordings

Article

Aug 2022

At many places, tectonic tremor is observed in relation to slow slip and can be used as a proxy to study slow slip events of moderate magnitude in which surface deformation is hidden in Global Navigation Satellite System (GNSS) noise. However, in subduction zones in which no clear relationship between tremor and slow slip occurrence is observed, these methods cannot be applied, and we need other methods to be able to better detect and quantify slow slip. Wavelets methods such as the Discrete Wavelet Transform and the Maximal Overlap Discrete Wavelet Transform (MODWT) are mathematical tools for analyzing time series simultaneously in the time and the frequency domains by observing how weighted differences of a time series vary from one period to the next. In this article, we use wavelet methods to analyze GNSS time series and seismic recordings of slow slip events in Cascadia. We use detrended GNSS data, apply the MODWT transform, and stack the wavelet details over several nearby GNSS stations. As an independent check on the timing of slow slip events, we also compute the cumulative number of tremor in the vicinity of the GNSS stations, detrend this signal, and apply the MODWT transform. In both the time series, we can then see simultaneous waveforms where timing corresponds to the timing of slow slip events. We assume that there is a slow slip event whenever there is a positive peak followed by a negative peak in the wavelet signal. We verify that there is a good agreement between slow slip events detected with only GNSS data and slow slip events detected with only tremor data for northern Cascadia. The wavelet-based detection method effectively detects events of magnitude higher than 6, as determined by independent event catalogs (e.g., Michel et al., 2019). As a demonstration of using the wavelet analysis in a region without significant tremor, we also analyze GNSS data from New Zealand and detect slow slip events that are spatially and temporally close to those detected previously by other studies.

Pre-seismic deformation detected from regional GNSS observation network: A case study of the 2013 Lushan, eastern Tibetan Plateau (China), Ms 7.0 earthquake

Article

Full-text available

May 2019
J ASIAN EARTH SCI

GNSS (Global Navigation Satellite System) observation is complicated by the presence of noise, local benchmark motion, un-modeled atmospheric and other effects. More subtle signals have existed the individual GNSS time series but have undetected due to its low signal-to-noise ratio (SNR). We introduce here a new method to extract spatially and temporally coherent signals by analyzing data from entire GNSS networks simultaneously using free-net solutions. Daily GNSS network is performed a similarity transformation by constraining one location and one direction. The deformation signal is extracted by analyzing the change of the GNSS geometry net-form integrating the location, baseline length and direction information of all GNSS stations from entire networks. Combined with area strain, the deformation characteristic are further interpreted. We apply the method for the 2013 Lushan Ms 7.0 earthquake and find a significant pre-seismic deformation anomaly with the locked status and negative acceleration in the first shear strain and direction change time series, implying a strong sinistral shear tectonic forces have happened in seismic source area and can intensify the pregnancy of the earthquake. The phenomena can be well supported and explained by the experiment results of rock fracture in the laboratory. It's inferred that a processes from stable locked status to negative acceleration change may be the most characteristic of pre-seismic deformation.

A Data-Adaptive Eof Based Method for Displacement Signal Extraction from Interferogram Time Series

Conference Paper

Jul 2018

Detection of transient aseismic slip signals from GNSS spatial-temporal data

Article

Jul 2015
CHINESE J GEOPHYS-CH

The aseismic slip or slow earthquakes is a very important component of the seismogenic process of faults. However, it is difficult to detect these small transient signals because of the low signal-to-noise ratio (SNR) in GNSS observation data. Based on the rich GNSS spatial-temporal data in active faults, a method of integrating processing of spatial-temporal noise and detection of transient aseismic slip signals is proposed. The transient signal and spatiotemporal correlated noise are expressed by the first-order Gaussian Markov process (FOGM). The SNR in the time domain is enhanced by Kalman filtering (KF). According to the high spatial coherence of fault deformation, the SNR in the space domain is enhanced by the principal component analysis (PCA). Combining spatiotemporal filtering of PCA with KF, the SNR of GNSS observation data is further improved. The result of simulations shows that the method can properly eliminate the effect of the linear trend, the year and half year cycle, further improve the space-time SNR of GNSS spatial-temporal data, and realize the detection of the transient aseismic slip signals even though the SNR is 1:1. Taking the slow slip event in Cascadia as an example, two aseismic slip events that happened in January 2007 and April 2008 were detected clearly. The interval of the two events was about 15 months and each event shows reverse displacements. The duration was about 18 days, and the displacement was about 8 mm, mainly distributed in the south edge of Cascadia subduction zone nearly in the range of 200 km. The slip characteristics were analyzed to be consistent with the relevant literature. Using the GNSS data in western Yunnan province from 2011 to 2013 provided by Crustal Movement Observation Network of China, the weak postseismic slip signals were detected. By principal component time response analysis, from March 2011 to June 2012, the displacements of NS and EW components exhibit obvious abnormal deviations, about 6 mm and 8 mm respectively. By space response analysis, the creep deformation is mainly distributed in the south section of the Lancangjiang fault, Red River fault and Xiaojiang fault. The spatial-temporal distribution corresponds to Burma MW7.2 earthquake on March 24, 2011. It is concluded that the fault activity in western Yunnan province is closely related to the Burma earthquake. Although it is difficult to improve the SNR of the single station GNSS observation data, using the spatial-temporal series of the whole GNSS network, according to the high spatial coherence of the fault deformation, based on KF combining with PCA, allows us to reduce the space-time irrelevant noise effectively and to reveal the temporal-spatial distribution characteristics of the transient aseismic slip. It would provide an important constraint and priori information for further fine inversion of fault parameters.

Toward a Continuous Monitoring of the Horizontal Displacement Gradient Tensor Field in Southern California Using cGPS Observations from Plate Boundary Observatory (PBO)

Article

May 2013

Detection and characterization of strain transients provides opportunities for better understanding of crust and mantle rheology (Freed and Burgman, 2004; Freed et al. , 2007; Pollitz et al. , 2012), fault zone behavior (McGuire and Segall, 2003; Miyazaki et al. , 2011), and the mechanics of parts of the earthquake cycle (Dragert and Wang, 2011; Bartlow et al. , 2011). The relatively dense coverage of continuous GPS (cGPS) within western North America, through EarthScope Plate Boundary Observatory (PBO), enables the possibility of detecting crustal strain transient phenomena on a variety of spatial and temporal scales (Ohtani et al. , 2010; Ji and Herring, 2011). In this paper we present a geodetic network‐processing tool for detection of anomalous strain transients in Southern California, or within any region with reasonably dense cGPS coverage. The modeling procedure determines time‐dependent displacement gradient fields from cGPS time series (Hernandez et al. , 2005; Hernandez, 2007) and determines the significance of any departures of this field relative to a geodetic reference model. The derived field satisfies strain‐rate compatibility relations everywhere within the plate boundary zone on a sphere. Given sufficient spatial coverage of cGPS, the method can detect anomalous strain, either from shear or volume sources, and either on or off the known faults. In Determination of a Geodetic Reference Model, we present a brief description of the derivation of the geodetic reference model. We show from a benchmarking exercise that GPS observations in southern California (Shen et al. , 2011) are sufficiently dense to provide a reliable estimate of a reference model. We also show that a joint fitting of reference model strains, together with the sparser distribution of GPS from PBO stations, yields minimal strain‐rate artifacts. In Method for Obtaining Time‐Dependent Displacement Estimates from cGPS Observations, we introduce the method for interpolating the …

Testing Kalman Smoothing/PCA Transient Signal Detection Using Synthetic Data

Article

May 2013

The analysis of the synthetic data from the SCEC transient-detection exercise allows us to make the following observations about the Kalman smoothing/PCA detection algorithm. In the analysis method we used here, we were able to mitigate some level of correlated noise in the station positions by using a network transformation to better isolate the relative motions of sites in small regions. Although a variety of temporally correlated noise processes were included in the simulated data sets, the FOGM process was able to approximate the correlated noise sufficiently well to allow the transient signals to be detected. When a large signal exists in the data, we allowed more variations during the time interval of the signal to minimize the smoothing of the signal. The use of the smoothing Kalman filter allowed interpolation across gaps in the data set thus increasing the total amount of data available for analysis. The analysis of the synthetic data also shows that the detection capability of PCA is improved when more sites record the signal. For large data sets, subsetting of the data in space and time reduces the noise contributions from other sites not affected by the transient signal. It is important to remove outliers (i.e., sites with anomalously large spatial amplitudes mostly due to site-specific noise) before the PCA. If large outliers are included, they tend to dominate the first PC. Careful calculation of the uncertainties of the PCs and sample eigenvectors allows the identification of signals in both space and time. We did find that relatively uniform large-scale signals in space are difficult to separate from network common errors, which tend to be removed by the network transformations that we applied. Multiple transient and/or propagating signals can be detected with our algorithm if the SNR is high. Both of these types of signals appear in multiple PCs and care must be used in interpreting these multiple components because they will blend the signals from either the multiple transients or the propagating signals. The larger transient signals can mask weaker ones thus careful analysis is necessary. Propagating and small transient signals can be difficult to detect and we are still developing the algorithm to better characterize the signals.

