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Seasonal, long-term, and short-term deformation in the Central Range of Taiwan induced by landslides
Abstract
Seasonal movement of GPS stations is often attributed to hydrological loading and other environmental factors. For the first time we observe seasonal motion associated with slow-moving landslides. Eight of 26 continuous GPS (cGPS) sites in the Central Range of Taiwan show long-term landslide-induced motion at rates of ∼3–15 mm/yr, ∼20%–60% of their tectonic interseismic velocities. The directions of movements after heavy rains and in the wet season are consistent with the slope directions derived from a high-resolution elevation model constructed by airborne lidar. Long-term and seasonal interseismic motions are modulated by slow-moving landslides. Seasonal motions of landslides at Lushan show peak to peak amplitudes of ∼3–19 mm. Estimates of interseismic crustal strain can be biased if surface processes are not taken into account. Preliminary analyses indicate that rainfall and topography play strong roles in the occurrence of landslides. Discrimination between surface processes and motion that has a tectonic origin is the key to natural hazard assessments.
... Here we refer geodetic vertical rates measured from GPS and leveling as rock uplift rates (England and Molnar, 1990). Geodetic vertical rates measured over the decadal-scale may not necessary reflect interseismic velocity but rather contain signals from deformation due to moderate to large earthquakes, long-duration slow slip events (Tsang et al., 2015), and surface processes (Hsu et al., 2014). Long-lasting and widespread postseismic deformation after the 1999 Chi-Chi M w 7.6 earthquake has been observed from geodetic data (Hsu et al., 2007, Rousset et al., 2012. ...
... Heavy precipitation and frequent earthquakes in Taiwan also lead to active landslides. Hsu et al. (2014) reported longterm, seasonal, and rainfall-induced short-term motions associated with slow-moving landslides. Several continuously-recording GPS (cGPS) sites affected by slow-moving landslides in the Central Range of Taiwan show 25%-40% differences in amplitudes of interseismic velocities compared with those in adjacent stable cGPS sites. ...
... To investigate whether these sites influenced by creeping landslides, we check the aspect of hillslope where GPS sites located and GPS moving directions during rainfall events. A small derivation of azimuth between the aspect and the GPS moving vector suggests the contribution of slope deformation in recorded GPS data as shown in previous studies (Hsu et al., 2014). ...
Rock uplift on the Earth surface is a key observation for studies of tectonics and geodynamic processes. Geodetic measurements from Global Positioning System (GPS) and leveling are able to track vertical deformation over a wide range of spatial and temporal scale. Taiwan mountain belt is subject to a rapid uplift rate of 20 mm/yr as revealed by GPS and leveling measurements in previous studies. The extremely high rates motivate us to analyze a set of newly processed GPS and leveling data along a NW-SE transect across southern Taiwan by considering the effects of the earthquake-related deformation as well as hydrological and surface processes. Our estimates of rock uplift rates from GPS and leveling are from −12 to +14 mm/yr across the southern Central Range of Taiwan. Vertical velocity changes on the small spatial scale are primary related to fault locking in the seismic cycle. The large spatial feature showing the mean uplift rate of 6 mm/yr between the eastern Central Range and the frontal thrust faults can be explained by a large-scale pop-up structure assuming the depth of 15–20 km in the orogenic wedge and the horizontal shortening of 30 mm/yr. Our results draw attention to carefully study geodetic vertical deformation in the mountain belts and seek relevant causes.
... As a consequence, the study of slow-moving landslides may lead a better understanding of the physical processes governing both slow and rapid landslides (Palmer, 2017). In recent years, various studies of slow-moving landslides (e.g., Bennett et al., 2016;Handwerger et al., 2013Handwerger et al., , 2015Hsu et al., 2014;Iverson & Major, 1987;Lacroix et al., 2014Lacroix et al., , 2015Reid, 1994;Schulz et al., 2009;Zerathe et al., 2016) have revealed a rich diversity in kinematics, with various forcing factors, for example, rainfall, earthquakes, glacial retreat, and anthropogenic activity. These forcings act over a wide variety of timescales, from seconds (earthquakes Lacroix et al., 2014) to several decades (glacial retreat Strozzi et al., 2010), thus making their study challenging. ...
... Landslide motion is often interpreted to occur as a direct consequence of external forcing factors (e.g., Handwerger et al., 2013Handwerger et al., , 2015Hsu et al., 2014;Lacroix et al., 2014;Reid, 1994;Zerathe et al., 2016), such as rainfall or ground shaking (i.e., from earthquakes). However, different internal processes may also influence landslide motion; for example, progressive failure is thought to play a key role in the initiation of landslide motion (Amitrano, 2004;Carey & Petley, 2014;Eberhardt et al., 2004;Gischig et al., 2016;Lacroix & Amitrano, 2013). ...
