Project

# ROMY-ERC: Rotational Motions in Seismology

Goal: Understanding all aspects of rotational ground motions, developing a first multicomponent ring laser for seismology and geodesy. Pushing for the development of a portable broadband rotation sensor for seismology.

Methods: Theoretical Seismology, Ring Laser Gyroscope, Computational Seismology, fibre-optic gyro, seismic array analysis

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## Project log

Though a standard procedure on a global scale, the retrieval of reliable seismic moment tensors on the regional/local scale is still hampered by several difficulties. Also, in the retrieval of kinematic source solutions, ambiguities and other problems still reduce the reliability of the results. So far, these source solutions are derived from only transla-tional ground motions. Newly available possibilities to also portably measure rotational ground motions in a broad frequency range raised the question of how the resolution of the source solutions would benefit. Here, we present the summarized results from several studies based on synthetic test cases considering six components of ground motion. We compare the results of waveform inversion from translational ground motion only versus including rotational ground motion. We focus on the number of stations, station distribution, source mechanism , and the influence of the underlying velocity model. We show that the resolution of the source components benefits drastically from the inversion of six component ground motion data. Especially, the depth-dependent components and the centroid depth have high potential to be resolved much more reliable than from three component ground motion data only. Trade-offs and ambiguities can be reduced drastically as well, making the solution more reliable for further studies.
Seismic moment tensors help us to increase our understanding about e.g. earthquake processes, tectonics, Earth or planetary structure. Based on ground motion measurements of seismic networks their determination is in general standard for all distance ranges, provided the velocity model of the target region is known well enough. For sparse networks in inaccessible terrain and planetary seismology, the waveform inversion for the moment tensor often fails. Rotational ground motions are on the verge of becoming routinely observable with the potential of providing additional constraints for seismic inverse problems. In this study, we test their benefit for the waveform inversion for seismic moment tensors under the condition of sparse networks. We compare the results of (1) inverting only traditional translational data with (2) inverting translational plus rotational data for the cases of only one, two, and three stations. Even for the single station case the inversion results can be improved when including rotational ground motions. However, from data of a single station only, the probability of determining the correct full seismic moment tensor is still low. When using data of two or three stations, the information gain due to rotational ground motions almost doubles. The probability of deriving the correct full moment tensor here is very high.
Ring Laser gyroscopes exploit the Sagnac effect and measure rotations absolute. They do not require an external reference frame and therefore provide an independent method to monitor Earth rotation. Large-scale versions of these gyroscopes promise to eventually provide a similar high resolution for the measurement of the variations in the Earth rotation rate as the established methods based on VLBI and GNSS. This would open the door to a continuous monitoring of LOD (Length of Day) and polar motion, which is not yet available today. Another advantage is the access to the sub-daily frequency regime of Earth rotation. The ring laser “G” (Grossring), located at the Geodetic Observatory Wettzell (Germany) is the most advanced realization of such a large gyroscope. This paper outlines the current sensor design and properties.
With the prospects of seismic equipment being able to measure rotational ground motions in a wide frequency and amplitude range in the near future we engage in the question how this type of ground motion observation can be used to solve the seismic inverse problem. In this paper, we focus on the question, whether finite source inversion can benefit from additional observations of rotational motion. Keeping the overall number of traces constant, we compare observations from a surface seismic network with 44 3-component translational sensors (classic seismometers) with those obtained with 22 6-component sensors (with additional 3-component rotational motions). Synthetic seismograms are calculated for known finite-source properties. The corresponding inverse problem is posed in a probabilistic way using the Shannon information content as measure how the observations constrain the seismic source properties. We minimize the influence of the source receiver geometry around the fault by statistically analyzing six-component (three velocity and three rotation rate) inversions with a random distribution of receivers. The results show that with the 6-C subnetworks the source properties are not only equally well recovered (even that would be benefitial because of the substantially reduced logistics installing half the sensors) but statistically some source properties are almost always better resolved. We assume that this can be attributed to the fact that the (in particular vertical) gradient information is contained in the additional motion components. We compare these effects for strike-slip and normal-faulting type sources and confirm that the increase in inversion quality for kinematic source parameters is even higher for the normal fault. This indicates that the inversion benefits from the additional information provided by the horizontal rotation rates, i.e. information about the vertical displacement gradient.