The Mw 6.5 Offshore Northern California Earthquake of January 10, 2010:ordinary stress drop on a high strength fault

Article

Sep 2014
GEOPHYS RES LETT

The January 10, 2010 Mw 6.5 earthquake offshore Northern California is one of the first intraplate earthquakes in oceanic lithosphere to be well captured by a GPS network. It presents an opportunity to evaluate rupture mechanics on a high-strength fault. Static inversion of the co-seismic displacements shows that the slip peaks at the same depth as the expected strength envelope, where the differential stresses can be as high as 600 MPa. Laboratory experiments on peridotite predict dramatic dynamic weakening at these conditions. The observed ordinary stress drop, 2–20 MPa, may indicate that the lithosphere is much weaker than strength envelope predicts or that the failure mechanisms seen in the laboratory are not occurring during the rupture. The GPS observations show very little post-seismic signal indicating that if a shear-zone exists beneath the co-seismic rupture, it operates at significantly greater stress levels than the co-seismic stress change.

A slow slip event in the south central Alaska Subduction Zone and related seismicity anomaly

Article

Aug 2012

We detected a slow slip event in the south central Alaska Subduction Zone by analyzing continuous GPS data from the Plate Boundary Observatory (PBO) network. The slow slip event started in early 2010 at a depth of 35 km beneath the Cook Inlet, near the down-dip end of the locked zone, and is ongoing as of November 2011 with an accumulated magnitude of Mw 6.9. Analysis of the earthquake catalog in the same area using the stochastic Epidemic Type Aftershock Sequence model (ETAS) shows a small but detectable seismicity increase during the slow slip event. We also find a change in seismicity rate around 1998, that may suggest an earlier slow slip event in the same region. Slow slip events in Alaska appear more widespread than previously thought but have remained undetected due to their long durations, the time intervals between them, and the limited time records from the continuous GPS.

Spatiotemporal filtering using principal component analysis and Karhunen-Love expansion approaches for regional GPS network analysis

Article

Full-text available

Mar 2006

1] Spatial filtering is an effective way to improve the precision of coordinate time series for regional GPS networks by reducing so-called common mode errors, thereby providing better resolution for detecting weak or transient deformation signals. The commonly used approach to regional filtering assumes that the common mode error is spatially uniform, which is a good approximation for networks of hundreds of kilometers extent, but breaks down as the spatial extent increases. A more rigorous approach should remove the assumption of spatially uniform distribution and let the data themselves reveal the spatial distribution of the common mode error. The principal component analysis (PCA) and the Karhunen-Loeve expansion (KLE) both decompose network time series into a set of temporally varying modes and their spatial responses. Therefore they provide a mathematical framework to perform spatiotemporal filtering. We apply the combination of PCA and KLE to daily station coordinate time series of the Southern California Integrated GPS Network (SCIGN) for the period 2000 to 2004. We demonstrate that spatially and temporally correlated common mode errors are the dominant error source in daily GPS solutions. The spatial characteristics of the common mode errors are close to uniform for all east, north, and vertical components, which implies a very long wavelength source for the common mode errors, compared to the spatial extent of the GPS network in southern California. Furthermore, the common mode errors exhibit temporally nonrandom patterns. (2006), Spatiotemporal filtering using principal component analysis and Karhunen-Loeve expansion approaches for regional GPS network analysis, J. Geophys. Res., 111, B03405, doi:10.1029/2005JB003806.

Multiscale Estimation of GPS Velocity Fields

Article

Full-text available

Dec 2008

We present a spherical-wavelet-based multiscale representation for three-component velocities on the earth's surface, as a tool to facilitate analysis of geodetic observations in dense GPS networks. We design an efficient inverse problem to determine a set of wavelet coefficients that describe the irregularly distributed observations. Once the velocity field is estimated, we readily compute spatial derivative quantities, such as the strain rate and dilatation rate. In comparison to previous GPS studies, novel aspects of our approach include: (1) an explicit and consistent decomposition of the velocity field into multiple scales at all locations; (2) a minimum scale at which we estimate the velocity field at a particular location that is controlled by local station coverage; (3) inclusion of the vertical velocity observations if they are available; and (4) use of spherical wavelets in representing the velocity field. The multiscale approach has the additional advantage of allowing one to compare estimated quantities in different locations, but at the same scale. We test the method for several synthetic examples, as well as for the NASA REASoN cGPS velocity field for southern California. Thus far we have only concentrated on the spatial representation of a time-dependent displacement field, showing that the multiscale representation is well-suited to identifying and characterizing geophysical signals. Our approach is a step toward monitoring of GPS time series, in hopes of identifying and characterizing transient deformation. By focusing on quantities inferred from multiple sites (e.g., strain rate), our approach does not rely on individual stations, and instead focuses on locally correlated behavior.

Time-dependent triggered afterslip following the 1989 Loma Prieta earthquake

Article

Full-text available

Mar 2000

We employ a modified version of the Network Inversion Filter to investigate time-dependent slip following the 1989 Loma Prieta earthquake. Previous analysis of Global Positioning System (GPS) and leveling data suggests afterslip on the Loma Prieta rupture as well as aseismic slip on a thrust fault northeast of the San Andreas fault which we identify with the Foothills thrust belt. We analyzed 173 daily GPS solution files at 62 stations collected from 1989.8 to 1998.3 (a total of 1,134 three-dimensional relative baseline determinations). The observed position changes are assumed to result from secular deformation, random benchmark motions, and temporally varying fault slip. The data reveal temporal variations in slip rate but poorly resolve spatial variations in fault slip. The amount of temporal smoothing is estimated by maximum likelihood. Conditional on this estimate, reverse slip on the Foothills thrust decays from 45±12 mm/yr immediately after the earthquake to zero by 1992. Reverse slip on the Loma Prieta rupture surface decays from 57±11 mm/yr to zero by 1994. Right-lateral slip on the Loma Prieta rupture surface decays monotonically from 30±10 mm/yr to zero by 1994. These results suggest that (1) triggered afterslip can occur off the main rupture zone on adjacent faults, (2) shallow afterslip dominated the postseismic deformation for the 8 years following the earthquake, and (3) postseismic slip on the Foothills thrust may account for a significant portion of its total slip budget.

Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica

Article

Full-text available

Nov 2004

New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (˜1 year) observation period. We also estimate northwest motion of a coastal "sliver block" at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with ˜8 cm/yr on the Cocos-Panama block boundary (including a component of permanent shortening across the Fila Costeña fold and thrust belt) and ˜1 cm/yr on the Panama block-Caribbean boundary. The GPS data suggest that the Cocos plate-Panama block boundary is completely locked from ˜10-50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence.