In mountainous environments, slow‐moving landslides (velocities <100 m/year) are a major concern for local populations. Rainfall is often the dominant forcing, and often result in major changes in kinematics which can mask smaller signals related to internal forcings. We focus here on a major (>40 Mm³) slow‐moving landslide in the desert of southern Peru and take advantage of this arid environment to study the internal processes affecting landslide kinematics. We first estimate the ground displacement from time series analysis of Landsat‐8 images, spanning a 5.7‐year period. Systematic artifacts in the optical time series are shown to correlate with topography, as well as vary seasonally. We apply a novel procedure for correcting these artifacts, which significantly reduces noise in the resulting time series, thereby allowing us to precisely resolve landslide displacements. We find landslide velocities of up to 35 m/year, with complex nonlinear interannual pattern, including a period of rapid acceleration. We validate our optically derived time series using Global Navigation Satellite System field measurements and find uncertainties (root‐mean‐square error) on the moving mass of 1.12 to 1.55 m. Sudden acceleration of the landslide body after March 2016 may originate from a mass collapse due to retrogression of the headscarp. By coupling sparse 3‐D Global Navigation Satellite System measurements with dense 2‐D optical time series data, we show that the headscarp retrogression acts like a wedge, resulting in domino‐like tilting of the downward blocks, and accelerates basal sliding over 2 years. These observations reveal that the dynamics of this retrogressive landslide are predominantly controlled by sediment supply and that succession of retrogressive and advancing motions is a self‐entrainment process.
... Recently, in Taiwan, with continuous global positioning system (GPS) network data, eight deepseated creeping landslide sites were identified and their causes were associated with hydrological loading, Hsu et al. (2014). The evidence was based on relating the creeping landslide events to the rainfall intensity (RI) or accumulated precipitation. ...
Rainfall is one of the most important triggers of both shallow debris flows and deep-seated landslides. The triggering mechanism involves the process of water infiltration into the failure zone. For deep-seated landslides, the deeper and more extensive failure surfaces delay the effect of the process and thus delay landslide initiations. The delay is difficult to assess, especially if the sites only have scarce or insufficient monitoring data. Under these circumstances, we illustrate that the occurrences of landslides can be estimated by their correlations with the phenomenological water storage index (WSI) of a given catchment. In the present study, a total of five deep-seated landslides in TienChih (4) and SiangYang (1) are investigated. The displacements of the landslides were recorded by global positioning system (GPS) and the WSI was modelled using the tank model. The result demonstrates that the WSI correlates closer in time to the landslide motion than the rainfall, and the WSI thresholds for the landslides are inferred. Thus, this technique can be applied as an associated method to evaluate landslide initiation.
... 2a and 2b), suggesting a dominant poroelastic mechanism in alluvial plains (e.g., Chaussard et al., 2014). As we are interested in the elastic loading effects, we remove GNSS sites located on alluvial fans and Quaternary basins (Text S2), stations greatly affected by multipath effects based on sky visibility in different years and field photos (Fig. S6), and stations affected by fault creep (Lee et al., 2003) and deep-seated landslides (Hsu et al., 2014). The remaining 176 stations are used to infer the changes of water storage in Taiwan. ...
We systematically investigate the spatiotemporal water storage changes in Taiwan using geodetic (GNSS and GRACE) and hydrological (precipitation, GLDAS and LSDM assimilation models, and in-situ groundwater level) datasets. We use GNSS-observed vertical deformation to estimate water storage changes based on elastic loading theory and weighted least-squares inversion, correcting for contributions from global loads using GRACE. The mean annual water-thickness change inferred from GNSS across Taiwan is 0.53 ± 0.17 m and the largest seasonal change of up to 0.91 m is estimated in southwest Taiwan. Comparison of the geodetic and hydrological data shows that the spatial pattern of annual water storage change estimated from GNSS, GLDAS, and precipitation data are generally consistent, indicating significant seasonal water-load fluctuations in Taiwan. However, the GRACE solution significantly underestimates the amplitude of water mass change in Taiwan due to leakage effect, but temporally correlates well with GNSS estimates. Hydrological assimilation model GLDAS, dominated by shallow soil moisture variations, predicts that the average seasonal variation of water thickness is only about 17% of GNSS estimates. This value is about half of the mean annual LSDM water storage change of 0.18 m including an estimate of both soil moisture and surface water. The discrepancy suggests that the contribution of groundwater is substantial and the total water storage change in the hydrological assimilation model is underestimated in Taiwan. The spatiotemporal distributions derived using independent component analysis (ICA) are generally consistent between the geodetic and hydrological data. However, comparisons of seasonal amplitudes and phases between all data pairs reveal different response times to precipitation, reflecting the complex nature of transient water storage due to variable rainfall patterns, infiltration rate, soil saturation, and runoff. The peak rainfall occurs in June-July, which is one-to-two months before the peak GNSS subsidence. Water storage of the GLDAS model also reaches its maximum in August, suggesting the water storage is controlled by the infiltration rate and capacity and the total water recharge from rainfall is generally larger than discharge in the summer. The highest groundwater levels lag one and two months behind the peak GNSS subsidence in western and eastern Taiwan, respectively, indicating a higher infiltration rate in western Taiwan.
... We select 267 CGPS stations that have continuously recorded more than 5 years of data and with data covered more than 70% of a year. We do not include stations located near areas subject to seasonal slow-moving landslide area in the Central Range (Hsu et al., 2014). ...