We report the first ground rotation observations on the seafloor from an experiment we carried out in the North Sea close to the island of Heligoland. A slightly modified commercial fiber-optic gyroscope was mounted on an ocean-bottom seismometer (OBS) platform together with a intermediate-period seismometer. The system was lowered to the seafloor for four days. To investigate a potential tilt contamination of horizontal translational recordings, we calculate the coherence between the corresponding motion components (rotations around x-axis and translations along y-axis, and vice versa). We find very high correlations in the period interval of 5-13 seconds, where the correlation coefficient reaches 0.94 over 8.5 hours. This clearly indicates that horizontal translational components are severely contaminated by rotations. We find that these rotational motions are caused by seafloor currents or deformation of the seafloor rather than by seismic waves. The ground rotation observations allow correcting for the cross-coupling effect thereby decreasing the power spectral density up to 11 dB at 10 s period on horizontal OBS components. We discuss general requirements for broadband rotation sensors for OBS applications as well as possible further applications.
Towards 6C seismology (translation and rotation)
It has been noted by theoretical seismologists for decades that—in addition to translations and strains—the rotational part of ground motions should also be recorded. It is expected that collocated measurements of translations and rotations may (1) allow transformation of translational seismograms to the complete ground motion of an observation point; (2) help to further constrain rupture processes and (3) provide additional hazard-relevant information to earthquake engineers. The lack of instrumental sensitivity used to be the main obstacle to observing rotational motions. Recently, ring laser technology has provided the means to develop instruments that allow in principle the observation of rotational motions in a wide frequency band and epicentral distance range. Here we investigate whether this technology— originally designed for geodesy—is capable of providing accurate and useful observations for seismology. We report observations of rotations around a vertical axis of several earthquakes obtained by a 4 × 4 m ring laser installed in SE-Germany and compare them to collocated broad-band translations. Assuming plane transverse wave propagation, acceleration and rota-tion rate should be in phase and their amplitude ratio proportional to horizontal phase velocity. Here we show that most of the observations can be explained under these assumptions and that the collocated observations allow the estimation of wavefield properties (e.g. phase veloc-ities, propagation directions), otherwise only accessible through seismic array measurements, polarization analysis, or additional strain measurements.
It has been noted by theoretical seismologists for decades that - in addition to translations and strains - the rotational part of ground motions should also be recorded. It is expected that collocated measurements of translations and rotations may (1) allow transformation of translational seismograms to the complete ground motion of an observation point; (2) help to further constrain rupture processes; (3) provide additional hazard- relevant information to earthquake engineers. The lack of instrumental resolution used to be the main obstacle to observing rotational motions. Recently, ring laser technology has provided the means to develop instruments that allow in principle the observation of rotational motions in a wide frequency band and epicentral distance range. Here we investigate whether this technology - originally designed for geodesy - is capable of providing accurate and useful observations for seismology. We report observations of rotations around a vertical axis of several earthquakes obtained by a 4x4m ring laser installed in SE-Germany and compare them to collocated broadband translations. Assuming plane transverse wave propagation, acceleration and rotation rate should be in phase and their amplitude ratio proportional to horizontal phase velocity. Here we show that most of the observations can be explained under these assumptions and that the collocated observations allow the estimation of wavefield properties (e.g., phase velocities, propagation directions), otherwise only accessible through seismic array measurements, polarization analysis, or additional strain measurements. We compare the observations with 3-D calculations of rotational ground motions for 3-D media. We further discuss investigate whether the Love-wave dispersion measurements are accurate enough to be useful for inversion.