The Southern California Integrated GPS Network (SCIGN)

Article

Full-text available

Jan 2001

This paper describes the fundamental scientific and observational goals of the Southern California Integrated GPS Network (SCIGN). It also reviews the organizational structure, the project management, and important policies. Recent accomplishments are also discussed in some detail, in particular the effective response following the October 16, 1999 Hector Mine earthquake (Mw7.1). The scientific motivation for establishing a continuous GPS network to observe time transient crustal deformation is discussed in detail, especially as it pertains to the 1990's sequence of fault interaction and postseismic deformation in the eastern California shear zone. As of March 2001, a total of 239 SCIGN stations are operating; 170 of the newest stations were installed by a contractor. The third phase of network construction was recently completed at an average rate of 2 new stations per week. Network implementation is continuing, and construction of the final 16 stations will be fully completed by July 1, 2001.

The 2006 aseismic slow slip event in Guerrero, Mexico: New results from GPS

Article

Full-text available

Jul 2007
GEOPHYS RES LETT

Crustal deformation measurements from Guerrero, Mexico were made with continuous GPS instrumentation. This network spans 75 km along the coast and a 275 km transect from the coast to Mexico City, which is perpendicular to the Middle America trench. A large aseismic slip transient occurred from April to December 2006, yielding horizontal displacements of nearly 6 cm in the direction opposite to that recorded interseismically. This transient slip episode closely follows a series of ~M5 deep intraplate events that occurred in the northwestern portion of the Guerrero region. Both horizontal and vertical displacements are inverted for the Guerrero GPS network to determine slip along the subduction interface. Using a simple fault model that accommodates changes in the subduction zone geometry, it is found that slip is concentrated on interior fault patches, and propagated to the southeast. This slip event has a minimum equivalent Mw of 7.5.

Coseismic and early postseismic slip for the 2003 Tokachi-oki earthquake sequence inferred from GPS data

Article

Full-text available

Feb 2008

Early afterslip following the 2003 Tokachi-oki earthquake is investigated using subdaily GPS time series. Afterslip results are compared with the coseismic slip for the M8 mainshock and the M7.4 aftershock. Afterslip between those two earthquakes is inferred at the southwestern adjacent region of the mainshock, between two epicentral regions, which possibly triggered the aftershock in the southwest. Subsequently, deeper slip occurs. The afterslip loci are distinct from the rupture regions. The non-uniform propagation of afterslip may reflect the depth-dependence of the effective normal stress and the distance between the closest unstable slip patches.

A large silent earthquake in the Guerrero seismic gap, Mexico

Article

Full-text available

Aug 2003
GEOPHYS RES LETT

Geodetic measurements from a network of permanent GPS stations along the Pacific coast of Mexico reveal a large “silent earthquake” along the segment of the Cocos-North American plate interface identified as the Guerrero seismic gap. The event began in October of 2001 and lasted for 6–7 months. Average slip of ∼10 cm produced measurable displacements over an area of ∼550 × 250 km2. The equivalent moment magnitude of the event was Mw ∼ 7.5. Recognition of this and previous slow event here indicate that the seismogenic portion of the plate interface is not loading steadily, as hitherto believed, but is rather partitioning the stress buildup with episodic, as opposed to steady-state or periodic, slip downdip of the seismogenic zone. This process increases the stress at the base of the seismogenic zone, bringing it closer to failure. These results call for a reassessment of the seismic potential of Guerrero and other seismic gaps in Mexico.

Slow slip events and seismic tremor at Circum-Pacific subduction zones

Article

Full-text available

Sep 2007

It has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements. However, only recently has a fuller spectrum of strain release processes, including normal, slow, and silent earthquakes (or slow slip events) and continuous and episodic slip, been observed and generated by numerical simulations of the earthquake cycle. Despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive. The concurrence of seismic tremor with slow slip episodes in Cascadia and southwestern Japan provides insight into the process of slow slip. A perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface. Alternatively, evidence is accumulating to support the notion that tremor results from shear failure during slow slip. Global observations of the location, spatial extent, magnitude, duration, slip rate, and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation. Most slow slip events occur just downdip of the seismogenic zone, consistent with rate- and state-dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation. At a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation. Slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault, bringing it closer to or farther from earthquake failure, respectively. This paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide, with a focus on circum-Pacific subduction zones. Trends in global observations of slow slip events suggest that (1) slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it, (2) different frictional properties likely control fast versus slow slip, (3) the depth range of slow slip may be related to the thermal properties of the plate interface, and (4) the equivalent seismic moment of slow slip events is proportional to their duration (Moατ), different from the Moατ3 scaling observed for earthquakes.

Characterizing an ``uncharacteristic'' ETS event in northern Cascadia

Article

Full-text available

Aug 2008
GEOPHYS RES LETT

GPS and borehole strainmeter data allowed the detection and model characterization of a slow slip event in northern Cascadia in November 2006 accompanying a brief episode of seismic tremor. The event is much smaller in area and duration than other well-known ETS events in northern Cascadia but is strikingly similar to typical ETS events at the Nankai subduction zone. The 30-45 km depth range and the 2-3 cm slip magnitude as interpreted for this event appear to be common to most ETS events in these two subduction zones, regardless of their sizes. We infer that the Nankai-type small ETS events must be abundant at Cascadia and that ETS events at the two subduction zones are governed by a similar physical process.

Southern California Permanent GPS Geodetic Array: Error Analysis of Daily Position Estimates and Site Velocities

Article

Full-text available

Aug 1997

We analyze time series of daily positions estimated from data collected by 10 continuously monitoring Global Positioning System (GPS) sites in southern California during the 19-month period between the June 28, 1992 (MW=7.3), Landers and January 17, 1994 (MW=6.7), Northridge earthquakes. Each time series exhibits a linear tectonic signal and significant colored noise. Spectral power at frequencies in the range 5yr-1 to 0.5d-1 is dominated by white noise or possibly fractal white noise and is several orders of magnitude higher than what would be expected from random walk noise (in this short-period range) attributed by others to geodetic monument motions. Estimating a single slope for the time series' power spectra suggests fractal white noise processes with spectral indices of about 0.4. Site velocity uncertainties assuming this fractal white noise model are 2-4 times larger than uncertainties obtained assuming a purely white noise model. A combination white noise plus flicker noise (spectral index of 1) model also fits the data and suggests that the velocity uncertainties should be 3-6 times larger than for the white noise model. We cannot adequately distinguish between these two noise models, nor can we rule out the possibility of a random walk signal at the lowest frequencies; these questions await the analysis of longer time series. In any case, reducing the magnitude of low-frequency colored noise is critical and appears to be best accomplished by building sites with deeply anchored and braced monuments. Otherwise, rate uncertainties estimated from continuous GPS measurements may not be improved significantly compared to those estimated from infrequent campaign-mode measurements.

Time-Space Variable Afterslip on and Deep Below the Izmit Earthquake Rupture

Article

Full-text available

Feb 2002

Surface deformation transients measured with the Global Positioning System during the 87 days between the 17 August 1999 İzmit earthquake and the 12 November 1999 Düzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture. Elastic model inversions for time-dependent distributed fault slip, using a network inversion filter approach, show that afterslip was highest between and below the regions of maximum coseismic slip and propagated downward to, or even below, the base of the crust. Maximum afterslip rates decayed from greater than 2 m/yr, immediately after the İzmit earthquake to about 1.2 m/yr just prior to the Düzce earthquake. Maximum afterslip occurred below the eastern Karadere rupture segment and near the İzmit hypocenter. Afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone. These observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone. Continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby, Mw 7.2, 12 November Düzce earthquake.

GPS constraints on 34 slow slip events within the Cascadia subduction zone, 1997–2005

Article

Full-text available

Apr 2008

Refinements to GPS analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the Cascadia subduction zone from 1997 through 2005. Timing of transient onset is determined with wavelet transformation of geodetic time series. Thirty continuous stations are included in this study, ranging from northern California to southwestern British Columbia. Our improvements in analysis better resolve the largest creep events and also identify many smaller events. At 48.5°N latitude, a 14-month average recurrence interval has been observed over eight events since 1997. Farther north along Vancouver Island a host of smaller events with a distinct 14-month periodicity also occurs. In southern Washington State, some of the largest transient displacements are observed but lack any obvious periodicity in their recurrence. Along central Oregon, an 18-month recurrence is evident, while in northern California an 11-month periodicity continues through 2005. We invert GPS offsets of the 12 best recorded events for thrust slip along the plate interface using a cross-validation scheme to derive optimal smoothing parameters. These 12 events have equivalent moment magnitudes between 6.3 and 6.8 and have 2–3 cm of slip. Unlike other subduction zones, no long-duration events are observed, and cumulative surface deformation is consistently less than 0.6 cm. The many newly resolved smaller transient events in Cascadia show that slow slip events occur frequently with GPS best capturing only the largest events. It is likely that slow slip events occur more frequently at levels not detectable with GPS.