Rapid convergence between the Eurasian plate and the Philippine Sea Plate makes Taiwan one of the most active tectonic regions in the world, and this strain is accommodated on a complex network of thrust and strike‐slip faulting. Here, we use ascending and descending Interferometric Synthetic Aperture Radar (InSAR) data integrated with continuous GPS measurements to monitor interseismic deformation in southwest Taiwan. Geodetic observations show rapid (20–40 mm/year) southwestward motion in the Pingtung plain in south Taiwan and more than 10 mm/year uplift in the southern part of the Western Foothills. We use combined InSAR and CGPS measurements from 2004 to 2010 to constrain a block model of horizontal interseismic crustal deformation. Total variation regularization (TVR) serves as a tool for block model selection to algorithmically assess the block model geometry based on geodetic observations, simultaneously estimating block rotations and fault slip rates on block‐bounding faults. The block model results suggest that the margins of the Western Foothills accommodate most of the interseismic deformation in southwestern Taiwan. Thirty‐five independent blocks are required to explain interseismic crustal deformation with a mean residual velocity of 3.6 mm/year. Based on this horizontal model, we then forward predict vertical displacement and find good consistency between the predicted uplift and the geodetic observations. Our approach shows an efficient and objective way to estimate fault activities and seismic hazards in the interseismic period based on dense geodetic measurements and their uncertainties.
... For K = 9, a single station in the mountain range is isolated as a cluster ( Figure S1h, labeled as LC, black). The data might be contaminated by local and seasonal effects such as landslides due to the heavy rainfall (e.g., Hsu et al., 2014). ...
Collision of the Luzon Volcanic Arc is the primary causative factor behind the creation of Taiwan Island. Additionally, the indention of the Peikang basement highs in the Taiwan Strait and the back arc spreading of the Okinawa Trough are responsible for the characteristic tectonics in Taiwan. Identification of active tectonic boundaries is important for understanding neotectonics in Taiwan. We analyzed Global Positioning System (GPS) horizontal velocity data in Taiwan with a hierarchical cluster analysis method. We also developed a statistical scheme introducing information entropy to quantitatively assess the fuzziness of the obtained cluster boundaries, which enables us to distinguish a trustworthy discontinuity. The hierarchical representation, which is expressed by a dendrogram, suggests the relative importance of tectonic sources in this region. The major clusters, which appear at higher hierarchy, are likely to represent major tectonic features such as the Longitudinal Valley fault and lateral extrusion in southwest Taiwan. Those at lower hierarchy suggest velocity discontinuities associated with local geological structures such as active faults. For example, boundaries in east offshore Taiwan and southern Taiwan are identified without using geological information by this analysis. In addition to the identification of these clear tectonic boundaries, our statistical cluster assessment shows the existence of fuzzy cluster boundaries along the Coastal Plain and the Western Foothills, reflecting continuum deformation in west Taiwan.
... While these uncertainties hinder a further interpretation of the seasonal trends of very slow moving slope sections, it must be emphasized that they do not explain the acceleration of the fastest moving 50% during the two winter periods which is visible despite the bias towards higher summer displacement rates. Numerous previous studies have already demonstrated close links between seasonal rainfall patterns, groundwater flows and the kinematics of slow-moving landslides (Bennett et al., 2016;Handwerger et al., 2013;Hsu et al., 2014;Iverson and Major, 1987;Malet et al., 2005b;Reid, 1994;Zerathe et al., 2016). In the absence of comprehensive groundwater data for all detected landslides (only pore water pressure measurements for the La Valette and Super-Sauze landslides are available) we consider the mean effective precipitation (mm • d −1 ) as a first order proxy for the hydrological conditions during each period. ...
Slow-moving landslides are widespread in many landscapes with significant impacts on the topographic relief, sediment transfer and human settlements. Their area-wide mapping and monitoring in mountainous terrain, however, is still challenging. The growing archives of optical remote sensing images offer great potential for the operational detection and monitoring of surface motion in such areas. This study proposes a multiple pairwise image correlation (MPIC) technique to obtain a series of redundant horizontal displacement fields, and different multi-temporal indicators for a more accurate detection and quantification of surface displacement. The technique is developed and tested on a series of monoscopic and stereoscopic Pléiades satellite images at a test site in the South French Alps. Empirical tests confirm that MPIC significantly increased detection accuracy (F−measure = 0.85) and that the measurement error can be reduced by averaging velocities from all pair combinations covering a given time-step (i.e. when stereo-pairs are available for at least one date). The derived inventory and displacement fields of 169 slow-moving landslides show a positive relationship between the landslide size and velocities, as well as a seasonal acceleration of the largest landslides in response to an increase in effective precipitation. The processing technique can be adapted to better exploit increasingly available time-series from a variety of optical satellites for the detection and monitoring of landslide displacement.
... Furthermore, geological events such as earthquakes did not occur near the change epoch. Because there are neither major man-made structures nor groundwater extraction near sites TCMS and TNML, the trend changes and modulation anomaly are probably of tectonic origin, perhaps associated with slowmoving landslides (Hwang et al., 2009;Hsu et al., 2014). However, further investigation of this issue is beyond the scope of this paper. ...