Exploring the internal structure of planetary objects is fundamental to understand the evolution of our solar system. In contrast to Earth, planetary seismology is hampered by the limited number of stations available, often just a single one. Classic seismology is based on the measurement of three components of translational ground motion. Its methods are mainly developed for a larger number of available stations. Therefore, the application of classical seismological methods to other planets is very limited. Here, we show that the additional measurement of three components of rotational ground motion could substantially improve the situation. From sparse or single station networks measuring translational and rotational ground motions it is possible to obtain additional information on structure and source. This includes direct information on local subsurface seismic velocities, separation of seismic phases, propagation direction of seismic energy, crustal scattering properties, as well as moment tensor source parameters for regional sources. The potential of this methodology will be highlighted through synthetic forward and inverse modeling experiments.
We present a full ray theory (FRT) method to simulate rotational motions of seismic surface waves in smooth, laterally heterogeneous Earth models. In the ray picture of wave propagation the vertical component of the rotational rate motion of fundamental mode Love waves is obtained by dividing the transverse component of ground acceleration by the Love wave local phase velocity beneath the seismic recording station. We illustrate the method with examples of theoretical calculations of T ~ 40s rotational rate ground motions of fundamental Love waves using the crust model CRUST2.0 combined with the mantle model S20RTS for the M8.1 September 25 2003 Tokachi-oki earthquake, Japan. FRT rotation synthetics match complete calculations using the spectral-element method very well and fit real data reasonably well. Furthermore, we show that the effect of realistic local structures beneath receivers on rotational motions is strong enough to be observable. FRT calculations could potentially help inferring Love wave local dispersion curves and thus estimating the 1-D local shear velocity structure beneath seismic stations from point measurements of rotational rate and acceleration ground motions. We support the theoretical work with applications to data from the ring laser rotation sensor at Wettzell, Germany. We estimate Love-wave dispersion curves by stacking spectral ratios of rotation rate and transverse acceleration.
Waveform inversion for the seismic moment tensor nowadays is a well-established standard method in teleseismic distances. Nevertheless, several difficulties remain, especially for shallow and/or regional/local distances. These difficulties include e.g. the resolution of the mechanism, especially the non-double-couple components and the resolution of the centroid depth but also the uncertainty of a determined moment tensor. During the last decade, the observation of rotational ground motions gained increasing attention amongst seismologists. So far, studies were based on one (vertical) component ring laser data but 3-component ring laser data and even data from portable rotation sensors are in reach. These new developments can contribute to solve the difficulties in waveform inversion for moment tensors. Here, we present results for moment tensors, mainly in the regional distance range, derived from collocated translational and rotational ground motion measurements. These results are based on numerical and real-data studies. We inverted the ground motions recorded by a network of stations but also addressed the question of how reliable the inversion for moment tensors is from a single 6-component measurement.
After some technical improvements to the ring laser system measuring the vertical component of rotation rate at Wettzell, Germany, in 2009 a marked improvement of the signal-to-noise ratio for the broad-band frequency range of seismic observations could be achieved. This led to the first direct observation of rotational ground motions induced by toroidal free oscillations of the Earth, following the Mw=8.1 Samoa earthquake on September 29, 2009 and the Mw=8.8 Chile earthquake on February 27, 2010. Observations are compared with synthetic seismograms computed by summing normal modes. Amplitude spectra of real and synthetic data are analyzed to interpret the observations. We show that several toroidal modes can be detected in the ring laser data and that our observations are in reasonable agreement with the synthetic spectra. This indicates that long-period seismology can benefit from measurements of rotational ground motion measurements using ring lasers in the future. In addition, analysis of earthquake-free time windows of the ring laser records leads us to the conclusion that we consistently observe Love waves generated in the two ocean-generated microseismic frequency bands and that the azimuth of the source areas can be estimated from joint analysis with translation records from a collocated broadband seismometer.