A slow earthquake sequence on the San Andreas Fault

Article

Full-text available

Sep 1996
NATURE

EARTHQUAKES typically release stored strain energy on timescales of the order of seconds, limited by the velocity of sound in rock. Over the past 20 years, observationsl - 13 and laboratory experi. ments14 have indicated that rupture can also occur more slowly, with durations up to hours. Such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones. The detection of events with larger timescales requires near-field deformation measurements. In December 1992, two borehole strainmeters close to the San Andreas fault in California recorded a slow strain event of about a week in duration, and we show here that the strain changes were produced by a slow earthquake sequence (equivalent magnitude 4.8) with complexity similar to that of regular earthquakes. The largest earthquakes associated with these slow events were small (local magnitude 3.7) and contributed negligible strain release. The importance of slow earthquakes in the seismogenic process remains an open question, but these observations extend the observed timescale for slow events by two orders of magnitude.

Segmentation in Episodic Tremor and Slip All Along Cascadia

Article

Full-text available

Nov 2006
GEOLOGY

Recent developments in the scientific study of the Cascadia subduction zone reveal new information about how plates are deforming and hold the promise of new insight into the associated hazards and geologic formations. The key advances have been the discovery of episodic tremor and slip (ETS) and forearc basins interpreted as manifestations of megathrust asperities. The processes that govern ETS and potential relationships with major earthquakes remain unknown, yet these events have been proposed to be an integral part of the subduction process and may have an impact on the likelihood of earthquakes. Using new single-station methods to identify non-volcanic tremor and slow slip episodes over broader regions and time periods than previous studies, we compile an ETS catalog for the entire Cascadia subduction zone over the past 10 years. Correlated GPS and seismic signals are found all along the convergent margin, suggesting that ETS is an inherent part of the subduction process. The expanded dataset reveals 3 broad coherent zones with different recurrence intervals (11 mo, 14 mo, 19 mo), where the interval duration and zone boundaries correlate best with upper plate topography and geologic terrains. In particular, the recurrence interval appears to be related to rheology dictated by the overriding plate, as the zones with lighter, weaker, and more continental-like crust have shorter "hold times" between events. The 3 broad zones of common recurrence interval are further divided into segments by the phase of ETS events. The 6 largest (100-200 km) segments correlate with locations of fore-arc basins interpreted as manifestations of megathrust asperities, revealing new links between ETS and seismogenic behavior.

Wavelet Analysis In Geophysics: An Introduction

Chapter

Full-text available

Jan 1994

Wavelet analysis is a rapidly developing area of mathematical and application-oriented research in many disciplines of science and engineering. The wavelet transform is a localized transform in both space (time) and frequency, and this property can be advantageously used to extract information from a signal that is not possible to unravel with a Fourier or even windowed Fourier transform. Wavelet transforms originated in geophysics in early 1980's for the analysis of seismic signal. After a decade of significant mathematical formalism they are now also being exploited for the analysis of several other geophysical processes such as atmospheric turbulence, space-time rainfall, ocean wind waves, seafloor bathymetry, geologic layered structures, climate change, among others. Due to their unique properties, well suited for the analysis of natural phenomena, it is anticipated that there will be an explosion of wavelet applications in geophysics in the next several years. This chapter provides a basic introduction to wavelet transforms and their most important properties. The theory and applications of wavelets is developing very rapidly and we see this chapter only as a limited basic introduction to wavelets which we hope to be of help to the unfamiliar reader and provide motivation and references for further study.

Transient fault slip in Guerrero, Southern Mexico

Article

Full-text available

Oct 2001

The Guerrero region of southern Mexico has ac-cumulated more than 5 m of relative plate motion since the last major earthquake. In early 1998, a continuous GPS site in Guerrero recorded a transient displacement. Modeling indicates that anomalous fault slip propagated from east to west along-strike of the subduction megathrust. Campaign GPS and leveling data corroborate the model. The moment release was equivalent to an Mw≥6.5 earthquake. No M>5 earthquakes accompanied the event, indicating the frictional regime is velocity-strengthening at the location of slip.

GPS Applications for Geodynamics and Earthquake Studies

Article

Full-text available

Jan 1998

Geodetic measurements obtained with the Global Positioning System (GPS) are increasingly more widely applied in geophysical studies. In this paper, we re-view the changes to the technology of GPS geodesy over the last five years that are responsible for this increased applicability. We survey geophysical inves-tigations employing GPS to measure coseismic, postseismic, and interseismic deformation; plate motion and crustal deformation at plate boundaries; volcano deformation; and the deformation associated with glacial isostatic adjustment and its application to sea-level studies. We emphasize the use of GPS determinations for the modeling of this wide variety of geophysical phenomena. We also discuss the recent advent of permanent GPS networks for regional geophysical studies, as well as the possible future of GPS surveying in light of the recent advances.

A decade of GEONET: 1994-2003-The continuous GPS observation in Japan and its impact on earthquake studies

Article

Full-text available

Aug 2004
EARTH PLANETS SPACE

Takeshi Sagiya

The dense continuous GPS network of Japan, now called GEONET, has been operated since 1994 by the Geo-graphical Survey Institute. GEONET provides precise daily coordinates of all the stations, with which displacement rates and strain rates are calculated nationwide. Various characteristics of tectonic deformation in the Japanese Is-lands have been revealed. GEONET is also quite useful in earthquake studies, precisely detecting co-seismic, post-seismic, and inter-seismic deformation signals. These observations are utilized to infer physical processes at earthquake sources. Slow slip events on plate boundaries have been found from GPS data. Such slow events provide an important constraint on the mechanism of faulting. On the other hand, there has been no success in detecting pre-seismic deformation. Lack of a precursory signal before the 2003 Tokachi-Oki (M8.0) earthquake has posed a serious question to short-term earthquake prediction. GEONET enables a good linkage between monitoring and modeling studies, opening a possibility of practical data assimilation. For further contribution to earthquake studies, it is necessary to continue GEONET with high traceability on the details in observation and analysis.

A slow thrust slip event following the two 1996 Hyuganada Earthquakes beneath the Bungo Channel, southwest Japan

Article

Full-text available

Nov 1999

We report a "slow thrust slip event" that occurred beneath the Bungo Channel region, southwest-ern Japan. On Oct. and Dec., 1996, two Hyuganada earthquakes (both M w = 6.7), followed by afterslips, occurred. In addition, a crustal movement character-ized by an extremely slow rise was observed around the Bungo Channel, about 200 km north from the epicen-ters, and continued for about 300 days long. Assuming a slow slip on the plate boundary, we estimate its du-ration and surface displacements from GPS time series data by curve-fitting, and then, determine the fault slip distribution. We found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the Hyuga-nada earthquakes. Occurrence of the slow thrust slip event suggests that this kind of event may be a charac-teristic mode of stress release at a transition region of interplate coupling.

Occurrence of quasi-periodic slow-slip off the east coast of the Boso peninsula, Central Japan

Article

Full-text available

Dec 2007
EARTH PLANETS SPACE

An analysis of Global Positioning System (GPS) data revealed south-southeastward transient deformation caused by the expected slow-slip near the Boso peninsula, central Japan, approximately 5 years after a similar event in 2002. An area of aseismic slip with a moment magnitude of 6.6 was estimated off the Boso peninsula, adjacent to the area of associated seismic activity. The 2007 aseismic slip started from around August 10, expanded slightly to the north, and gradually ceased activity over a period of about 10 days. This rupture process is different from those of the last two events in which the slow-slip area moved from north to south. However, the three slow-slip events detected by the GPS array verify the hypothesis that the Boso slow-slips occur quasi-periodically in a certain area, accompanied by seismic swarms. The factors causing the slight differences in the rupture process, magnitude, recurrence time, and slip area among the observed slow-slip events remain unclear.