The seasonal signal and long-term trend in the height time series of 10 IGS sites in China are investigated in this paper. The offset detection and outlier removal as well as the removal of common mode error are performed on the raw GPS time-series data developed by the Scripps Orbit and Permanent Array Center (SOPAC). The seasonal-trend decomposition procedure based on LOESS (STL) is utilized to extract precise seasonal signals, followed by an estimation of the long-term trend with the application of maximum likelihood estimation (MLE) to the seasonally adjusted time series. The Up-components of all sites are featured by obvious seasonal variations, with significant phase and amplitude modulation on some sites. After Kendall’s tau test, a significant trend (99% confidence interval) for all sites is achieved. Furthermore, the trends at sites TCMS and TNML have significant changes at epochs 2009.5384 and 2009.1493 (95% confidence interval), respectively, using the Breaks For Additive Seasonal and Trend test. Finally, the velocities and their uncertainties for all sites are estimated using MLE with the white noise plus flicker noise model. And the results are analyzed and compared with those announced by SOPAC. The results obtained in this paper have a higher precision than the SOPAC results.
The study of the disturbance process of hydrologic load on crustal stressis helpful in clarifying the relationship between hydrologic load and seismic activity. Using the seasonal signal of GPS time series in Nepal, the seasonal three-dimensional displacement field and stress-strain field model in this region is constructed. The modulation effect of seasonal hydrological load on seismicity in this area is studied by combining rain fall and seismic catalogue. Theresultsshowthat:1) the reis a strong spatiotemporal correlation between seasonal surface displacement and rain fall; 2) the rain fall in monsoon period disturbs the long-term movement trend of the fault and releases the Coulomb stress to a certain extent, thus inhibiting the seismic activity and affecting the time of the earthquake
Due in large part to the advent of GPS, geodesy has become an important discipline within the Earth sciences. It is practiced and taught at a growing number of universities and research institutions worldwide, and provides the underpinnings for geographical information and locational awareness in modern life and commerce. The main advantage of GPS geodesy is the ability to directly measure very precise static, kinematic‚ and dynamic positions and displacements with respect to a global reference frame. GPS geodesy attracted the attention of geophysicists in the early 1980s when the potential for significant advances in the understanding of tectonic motion, crustal deformation‚ and geodynamics became apparent. Since then – something that was certainly not anticipated by the pioneers of the GPS in the 1970s – it has been applied to investigations of natural and anthropogenic processes and hazards, including earthquakes, tsunamis, volcanoes, the cryosphere, extreme weather, sea level rise, climate change‚ and hydrology. Contributing to its success have been advances in technology and the development of a global GPS infrastructure consisting of thousands of continuous stations spanning nearly all of the tectonic plate boundaries and hundreds of global stations to provide precise orbits and access to a global reference frame. GPS as a measuring tool is complementary to other terrestrial, ocean, atmospheric‚ and spaceborne instrumentation including seismometers, synthetic aperture radars, GPS/acoustic methods, gravimeters, radiometers, and UAV and LiDAR imaging. This chapter provides a historical perspective of GPS geodesy through to its current practice.
The Chinese Loess Plateau (CLP) is still undergoing development. Nearly one-third of the geological disasters take place in this region in China each year. Prior to assessing the spatial characteristics of the rainfall-induced landslides in the CLP, it is fundamental to establish a detailed landslide inventory map. This study utilizes GLDAS (Global Land Data Assimilations Systems) rainfall distribution data in the CLP based on yearly, monthly, and the daily precipitation data from 57 rain gauge stations. Geostatistics analysis and Geographic Information System technique combined with information on geological disasters within the region were used to study the spatial characteristics and trend variations of rainfall and geological disaster distribution in the region. The results indicate that a positive relationship exists between geological disaster and the average annual rainfall, correlation coefficient reaches 0.82, with zonality disasters (account for more than 90%) in the loess gullies, hills, and ridges. Seven zones of loess landside distribution were identified based on their landside densities. The spatial distribution of the geological disasters is characterized by a “linear and zone” shape in the CLP. These results provide a useful reference for the study of the mechanism of water-induced loess landslides.
Applications of low-cost single-frequency continuous GPS receivers for monitoring volcano and landslide activities as well as to complement dual-frequency receivers have been demonstrated to produce stable and accurate positioning. In studies of crustal deformation, the relative distance between monitoring stations may vary from several kilometers to tens of kilometers, hence the differential single-frequency observations cannot model the ionospheric delay or other distance dependent errors. The 55 low-cost single-frequency continuous stations have been deployed together with 52 continuous dual-frequency stations in southeastern Taiwan since 2008. All of the single-frequency stations have applied corrections using dual-frequency stations to eliminate the distance dependent errors. Comparing velocity estimates from 8 co-located, the differences in horizontal and vertical components are less than 3 mm/yr and 6 mm/yr, respectively. Our study shows that the combination of single- and dual-frequency GPS data can provide robust results to study the fault slip behavior on the Longitudinal Valley fault.