Microzonation, the estimation of (shear) wave velocity profiles of the upper few 100 m in dense 2D profiles, is one of the key methods for understanding the variation in seismic damage caused by ground-shaking events and thus for mitigating the risk of damage in the future. In this article, we present a novel method for estimating the Love-wave phase velocity dispersion using ambient noise recordings. We use the vertical component of rotational motions inherently present in ambient noise and the established relation to simultaneous recordings of transverse acceleration, in which the phase velocity of a plane SH (or Love)-type wave acts as a proportionality factor. We demonstrate that the developed inversion technique shows comparable results to more classical, array-based methods. Furthermore, we demonstrate that if portable weak-motion rotational motion sensors are available and the installation of a seismic network or array is not possible, a single point, multicomponent approach for estimating the dominant direction of the incident wavefield and the local velocity structure will be feasible with similar performance compared to more classical techniques.
Kürzlich veröffentliche theoretische Studien legen nahe, dass ein portabler breitbandiger Rotationssensor erhebliche Bedeutung/Verbesserungen auf den Gebieten der Nahfeld-Seismologie, Vulkanologie, marinen Seismologie, seismischen Tomographie sowie planetaren Seismologie haben könnte, sofern ein solches Instrument mit der entsprechenden Güte und Spezifikation zur Verfügung steht. In diesem Zusammenhang präsentieren wir die Kennlinien des BlueSeis-3A, dem weltweit ersten Faser-Optischen Gyroskop (FOG), der rein für seismologische Zwecke konstruiert wurde. Der BlueSeis-3A wird von der in Frankreich beheimateten Fa. iXblue und als direkte Folge vom ‘European Research Council’ finanzierten Projekt ‘ROMY’ (Rotational Motions: a new Observable for seismologY) hergestellt. Wir stellen zunächst das Eigenrauschen des Sensors in Form von Leistungsdichtespektren (drei Instrumentenmethode), ‘Operating Range Diagram’ (ORD) sowie ‘Allan Deviation’ vor, um danach die ersten Ergebnisse dreier Feldexperimente, die am Vulkan Stromboli, in der Nachbebenregion von Norcia, sowie im Glockenturm Giotto in Florenz durchgeführt worden sind, zu präsentieren.
The unique instrument setting at the Piñon Flat Observatory in California is used to simultaneously measure 10 out of the 12 components, completely describing the seismic‐wave field. We compare the direct measurements of rotation and strain for the 13 September 2015 Mw 6.7 Gulf of California earthquake with array‐derived observations using this configuration for the first time. In general, we find a very good fit between the observations of the two measurements with cross‐correlation coefficients up to 0.99. These promising results indicate that the direct and array‐derived measurements of rotation and strain are consistent. For the array‐based measurement, we derived a relation to estimate the frequency range within which the array‐derived observations provide reliable results. This relation depends on the phase velocity of the study area and the calibration error, as well as on the size of the array.
Using a co-located ring laser and an STS-2 seismograph, we estimate the ratio of Rayleigh-to-Love waves in the secondary microseism at Wettzell, Germany, for frequencies between 0.13 and 0.30 Hz. Rayleigh-wave surface acceleration was derived from the vertical component of STS-2 and Love-wave surface acceleration was derived from the ring laser. Surface wave amplitudes are comparable; near the spectral peak about 0.22 Hz, Rayleigh-wave amplitudes are about 20 percent higher than Love-wave amplitudes but outside this range, Love-wave amplitudes become higher. In terms of the kinetic energy, Rayleigh-wave energy is about 20-35 percent smaller on average than Love-wave energy. The observed secondary microseism at WET thus consists of comparable Rayleigh and Love waves but contributions from Love waves are larger. This is surprising as the only known excitation mechanism for the secondary microseism, described by Longuet-Higgins [1950], is equivalent to a vertical force and should mostly excite Rayleigh waves.
Monthly variations in the ratio of Rayleigh-to-Love waves in the secondary microseism are obtained from a co-located ring laser and an STS-2 seismograph at Wettzell, Germany. Two main conclusions are derived for the Rayleigh-to-Love wave kinetic energy ratios in the secondary microseism; first, the energy ratio is in the range 0.8-0.9 (<1.0) throughout a year except for June and July. It means that Love-wave energy is larger than Rayleigh-wave energy most of the year by about 10-20 percent. Second, this ratio suddenly increases to 1.0-1.2 in June and July, indicating a larger fraction of Rayleigh-wave energy. This change suggests that the locations and behaviors of excitation sources are different in these months.