Transient crustal deformation in Tokai region, Central Japan, until May 2004

Article

Full-text available

Oct 2005

An analysis of Global Positioning System (GPS) data reveals a continuation of the transient crustal deformation that was first reported by Ozawa et al. (2002) in the western Tokai region. Kalman filtering following the time-dependent inversion shows a slow-thrust slip occurring on the plate boundary in the western Tokai region, with its center around Lake Hamana, which is adjacent to the anticipated Tokai earthquake source area. The moment release from the Tokai silent earthquake was observed to be continuous surpassing M w 7.0 in May 2004, with relative slip increases to the northeast and north of Lake Hamana from 2002. The moment release rate decreased in 2002, and then increased in 2003, reaching the rate observed in 2001.

Rapid deformation of Kilauea Volcano- Global Positioning System measurements between 1990 and 1996

Article

Full-text available

Aug 2000
J GEOPHYS RES

Campaign Global Positioning System (GPS) measurements from 1990 to 1996 are used to calculate surface displacement rates on Kilauea Volcano, Hawaii. The GPS data show that the south flank of the volcano, which has generated several large earthquakes in the past 3 decades, is displacing at up to 8 cm/yr to the south-southeast. The summit and rift zones are subsiding, with maximum subsidence rates of 8 cm/yr observed a few kilometers south of the summit caldera. Elastic dislocation modeling of the GPS data suggests that the active sources of deformation include deep rift opening along the upper east and east rift zone, fault slip along a subhorizontal fault near the base of the volcano, and deflation near the summit caldera. A nonlinear optimization algorithm was used to explore the parameter space and to find the best fitting source geometry. There is a broad range of model geometries that fit the data reasonably well. However, certain models can be ruled out, including those that have shallow rift opening or shallow fault slip. Some offshore, aseismic slip on a fault plane that dips between 25 north-northwest and 8 south-southeast is required. Best fitting slip and rift opening rates are 23–28 cm/yr, although rates as low as 10 cm/yr are permitted by the data.

Repeating short- and long-term slow slip events with deep tremor activity around the Bungo Channel region, southwest Japan

Article

Full-text available

Oct 2005

We report the repeating occurrence of short-and long-term slow slip events (SSE) which are accompanied by deep tremor activity around the Bungo channel region, southwest Japan. Both of these activities are detected by NIED Hi-net, which is composed of densely distributed observatories equipped with a set of tiltmeter and a high-sensitivity seismograph. Since the short-term SSE is small in magnitude, GPS can detect only the long-term SSE. Some of these episodes have nearly the same surface deformation pattern. This shows the existence of 'slow slip patches' on a plate interface, where the episodic slow slip is the characteristic slip behavior. We observe a change in periodicity and size of the short-term episode after the onset of the long-term SSE. Moreover, the long-term slow slip accelerates when the short-term activity takes place. This suggests that there is an interaction between these two types of SSEs.

Wavelet Analysis for Geophysical Application

Article

Full-text available

Nov 1997

Wavelet transforms originated in geophysics in the early 1980s for the analysis of seismic signals. Since then, significant mathematical advances in wavelet theory have enabled a suite of applications in diverse fields. In geophysics the power of wavelets for analysis of nonstationary processes that contain multiscale features, detection of singularities, analysis of transient phenomena, fractal and multifractal processes, and signal compression is now being exploited for the study of several processes including space-time precipitation, remotely sensed hydrologic fluxes, atmospheric turbulence, canopy cover, land surface topography, seafloor bathymetry, and ocean wind waves. It is anticipated that in the near future, significant further advances in understanding and modeling geophysical processes will result from the use of wavelet analysis. In this paper we review the basic properties of wavelets that make them such an attractive and powerful tool for geophysical applications. We discuss continuous, discrete, orthogonal wavelets and wavelet packets and present applications to geophysical processes.

Improvement of the analysis strategy of GEONET

Article

Jan 2003

Transient Crustal Deformation in the Tokai District

Article

Jan 2009

We investigated the transient crustal deformation in the Tokai district, central Japan, based on the GPS data in 2000-2008. Our modeling showed that observed transient deformation in the Tokai district can be explained by three different mechanisms, slow slip on the plate interface, viscoelastic response and an afterslip following the 2004 off southeast Kii peninsula earthquake. In our modeling, we first evaluated the effect of postseismic deformation due to the viscoelastic response of the 2004 off southeast Kii peninsula earthquake, and obtained the reasonable value of viscosity 1.0×1019 Pa·s. Then we estimated the afterslip model of the 2004 off southeast Kii peninsula earthquake, and obtained that the time constant of the postseismic deformation caused by the afterslip is 62.65 days. After removing the viscoelastic response and the effect of afterslip from original transient deformation, we modeled this transient deformation as a slow slip on the plate interface, which we call here as Tokai slow slip event. Our result revealed that the Tokai slow slip which occurred mainly under the Lake Hamana ended in July 2005. The maximum slip of this event was estimated at 25cm under the Lake Hamana. This maximum slip accounts for the backslip accumulated in the past 14 years, assuming that the backslip rate has been constant. The total moment release was equivalent to that of an Mw 7.1 earthquake. On the other hand, the aseismic slip still continues in the northeast region of the Lake Hamana at the same rate as that before July 2005. The maximum slip in this region since July 2005 reaches 5cm, and total moment release is equivalent to that of an Mw 6.5 earthquake.

A New Approach to Time-Dependent Inversion of Geodetic Data Using Monte Carlo Mixture Kalman Filter

Article

Dec 2003

It is important to precisely clarify time history of transient fault slip or magma intrusion (hereafter we simply call them fault slip) from surface deformation measurements to understand the physical processes of earthquake generation and volcano eruption. We have developed a new time dependent inversion method for imaging detailed time history of fault slip from geodetic data. We employed a stochastic model that enables non-parametric description of fault slip evolution. In this framework, temporal smoothness of slip is determined by a scaling parameter (temporal smoothing parameter). In order to reconstruct detailed time history of fault slip, we assumed that the temporal smoothing parameter is variable in space and time. In this case, Kalman filter that has been used in past studies cannot be used because spatio-temporal distribution of the temporal smoothing parameter is unknown. In order to estimate slip history under this condition, we have developed a new filtering technique, Monte Carlo mixture Kalman filter (MCMKF), and applied it to the time dependent inversion. The validity of the MCMKF based inversion scheme is investigated through numerical experiments. In the numerical experiments, the MCMKF based inversion scheme is applied to the simulated displacement time series generated by 4 sources: 1 Mogi source, 2 tensile faults, and 1 shear fault. In a source, the temporal smoothing parameter and the slip are assumed to be temporally variable and spatially uniform, and the temporal evolutions of the temporal smoothing parameter and the slip are allowed to take different values in respective sources. Then results are compared with those obtained by the conventional Kalman filter based scheme. The results show that the MCMKF based method is capable of imaging spatio-temporal variation of fault slip together with the temporal smoothing parameter that is variable in space and time. We also find that the MCMKF is able to image whole time history of transient slip events more precisely than Kalman filter. Especially, the MCMKF is superior to Kalman filter in precisely reproducing slip rate changes and propagating slip. Furthermore, the MCMKF gives significantly smaller AIC (Akaike information criterion) values than Kalman filter. These results indicate that the MCMKF yields better slip estimates than Kalman filter.