Unlike the past International Terrestrial Reference Frame (ITRF) versions where global long-term solutions were combined, the ITRF2005 uses as input data time series (weekly from satellite techniques and 24-h session-wise from Very Long Baseline Interferometry) of station positions and daily Earth Orientation Parameters (EOPs). The advantage of using time series of station positions is that it allows to monitor station non-linear motion and discontinuities and to examine the temporal behavior of the frame physical parameters, namely the origin and the scale. The ITRF2005 origin is defined in such a way that it has zero translations and translation rates with respect to the Earth center of mass, averaged by the Satellite Laser Ranging (SLR) time series spanning 13 years of observations. Its scale is defined by nullifying the scale and its rate with respect to the Very Long Baseline Interferometry (VLBI) time series spanning 26 years of observations. The ITRF2005 orientation (at epoch 2000.0) and its rate are aligned to the ITRF2000 using 70 stations of high geodetic quality. The estimated level of consistency of the ITRF2005 origin (at epoch 2000.0) and its rate with respect to the ITRF2000 is respectively 0.1, 0.8, 5.8 mm and 0.2, 0.1, 1.8 mm/yr along the X, Y and Z-axis. We estimate the formal errors on these components to be 0.3 mm and 0.3 mm/yr. We believe that this low level of agreement between the two frame origins is most probably due to the poor SLR network geometry and its degradation over time. The ITRF2005 combination involving 84 co-location sites revealed a scale inconsistency of 1 ppb (6.3 mm at the equator), at epoch 2000.0, and 0.08 ppb/yr between the SLR and VLBI long-term solutions as obtained by the stacking of their respective time series. Possible causes of this inconsistency may include the poor SLR and VLBI networks and their co-locations, local tie uncertainties, systematic effects and possible inconsistent model corrections used in the data analysis of both techniques. For the first time of the ITRF history, the ITRF2005 rigorous combination provides self-consistent series of EOPs, including Polar Motion from VLBI and satellite techniques and Universal Time and Length of Day from VLBI only. A velocity field of 152 sites with an error less than 1.5 mm/yr is used to estimate absolute rotation poles of 15 tectonic plates that are consistent with the ITRF2005 frame. This new absolute plate motion model supersedes and significantly improves that of the ITRF2000 which involved six major tectonic plates.
The rainfall regimes of Taiwan are investigated using the 18-yr European Centre for Medium-Range Weather Forecasts (ECMWF) data (1980-97), the available 38-yr daily rainfall data from 25 conventional surface stations around Taiwan (1961-98), and the 5-yr hourly rainfall data (1994-98) from 249 high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System (ARMTS) stations. Rainfall over the island is usually generated either by transient disturbances embedded in the prevailing monsoon flow or local rainshowers related to terrain or local winds. With the change in the direction of the prevailing winds between the warm and cold seasons as well as a variety of transient subsynoptic disturbances occurring in different seasons (e.g., winter monsoon cold surges, springtime cold fronts, mei-yu fronts in the early summer, typhoons in summer months, and cold fronts in fall) and the presence of the Central Mountain Range, the regional rainfall climate over the island shows large spatial and temporal variabilities. Nevertheless, despite the presence of these transient disturbances, it is shown that the horizontal distributions of climatological rainfall patterns for different rainfall regimes are strongly dependent on the direction of the low-level prevailing flow with much higher rainfall on the windward side. Furthermore, the seasonal variations in rainfall amount and type (light precipitation versus convective precipitation) are also dependent on the thermodynamic stratification and the availability of moisture.
Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the ``peering'' approach. Peering is a way to depict the contributions of the comparatively well-known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that ~40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales.
The seasonal variations of heavy rainfall days over Taiwan are analyzed using 6-yr (1997-2002) hourly rainfall data from about 360 rainfall stations, including high-spatial-resolution Automatic Rainfall and Meteorological Telemetry System stations and 25 conventional stations. The seasonal variations and spatial variations of nontyphoon and typhoon heavy rainfall occurrences (i.e., the number of rainfall stations with rainfall rate 15 mm h1 and daily accumulation 50 mm) are also analyzed. From mid-May to early October, with abundant moisture, potential instability, and the presence of mountainous terrain, nonty- phoon heavy rainfall days are frequent (60%), but only a few stations recorded extremely heavy rainfall (130 mm day1) during the passage of synoptic disturbances or the drifting of mesoscale convective systems inland. During the mei-yu season, especially in early June, these events are more widespread than in other seasons. The orographic effects are important in determining the spatial distribution of heavy rainfall occurrences with a pronounced afternoon maximum, especially during the summer months under the southwesterly monsoon flow. After the summer-autumn transition, heavy rainfall days are most fre- quent over northeastern Taiwan under the northeasterly monsoon flow. Extremely heavy rainfall events (130 mm day1) are infrequent during the winter months because of stable atmospheric stratification with a low moisture content. Typhoon heavy rainfall events start in early May and become more frequent in late summer and early autumn. During the analysis period, heavy rainfall occurrences are widespread and dominated by extremely heavy rainfall events (130 mm day1) on the windward slopes of the storm circulations. The spatial distribution of typhoon heavy rainfall occurrences depends on the typhoon track with very little diurnal variation.