Using closely located seismographs at Piñon Flat (PFO), California, for one-year long record (2015), we estimated the Rayleigh-to-Love wave energy ratio in the secondary microseism (0.1-0.35 Hz) in four seasons. Rayleigh-wave energy was estimated from a vertical component seismograph. Love-wave energy was estimated from rotation seismograms that were derived from a small array at PFO. Derived ratios are 2-2.5, meaning that there is 2-2.5 times more Rayleigh-wave energy than Love-wave energy at PFO. In our previous study at Wettzell, Germany, this ratio was 0.9-1.0, indicating comparable energy between Rayleigh waves and Love waves. This difference suggests that the Rayleigh-to-Love wave ratios in the secondary microseism may differ greatly from region to region. It also implies that an assumption of the diffuse wavefield is not likely to be valid for this low frequency range as the equipartition of energy should make this ratio much closer.
We introduce a new event database for rotational seismology. The rotational seismology website (see Data and Resources) grants access to 17,000+ processed global earthquakes starting from 2007. For each event, it offers waveform and processed plots for the seismometer station at Wettzell and its vertical-component ring laser (G-Ring), as well as extensive metrics (e.g., peak amplitudes, signal-to-noise ratios). Tutorials and illustrated processing guidelines are available and ready to be applied to other data sets. The database strives to promote the use of joint rotational and translational ground-motion data demonstrating their potential for characterizing seismic wavefields.
Report on ROMY project, ring laser, rotatoional seismology, Earth's rotation See movie here: https://www.youtube.com/watch?v=MXYV6wNdZm8&feature=youtu.be
Seismology is no longer based only on the analysis of observations made with classical seismometers measur-ing translational motions or their time derivatives. More and more observables like strain, rotations, GPS-based measurements and others are used to constrain either the Earth's structure or to understand the nature of seismic sources and observed ground shaking. This special issue focuses on recent developments in the area of rotational seismology and engineering applications. The analysis of rotational ground motions has re-cently emerged as a new branch in seismology and earthquake engineering. It does not seem to be com-mon, seismological knowledge that particularly at the Earth's surface, with (almost any) translational ground motion there is an associated rotational ground motion as well. This has several implications: (1) depending on frequency and amplitude range translational seis-mometer records are contaminated by rotational motions; (2) when both motion types (or additional strain measurements) are available the collocated records contain direct information on subsurface struc-ture; (3) the additional motion components can con-tribute to the excitation of structures and structures can radiate additional rotational motions and (4) rotational motion observations can help in decomposing seismic wave fields. The neglect of the rotational motion type so far is basically explained with the tremendous dif-ficulty in measuring these motions with high accuracy in the broad frequency band required in seismology. This is also true for strong motion, rotational meas-urements and their engineering applications, which await adequate records from epicentral areas of major earthquakes. Progress has been made possible with recent developments in rotation sensors, producing the first direct measurements that could be compared with theoretical expectations or used to predict the responses of structures.
Recent advances in large ring laser gyroscopes (RLG) technologies opened the possibility to observe rotations of the ground with sensitivities up to $10^{-11}$ $\frac{rad}{s}$ over the frequency band of seismological interest (0.01-1Hz), thus opening the way to a new geophysical discipline, i.e. rotational seismology. A measure of rotations in seismology is of fundamental interest for (a) the determination of all the six degrees of freedom that characterize a rigid body motion, and (b) the quantitative estimate of the rotational motions contaminating ground translation measurements obtained from standard seismometers. Within this framework, this paper presents and describes GINGERino, a new large observatory-class RLG located in Gran Sasso underground laboratory (LNGS), one national laboratories of the INFN (Istituto Nazionale di Fisica Nucleare). We also report unprecedented observations and analyses of the roto-translational signals from a tele-seismic event observed in such a deep underground environment.