Geodetic inversion for space-time distribution of fault slip with time-varying smoothing regularization

Article

Apr 2008

We have developed a new geodetic inversion method for space-time distribution of fault slip velocity with time-varying smoothing regularization in order to reconstruct accurate time histories of aseismic fault slip transients. We introduce a temporal smoothing regularization on slip and slip velocity through a Bayesian state space approach in which the strength of regularization (temporal smoothness of slip velocity) is controlled by a hyperparameter. The time-varying smoothing regularization is realized by treating the hyperparameter as a time-dependent stochastic variable and adopting a hierarchical Bayesian state space model, in which a prior distribution on the hyperparameter is introduced in addition to a conventional Bayesian state space model. We have tested this inversion method on two synthetic data sets generated by simulated aseismic slip transients. Results show that our method reproduces well both rapid changes of slip velocity and steady-state velocity without significant oversmoothing and undersmoothing, which has been hard to overcome by the conventional Bayesian approach with time-independent smoothing regularization. Application of this method to transient deformation in 2002 caused by a silent earthquake off the Boso peninsula, Japan, also shows similar advantages of this method over the conventional approach.

Low Frequency Earthquake Swarms and Non-Volcanic Tremor under Shikoku, Japan

Article

Jan 2006

Low frequency earthquakes (LFEs) are relatively energetic and isolated portions of non-volcanic tremor with identifiable phases that are routinely identified by the Japan Meteorological Agency. Compared to regular earthquakes from a similar hypocentral region, LFE are enriched in low frequencies (1-5 Hz) and depleted in higher frequencies (above 10 Hz). We precisely located 1180 LFEs using waveform cross-correlations and found them to lie along the subduction interface at a depth of 30-35 km (Shelly et al., 2006). Based on their locations on the plate boundary and the character of their waveforms, we hypothesized that LFEs may represent fluid-enabled shear slip on the plate boundary as part of concurrently observed slow slip events. Subsequent analysis indicates LFE source mechanisms are consistent with this interpretation (Ide et al., 2006). Since these LFEs represent only a very small portion of tremor, their relationship to continuous tremor has remained uncertain. In this study we use waveforms from NIED Hi-net high sensitivity borehole seismometers to examine the relationship between LFEs and the rest of the tremor. We search continuous waveform records during non-volcanic tremor activity for events similar to previously recorded and located "template events" using a matched-filter technique. As template events, we employ 677 well-recorded LFEs that occurred in this region between 2002 and 2005. We then search tremor episodes systematically for events resembling these template events by cross-correlating corresponding stations and components between the template events' S-waves and continuous data. We register a detection when the pattern of waveforms across stations in the continuous data is similar to that produced by the template event. Using this procedure, we can detect events within continuous tremor many times weaker than our (already weak) template events even when multiple sources are active concurrently. We find that long duration tremor is often composed of the superposition of signals from a swarm-like sequence of many small LFEs. This is particularly apparent during the larger amplitude bursts within the tremor waveforms. Moreover, spectra for the rest of the tremor and spectra for the LFEs are indistinguishable from one another and distinct from the spectra of earthquakes or background noise. We conclude that much of the tremor, including the most energetic bursts, can be thought of as a swarm of LFEs on the plate interface. Although the mechanism of the rest of the tremor is more difficult to determine, its spectra appears consistent with this explanation as well.

Transient subduction slip episodes in Japan observed by the nationwide GPS array

Article

We observe slow thrust slip events along the Suruga-Nankai Trough where the Philippine Sea plate is subducting beneath the Japanese Islands. The first event initiated around 1997.0 at the Bungo Channel, following two successive M 6.7 earthquakes off Hyuganada. The second event occurred just west (down dip) of the Tokai seismic gap. We utilize the Network Inversion Filter, recently modified by McGuire and Segall [2002] to invert the GPS observations. The improved method enables us to implement non-negativity constraints and automatic estimation of hyperparameters, including spatial and temporal smoothing. The results of the Bungo event show that the slip initiated just south of southwesternmost Shikoku Island and propagated west to the Bungo Channel. The result clearly demonstrates that the slow event has no connection with the Hyuganada earthquakes, though it may have been triggered by those preceding events. The total aseismic moment release corresponds to Mw 7.2. The Tokai event initiated following major volcanic activity in the Izu Islands (since the end of June, 2000). The slip initially accelerated during the first 8 months. The slow slip event is not yet over; the present slip-rate is the same as in 2001. Currently the cumulative moment release corresponds to Mw 6.8. We note that both events occurred at sites where the plate interface shows significant lateral bending. Such a geometry, as well as heterogeneity of fault constitutive parameters, may be important in generating slow events.

Introduction to random signals and applied Kalman filtering : with MATLAB exercises and solutions

Article

Jan 1997

Characteristic silent earthquakes in the eastern part of the Boso peninsula, Central Japan

Article

Mar 2003
GEOPHYS RES LETT

Analysis of Global Positioning System (GPS) data shows transient crustal deformation in the Boso peninsula, central Japan, for about fifty days from October 2002. The southeastward horizontal displacements detected suggest the occurrence of interplate aseismic slip between the Philippine Sea plate and the North American plate, six years after a similar event in 1996. We estimate an area of aseismic slip with moment magnitude (Mw) 6.6 centered off the Boso peninsula, adjacent to associated seismic activity. Slip evolution of the 2002 aeismic slip started on October 4 and migrated from north to south for about ten days, followed by gradual subsidence and restarting of slip which lasted until ∼December 2, propagating more southward. Similarities in focal area and slip process between the 1996 and the 2002 events suggest the Boso aseismic slips as characteristic silent earthquakes, together with the repetitive occurrence of seismic swarm in a similar region every six to seven years. A more tightly coupled southern area that did not release enough energy in the 1996 event may have caused the differences in magnitude, central slip area, and duration between the 1996 and the 2002 events, if the hypothesized recurrence interval for strain accumulation is true, based on swarm seismicity.

Time-dependent inversion study of the slow thrust event in the Nankai Trough subduction zone, southwestern Japan

Article

Jan 2001

We estimate time variation of interplate coupling between the subducting Philippine Sea and the overriding continental plates along the western part of the Nankai trough, Japan, during the period between 1996 and 1999. After subtracting annual components and average annual velocities, deviation from steady crustal deformation in the Shikoku and Kyushu islands is examined. Around 15 GPS stations facing the Bungo channel detected the following three-stage ground displacements of up to 3 cm in 1997, with a duration of ~1 year: (1) increased displacement rate associated with the 1996 Hyuga-nada earthquakes; (2) an increasingly rapid anomalous period of deformation from June 1997 to the end of 1997; and (3) a slowdown period and return to normal rates of deformation after the end of 1997. Linearized least squares locate the slip area of this slow event near the plate boundary beneath the Bungo channel. The estimated slip is opposite the motion of the Philippine Sea plate. These results support the hypothesis that the observed anomalous crustal deformation is due to a rebound process of the Philippine Sea plate. Time-dependent inversion analysis shows the following slip history. During the initial period of increased displacement rate, a visible slip area is resolved beneath the coastal area of southwestern Shikoku. In the next stage of accelerated displacements, the slip area expanded southwestward to the Bungo channel beginning around June 1997, when postseismic slip of the 1996 Hyuga-nada earthquakes almost ended. After this accelerated period, the estimated thrust slip ends with time.

Imaging of aseismic slip transients recorded by dense geodetic networks

Article

Dec 2003

The large-scale continuous GPS networks that have emerged in the last decade have documented a wide-range of aseismic deformation transients that resulted from physical processes such as aseismic fault slip and magma intrusion. In particular, a new class of (M ~ 7) slow earthquakes with durations ranging from days to months have been observed with GPS arrays located above the downdip portion of subduction zone thrust interfaces. Interpretation of the displacement time-series resulting from these events is not straightforward owing to the contaminating effects of multiple contributing signals such as fault-slip, local benchmark motion, seasonal effects, and reference frame errors. We have developed a time-dependent inversion algorithm based on the extended Kalman filter which can separate the various signals and allow the space-time evolution of these slow-slip transients to be studied in detail. We applied the inversion algorithm to the 1999 Cascadia slow earthquake. This event had two primary episodes of moment-release separated by a two week period in which relatively little moment-release occurred. The Cascadia event and other slow earthquakes share numerous similarities with both ordinary earthquakes and afterslip transients suggesting that they may represent fault slip under a conditionally stable regime.