Global Positioning System (GPS) receivers have been deployed worldwide to study inter-and intraplate crustal motions and local deformations associated with earthquakes and volcanic activities. A dense array of GPS is also useful for study-ing seasonally changing load through periodic components in crustal movements. This article reviews observed and predicted seasonal crustal movements in the Japanese Islands, where both nationwide dense GPS array and meteorological sen-sor network are available. From comprehensive evaluation of various sources con-tributing to seasonal signals, the largest factor in Japan is found to be snow, weighing over 1000 kg per square meter in some regions. This is followed by various kinds of loads on the land area, such as atmosphere, soil moisture and water impoundment in reservoirs, and non-tidal ocean loads also cause certain seasonal signatures. Sea-sonal crustal deformations are calculated by synthesizing all these seasonal load changes, some of which are directly measured meteorologically and others are inferred through models. They are compared with real data observed by the dense GPS array in Japan, and their agreement was examined. The seasonal signals observed by GPS also include artifacts, such as those caused by atmospheric delay gradients and scale changes due to atmospheric refraction. We often discuss subtle crustal deformation signals, e.g. those associated with silent earthquakes, isolating them by removing secular and periodic components. Understanding seasonal signals and their interannual variability is crucial in removing these unwanted signals. The article discusses the Japanese case, but the methods proposed here will be useful worldwide to study seasonal mass redistributions. Dense GPS arrays may play a complementary role to satellite gravity missions in studying seasonal mass redis-tribution on the Earth in a regional scale.
Recent researches have demonstrated the applicability of using Global Positioning System (GPS) techniques to precisely determine the 3-D coordinates of moving points in the field of natural hazards. Indeed, the detailed analysis of the motion of a landslide, in particular for a near real-time warning system, requires the combination of accurate positioning in three dimensions (infracentimetric) and fine temporal resolution (hourly or less). The monitoring of landslides with the GPS is usually performed using repeated campaigns, as a complement to conventional geodetic methods. Continuous monitoring of landslides with GPS is usually not performed operationally, mostly because of the cost of such a system compared to conventional deformation monitoring techniques. In addition, if GPS measurements can reach a millimetre-level accuracy for long observation sessions (typically 24 h), their accuracy decreases with the duration of the observation sessions, because of errors introduced by variations of the satellite constellation and multipath effects at the sites. This study aims at determining the experimental accuracy of GPS measurements for the continuous monitoring of landslides with GPS. In particular, we want to calibrate the variation of the measurement accuracy as a function of the duration of the observation sessions. The study was carried out on the Super-Sauze earthflow (Southern Alps, France) which evolves in a channelized flow with surface displacements reaching a few tens of centimetres to a few metres per year. The GPS data were acquired during two campaigns in May and October 1999 (two reference stations were installed outside the flow and four “moving” stations distributed on the flow). The maximal 3-D cumulative displacement reaches 2.1 m during 3 weeks in May 1999. The accuracy for a 1-h session reaches 2.7, 2.2 and 5.0 mm for the north–south, east–west and vertical components, respectively. The detectability threshold for a significant motion and a given temporal resolution stands between 3.5 mm/24 h and 8.5 mm/h in planimetry, between 6 mm/24 h and 19.5 mm/h in altimetry. Thus, the motion of the flow is clearly detected by the GPS measurements and the results have been compared with those obtained with conventional geodetic methods (theodolite and electronic distance-meters) or with a wire extensometer device. In addition, combination of periodical topometric measurements, continuous extensometric and GPS measurements allows us to identify seasonal and episodic transient variations in the surficial velocity of the flow. The analysis of the relationships between rainfall (and snowfall), groundwater level, and displacements permits us to understand the behaviour of the flow and to determine pore water pressures (PWP) thresholds initiating an acceleration of the movement. GPS therefore appears applicable to the continuous monitoring of geophysical objects or of man-made structures with small and slow displacements (∼5 mm/day). This technique does not require direct line of sight between the “moving” sites and the reference stations. Measurements can be carried out in all weather and at night. GPS processing can be performed in near real time without loss of accuracy. The use of GPS is, however, limited by the environmental characteristics of the geophysical object (mountains, vegetation), which can constitute masks limiting the visibility of the sky and create multipaths effects.
A combination of digital photogrammetry and Global Positioning Systems (GPS) surveying has been used to measure landslide surface displacements and to estimate the volume involved in the movement. Ninety-eight percent of landslides mapped as active in the Emilia-Romagna region of northern Italy are reactivations of dormant mass movements. The Ca' di Malta landslide, south of Bologna, was chosen to test this integrated system. A 0.5-m cellsize Digital Elevation Model (DEM) with vertical accuracy of 0.1 m was generated using digital photogrammetric techniques.We have used a combination of digital photogrammetry and three GPS techniques to monitor landslide movement. Rapid static surveying in which the survey observations are made for a period of minutes yielded sub-centimetre positions for several marked points distributed on a longitudinal axis along the landslide. Kinematic surveying, in which the observations are carried out continuously, in this case by a walking person, provided models of the surface (1×1 m grid) by measuring the position of irregularly distributed points. Continuous observations over 7 months were made between two GPS receivers. One positioned within the moving landslide body and the other at a fixed location outside the landslide. An automatic procedure was developed to download, process and compute relative movement at constant time intervals. The accuracy achieved with the GPS measurements ranges between several millimetres to a few centimetres for static and kinematic observations, respectively.This integrated survey technique is a cost-effective method that could be applied to other structural and morphological settings. The real-time monitoring could be coupled with a warning system for landslide hazard management and the repeated kinematic GPS surveys derive precise DEMs of a landslide, providing information on geometry, volumes and evolution of the phenomenon.
The Newport-Inglewood fault zone (NIFZ) displays interferometric synthetic aperture radar (SAR) phase features along most of its length having amplitudes of up to 60 mm. However, interpretation in terms of right-lateral, shallow slip along the fault fails to match the range of geologic estimates of slip. Recently, Bawden et al. proposed that these phase features, as well as a broader deformation pattern in the Los Angeles basin, are due to vertical motion related to annual variations in the elevation of the water table. We confirm this hypothesis through the analysis of a longer span of data consisting of 26 SAR images collected by the ERS-1 and ERS-2 spacecraft between June 1992 and June 2000. Moreover, we use continuous GPS measurements from 1995 to the present to establish the amplitude and phase of the vertical deformation. The Los Angeles basin becomes most inflated one quarter of the way through the year, which is consistent with water table measurements as well as with the end of the rainy season when the aquifer should be at a maximum. The spatial pattern of the amplitude of the annual signal derived from continuous GPS measurements is consistent with the shape of the interferometric fringes. GPS sites both near the NIFZ and in a 20 by 40 km zone within the basin also show significant N-S annual variations that may be related to the differential expansion across the fault. Since these horizontal signals have peak-to-trough amplitudes of 6 mm, they mask the smaller tectonic signals and need to be taken into account when interpreting GPS time series of site position. Moreover, since the groundwater signal appears to have a longterm vertical trend which varies in sign depending on location, it will be difficult to distinguish interseismic tectonic slip along the NIFZ and within the affected areas in the basin.
Distinct periodic variations with annual frequencies are often found in the time series of continuous Global Positioning System
(GPS) site coordinates in northeastern Japan. They show maximum arc-normal contraction of a few millimeters as well as maximum
subsidence of 1 to 2 centimeters, both in March. In northeastern Japan, it snows heavily on the western flank of the backbone
range, attaining a maximum depth of several meters in March. When observed snow depths were compared with the load distribution
estimated from the GPS data, the surface loads caused by the snow were found to be largely responsible for the annual displacement
of GPS sites. The snow load modulates secular strain buildup in northeastern Japan due to the Pacific Plate subduction, but
its relevance to the seasonal change of earthquake occurrences remains uncertain.
After the 1987 Whittier Narrows and 1994 Northridge earthquakes revealed that blind thrust faults represent a significant threat to metropolitan Los Angeles, a network of 250 continuously recording global positioning system (GPS) stations was deployed to monitor displacements associated with deep slip on both blind and surface faults. Here we augment this GPS data with interferometric synthetic aperture radar imagery to take into account the deformation associated with groundwater pumping and strike-slip faulting. After removing these non-tectonic signals, we are left with 4.4 mm yr-1 of uniaxial contraction across the Los Angeles basin, oriented N 36 degrees E (perpendicular to the major strike-slip faults in the area). This indicates that the contraction is primarily accommodated on thrust faults rather than on the northeast-trending strike-slip faults. We have found that widespread groundwater and oil pumping obscures and in some cases mimics the tectonic signals expected from the blind thrust faults. In the 40-km-long Santa Ana basin, groundwater withdrawal and re-injection produces 12 mm yr-1 of long-term subsidence, accompanied by an unprecedented seasonal oscillation of 55 mm in the vertical direction and 7 mm horizontally.
The deep-seated gravitational slope deformation (DGSD) on mountain areas is widespread and overexploitation has increased the frequency of natural disasters such as landslides, rockslides, and debris flows. In Taiwan, the observation and the analysis of the deep-seated landslides suggest that these are mainly controlled by tectonic structures, which play a dominant role in the deformation of massif slopes. Considerable enhancement for morphometric interpretation can be obtained by means of the integration with Airborne Light Detection And Ranging (LiDAR) technology. On the other hand, surface displacements ranging from a few millimeters to several centimeters can be easily measured by continuous Global Positioning System (GPS). Our study analyzes the characteristics of deep-seated gravitational slope deformation using a multi-technique approach by integrating data from the photogrammetry, LiDAR, GPS, rain gauges and field observations. A continuous GPS site, TENC, located along the southern cross-island highway, Taiwan, shows a huge displacement of about 240 mm during the Morakot typhoon in 2009.
Groundwater use in California's San Joaquin Valley exceeds replenishment of the aquifer, leading to substantial diminution of this resource and rapid subsidence of the valley floor. The volume of groundwater lost over the past century and a half also represents a substantial reduction in mass and a large-scale unburdening of the lithosphere, with significant but unexplored potential impacts on crustal deformation and seismicity. Here we use vertical global positioning system measurements to show that a broad zone of rock uplift of up to 1-3 mm per year surrounds the southern San Joaquin Valley. The observed uplift matches well with predicted flexure from a simple elastic model of current rates of water-storage loss, most of which is caused by groundwater depletion. The height of the adjacent central Coast Ranges and the Sierra Nevada is strongly seasonal and peaks during the dry late summer and autumn, out of phase with uplift of the valley floor during wetter months. Our results suggest that long-term and late-summer flexural uplift of the Coast Ranges reduce the effective normal stress resolved on the San Andreas Fault. This process brings the fault closer to failure, thereby providing a viable mechanism for observed seasonality in microseismicity at Parkfield and potentially affecting long-term seismicity rates for fault systems adjacent to the valley. We also infer that the observed contemporary uplift of the southern Sierra Nevada previously attributed to tectonic or mantle-derived forces is partly a consequence of human-caused groundwater depletion.