Displacement of Surface Monuments: Vertical Motion

Article

Feb 1989

Frank K. Wyatt

Several measurements of vertical ground motion at Piñon Flat Observatory, California, indicate the overall weakness and instability of the Earth's weathered surface with respect to the underlying rock. Cumulative long-period motions of order 0.5 mm per year dominate these records, though smaller elastic deformations caused by precipitation loading, atmospheric loading, and tidal strains are evident at higher frequencies; all of these help to characterize the near-surface material. The long-period records suggest that near-surface weathering is the dominant influence on monument motion, at least at this site on crystalline rock in a semi-arid environment. Rainfall loading gives an average vertical modulus of 2.6 GPa for the material in the uppermost 26 m of the ground, compared with 88 GPa for granite under moderate confining stress; atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours. Earth tide records show Poisson's ratio to be 0.09 in contrast to the normative range of 0.2–0.25, establishing that horizontal strains couple only weakly into vertical ones, so that vertical strains near the surface are a poor measure of areal strain. The form of the ground-surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument-positioning error budget that increases with time. Because of ground instability, and the generally small rate of crustal tectonic motions, deeply emplaced monuments will be needed for observational programs designed to detect short-term changes in crustal deformation over baselengths of order 10 km and less.

Detection and monitoring of ongoing aseismic slip in the Tokai Region

Article

Jan 2002

Introduction to random signals and applied kalman filtering (3rd ed

Article

Jan 1997

Slow slip on the northern Hikurangi subduction interface, New Zealand

Article

Aug 2005

In October 2002, a surface displacement episode of 20-30 mm magnitude was observed over a ~10 day period on two continuous Global Positioning System (GPS) instruments near Gisborne, North Island, New Zealand. We interpret this to result from slow slip on the northern Hikurangi subduction interface. Using ten years of regional campaign GPS (1995-2004) and recent continuous GPS data, we estimate the recurrence interval for similar events to be 2-3 yrs. In November 2004, a similar slow slip event occurred within this recurrence period. The 2002 event can be modeled by ~18 cm of slow slip near the down-dip end of the seismogenic zone on the subduction interface offshore of Gisborne. The campaign GPS data show that the 2002 slow slip event had little effect on regional strain patterns.

Time dependent inversion of geodetic data. J Geophys Res 102(B10):22391

Article

Oct 1997

The recent expansion of permanent Global Positioning System (GPS) networks provides crustal deformation data that are dense in both space and time. While considerable effort has been directed toward using these data for the determination of average crustal velocities, little attention has been given to detecting and estimating transient deformation signals. We introduce here a Network Inversion Filter for estimating the distribution of fault slip in space and time using data from such dense, frequently sampled geodetic networks. Fault slip is expanded in a spatial basis set Bk(x) in which the coefficients are time varying, s(x,t)=∑k=1Mck(t)Bk(x). The temporal variation in fault slip is estimated nonparametrically by taking slip accelerations to be random Gaussian increments, so that fault slip is a sum of steady state and integrated random walk components. A state space model for the full geodetic network is formulated, and Kalman filtering methods are used for estimation. Variance parameters, including measurement errors, local benchmark motions, and temporal and spatial smoothing parameters, are estimated by maximum likelihood, which is computed by recursive filtering. Numerical simulations demonstrate that the Network Inversion Filter is capable of imaging fault slip transients, including propagating slip events. The Network Inversion Filter leads naturally to automated methods for detecting anomalous departures from steady state deformation.

The Illustrated Wavelet Transform Hand-Book

Article

Jan 2002

P.S. Addison

Mechanism of the 1991 eruption of Hekla from continuous borehole strain monitoring

Article

Oct 1993

VOLCANOES erupt when the pressure in a magma chamber several kilometres below the edifice overcomes the strength of the intervening rock. Seismic activity may accompany and precede eruptions, allowing (in favourable circumstances) the location and movement of magma to be traced. Ground deformation near volcanoes can provide more direct evidence for magma movement, but continuous monitoring is necessary to ensure that all the essential aspects of an eruption are recorded. Here we report dilatational strain data collected continuously during the January 1991 eruption of Hekla volcano by five borehole strainmeters located 1545 km from the volcano. The data record the upward propagation of magma, as well as the deflation of a deep reservoir. In only 30 minutes the magma forced open a conduit to the surface from a depth of ~4km. Although other volcanoes might behave differently, our results suggest the possibility of using continuous deformation measurements to monitor conduit formation at other sites, perhaps providing short-term warnings of impending eruptions.

Maximum Likelihood Estimates of Linear Dynamic Systems

Article

Aug 1965

A transient subduction zone slip episode in southwest Japan observed by the nationwide GPS array

Article

Feb 2003

We investigate the spatiotemporal variation of fault slip on the subduction interface in southwest Japan following the two 1996 Hyuganada earthquakes (M 6.7) using continuous GPS observations. A silent thrust slip event has been reported by Hirose et al. [1999], following these earthquakes approximately 100 km to the north beneath the Bungo channel. We employ the Network Inversion Filter to rigorously invert for the fault slip distribution in space and time. The GPS time series is modeled as a sum of: (1) secular deformation; (2) coseismic station displacements; (3) transient fault slip; (4) random benchmark motion; (5) reference frame shifts; and (6) observational errors. Nonnegativity constraints effectively prohibit normal faulting on the subduction interface. Spatial and temporal smoothing parameters are estimated by the extended Kalman filter. The estimated fault slip-rate images postseismic slip following the two Hyuganada earthquakes. Slip associated with the silent thrust event began roughly 1 month after the second earthquake but initiated ∼100 km to the northeast of the Hyuganada earthquakes. The slow event propagated southwest and downdip beneath the Bungo channel, lasting approximately 1 year with maximum slip-rates of 0.6 m/yr. The equivalent seismic moment was Mw ∼ 7.2. We can rule out the possibility that slip propagated from the source region of the Hyuganada earthquakes to the location of the silent earthquake. However, the timing of the events suggests a causal connection, perhaps due to static stress transfer. This kind of transient event, or silent earthquake, may reflect different fault zone characteristics and may be responsible for accommodating strain that would otherwise be released in large earthquakes.

Short-term slow slip and correlated tremor episodes in the Tokai region, central Japan

Article

Sep 2006

We observe the related phenomenon of short-term slow slip event (SSE) and low-frequency nonvolcanic tremor on the deeper extension of the 1944 Tonankai and the antici-pated Tokai earthquake rupture areas, central Japan. Japan nation-wide seismometer and tiltmeter array, NIED Hi-net has recorded these correlated episodes of SSE and tremor, known as episodic tremor and slip (ETS) in 2004 and 2005. The events are located on two areas in the Tokai region, one is near the Shima peninsula, and the other is around the Aichi area. Both episodes have a typical duration of 2-3 days and released seismic moment corresponding to Mw ∼ 6.0. Moreover, these episodes repeatedly occur with a recur-rence interval of about six months in both areas. This ev-idence suggests that the coincidence between the SSE and the tremor is a characteristic behavior in the whole tremor belt-like zone in the Nankai trough subduction zone.

Spatiotemporal evolution of a transient slip event on the San Andreas fault near Parkfield, California

Article

Sep 2005
J GEOPHYS RES

1] In 1993 several baselines of the two-color electronic distance meter (EDM) network at Parkfield, California, deviated from their long-term rates, coincident with anomalous observations from nearby strain meters and a creep meter, as well as an increase in microseismicity. Between October 1992 and December 1994, three M

4.5 earthquakes occurred beneath Middle Mountain, near the hypocenter of the 1934 and 1966 Parkfield M6 events. We analyzed the two-color EDM data using a Kalman-filtering based technique to image the spatiotemporal evolution of slip on the fault at Parkfield between the mid-1980s and 2003. This method accounts for localized random walk motion of the geodetic monuments and a prominent seasonal signal that affects many baselines. We find that a slip rate increase occurred between January 1993 and July 1996 on the upper 8 km of the fault near Middle Mountain. The peak estimated slip rate during this time was 49 mm/yr, which exceeds the long-term geologic rate of

35 mm/yr. The slip rate evolution appears episodic, with an initial modest increase after the M4.3 earthquake and a much larger jump following the shallower M4.7 event in December 1994. This temporal correlation between inferred slip and seismicity suggests that the moderate earthquakes triggered the aseismic fault slip. The EDM data cannot resolve whether transient slip propagated across the nucleation zone of the 1934 and 1966 M6 Parkfield earthquakes. However, transient slip and its associated stress release in the hypocentral area of previous Parkfield events is consistent with the nucleation of the 2004 M6 Parkfield earthquake elsewhere on the fault.

Interplate Coupling in the Kanto District, Central Japan, and the Boso Peninsula Silent Earthquake in May 1996

Article

Dec 2004

Takeshi Sagiya

— I studied crustal deformation in the Kanto district, central Japan, based on continuous GPS data. Horizontal as well as vertical displacement rate demonstrate significant interaction between the landward Kanto block and the Philippine Sea plate. Although the subduction effect of the Pacific plate is not apparent, it is reasonable to consider the entire Kanto district is displaced westward due to the interaction with the Pacific plate. The GPS velocity data were inverted to estimate the slip deficit distribution on the Sagami Trough subduction zone. The result delineates a strongly coupled region on the plate interface, part of which corresponds to the 1923 Kanto earthquake. The strongly coupled region is located shallower than 20 km. In addition, the plate interaction is laterally heterogeneous even in the same depth range, implying thermal structure is not the only factor controlling interplate coupling. The GPS data also detected a silent earthquake event on the interface of the Philippine Sea slab east of the Boso Peninsula in the middle of May, 1996. The silent rupture propagated over a 50 km * 50 km wide area during about a week. The maximum slip was approximately 50 mm and the released seismic moment was 4.7*1018Nm (M w 6.4). There was a small seismicity triggered by this silent event. The silent slip was located in the peripheral of the strongly coupled area, suggesting that frictional properties and/or stress conditions are inhomogeneous on the plate boundary interface.

A Wavelet Tour of Signal Processing

Chapter

Jan 1999

Stéphane Georges Mallat

Mallat's book is the undisputed reference in this field - it is the only one that covers the essential material in such breadth and depth. - Laurent Demanet, Stanford University The new edition of this classic book gives all the major concepts, techniques and applications of sparse representation, reflecting the key role the subject plays in today's signal processing. The book clearly presents the standard representations with Fourier, wavelet and time-frequency transforms, and the construction of orthogonal bases with fast algorithms. The central concept of sparsity is explained and applied to signal compression, noise reduction, and inverse problems, while coverage is given to sparse representations in redundant dictionaries, super-resolution and compressive sensing applications. Features: * Balances presentation of the mathematics with applications to signal processing * Algorithms and numerical examples are implemented in WaveLab, a MATLAB toolbox * Companion website for instructors and selected solutions and code available for students New in this edition * Sparse signal representations in dictionaries * Compressive sensing, super-resolution and source separation * Geometric image processing with curvelets and bandlets * Wavelets for computer graphics with lifting on surfaces * Time-frequency audio processing and denoising * Image compression with JPEG-2000 * New and updated exercises A Wavelet Tour of Signal Processing: The Sparse Way, third edition, is an invaluable resource for researchers and R&D engineers wishing to apply the theory in fields such as image processing, video processing and compression, bio-sensing, medical imaging, machine vision and communications engineering. Stephane Mallat is Professor in Applied Mathematics at École Polytechnique, Paris, France. From 1986 to 1996 he was a Professor at the Courant Institute of Mathematical Sciences at New York University, and between 2001 and 2007, he co-founded and became CEO of an image processing semiconductor company. Includes all the latest developments since the book was published in 1999, including its application to JPEG 2000 and MPEG-4 Algorithms and numerical examples are implemented in Wavelab, a MATLAB toolbox Balances presentation of the mathematics with applications to signal processing.

Microseismicity but no tremor accompanying slow slip in the Hikurangi subduction zone, New Zealand

Article

Jan 2009
EARTH PLANET SC LETT

Geodetically-detected episodes of slow slip appear in several subduction zones to be accompanied by bursts of low-frequency coherent noise known as seismic tremor, but whether a single physical process governs this association or even whether slow slip is invariably accompanied by tremor remains unresolved. Detailed analysis of broadband seismic data spanning a slow slip episode in the Hikurangi subduction zone, New Zealand, reveals that slow slip was accompanied by distinct reverse-faulting microearthquakes, rather than tremor. The timing, location, and faulting style of these earthquakes are consistent with stress triggering down-dip of the slow slip patch, either on the subduction interface or just below it. These results indicate that tremor is not ubiquitous during subduction zone slow slip, and that slow slip in subduction zone environments is capable of triggering high-frequency earthquakes near the base of the locked subduction thrust. In this and other locations (Hawaii, Boso Peninsula) where slow slip is accompanied by triggered microseismicity, the estimated upper extent of the slow slip is shallower (less than ~ 20 km) than in those locations from which tremor has been reported. This suggests that ambient temperature- or pressure-dependent factors govern the character of the seismic response to slow slip on subduction thrusts and other large faults, with rheological or lithological conditions at shallow depths triggering high-frequency microearthquakes and those at greater depths triggering seismic tremor.

Periodic slow earthquakes on the flank of Kīlauea volcano, Hawaíi

Article

Jun 2006
EARTH PLANET SC LETT

We analyze 8 years of continuous GPS data from the Hilina slump (HS) on Kīlauea volcano's south flank and identify 3 new slow earthquake (SE) events. The new SEs are very similar to the previously identified one from November 2000, suggesting they share a common source. The series of SEs are separated by regular periods of 774 (± 7) days. None of the newly identified events are associated with increased rainfall rates, precluding rainfall as a necessary SE trigger. All of the SEs are followed by increased microseismicity in a ∼NW–SE trending band in the southern HS, suggesting that the SEs trigger seismicity. SE location and source parameters are not well-constrained by the CGPS network, although moment can be constrained adequately for the January 2005 event. Using this as a reference, we calculate for the four events equivalent moment magnitude values of 5.6, 5.7, 5.5, and 5.8, in their order of occurrence.

The illustrated wavelet transform handbook. Introductory theory and applications in science, engineering, medicine and finance

Book

Jan 2002

P.S. Addison

PREFACE GETTING STARTED THE CONTINUOUS WAVELET TRANSFORM Introduction The Wavelet Requirements for the Wavelet The Energy Spectrum of the Wavelet The Wavelet Transform Identification of Coherent Structures Edge Detection The Inverse Wavelet Transform The Signal Energy: Wavelet-Based Energy and Power Spectra The Wavelet Transform in Terms of the Fourier Transform Complex Wavelets: The Morlet Wavelet The Wavelet Transform, Short Time Fourier Transform and Heisenberg Boxes Adaptive Transforms: Matching Pursuits Wavelets in Two or More Dimensions The CWT: Computation, Boundary Effects and Viewing Endnotes THE DISCRETE WAVELET TRANSFORM Introduction Frames and Orthogonal Wavelet Bases Discrete Input Signals of Finite Length Everything Discrete Daubechies Wavelets Translation Invariance Biorthogonal Wavelets Two-Dimensional Wavelet Transforms Adaptive Transforms: Wavelet Packets Endnotes FLUIDS Introduction Statistical Measures Engineering Flows Geophysical Flows Other Applications in Fluids and Further Resources ENGINEERING TESTING, MONITORING AND CHARACTERISATION Introduction Machining Processes: Control, Chatter, Wear and Breakage Rotating Machinery Dynamics Chaos Non-Destructive Testing Surface Characterisation Other Applications in Engineering and Further Resources MEDICINE Introduction The Electrocardiogram Neuroelectric Waveforms Pathological Sounds, Ultrasounds and Vibrations Blood Flow and Blood Pressure Medical Imaging Other Applications in Medicine FRACTALS, FINANCE, GEOPHYSICS AND OTHER AREAS Introduction Fractals Finance Geophysics Other Areas APPENDIX: USEFUL BOOKS, PAPERS AND WEBSITES Useful Books and Papers Useful Websites REFERENCES INDEX

Network strain filter: A new tool for monitoring and detecting transient deformation signals in GPS arrays

Abstract

No full-text available

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