We analyze continuous GPS measurements in Nepal, southern side of the
Himalaya, and compare GPS results with GRACE observations in this area.
We find both GPS and GRACE show significant seasonal variations. Further
comparison indicates that the observed seasonal GPS height variation and
GRACE-derived seasonal vertical displacement due to the changing
hydrologic load exhibit very consistent results, for both amplitude and
phase. For continuous GPS stations whose observation time span are
longer than 3 years, the average WRMS reduction is ˜45% when we
subtract GRACE-derived vertical displacements from GPS observed time
series. The comparison for annual amplitudes between GPS observed and
GRACE-derived seasonal displacements also shows consistent correlation.
The good seasonal correlation between GPS and GRACE is due to the
improved GPS processing strategies and also because of the strong
seasonal hydrological variations in Nepal. Besides the seasonal signal,
GRACE also indicates a long-term mass loss in the Himalaya region,
assuming no GIA effect. This mass loss therefore will lead to crustal
uplift since the earth behaves as an elastic body. We model this effect
and remove it from GPS observed vertical rates. With a 2D dislocation
model, most GPS vertical rates, especially in the central part of Nepal,
can be interpreted by interseismic strain from the Main Himalayan
Thrust, and several exceptions may indicate the complexity of vertical
motion in this region and some potential local effects.
1] We suggest that strain in the elastic part of the Earth's crust induced by surface temperature variations is a significant contributor to the seasonal variations observed in the spatially filtered daily position time series of Southern California Integrated GPS Network (SCIGN) stations. We compute the predicted thermoelastic strain from the observed local atmospheric temperature record assuming an elastically decoupled layer over a uniform elastic half-space and compare the seasonal variations in thermoelastic strain to the horizontal GPS position time series. We consider three regions (Palmdale, 29 Palms, and Idyllwild), each with one temperature station and three to six GPS stations. The temperature time series is used to compute thermoelastic strain at each station on the basis of its relative location in the temperature field. For each region we assume a wavelength for the temperature field that is related to the local topography. The depth of the decoupled layer is inferred from the phase delay between the temperature record and the GPS time series. The relative amplitude of strain variation at each GPS station, calculated to be on the order of 0.1 mstrain, is related to the relative location of that station in the temperature field. The goodness of fit between model and data is evaluated from the relative amplitudes of the seasonal signals, as well as the appropriateness of the chosen temperature field wavelength and decoupled layer depth. The analysis shows a good fit between the predicted strains and the GPS time series. This suggests that the model captures the key first-order ingredients that determine the thermoelastic strain in a given area. The results can be used to improve the signal/noise ratio in GPS data.
We develop a spectral inversion method for mass redistribution on the Earth's surface given geodetic measurements of the solid Earth's geometrical shape, using the elastic load Love numbers. First, spectral coefficients are geodetically estimated to some degree. Spatial inversion then finds the continental surface mass distribution that would force geographic variations in relative sea level such that it is self-consistent with an equipotential top surface and the deformed ocean bottom surface and such that the total (ocean plus continental mass) load has the same estimated spectral coefficients. Applying this theory, we calculate the contribution of seasonal interhemispheric (degree 1) mass transfer to variation in global mean sea level and nonsteric static ocean topography, using published GPS results for seasonal degree-1 surface loading from the global IGS network. Our inversion yields ocean-continent mass exchange with annual amplitude (2.92 ± 0.14) × 1015 kg and maximum ocean mass on 25 August ±3 days. After correction for the annual variation in global mean vertical deformation of the ocean floor (0.4 mm amplitude), we find geocentric sea level has an amplitude of 7.6 ± 0.4 mm, consistent with TOPEX-Poseidon results (minus steric effects). The seasonal variation in sea level at a point strongly depends on location ranging from 3 to 19 mm, the largest being around Antarctica in mid-August. Seasonal gradients in static topography have amplitudes of up to 10 mm over 5000 km, which may be misinterpreted as dynamic topography. Peak continental loads occur at high latitudes in late winter at the water-equivalent level of 100-200 mm.
Two-thirds of the land mass of Taiwan island is mountainous, which affects the airflow and precipitation systems over the island. In this study, we discuss the characteristics of precipitation systems when the prevailing wind direction is from the north-east during winter. Observations indicate that rainfall amounts were higher in northeastern Taiwan (the upstream side of the mountains) and that a rainfall shadow occurred in southwestern Taiwan.
Simulation results from a non-hydrostatic model indicate that airflow was deflected in eastern Taiwan, while relatively high (low) pressure areas formed in eastern (western) Taiwan. A higher mixing ratio of rainfall occurred over northeastern Taiwan while lighter rainfall occurred in the eastern, and northwestern areas and the southern tip of Taiwan. This was consistent with the observational data except for the southern tip of Taiwan. Uplift due to the topography near the mountainous areas, as well as low level convergence near the coastal areas (due to the deceleration of an easterly wind in northeastern Taiwan), helped form the mixing ratio of rain. Transportation of the mixing ratio of rainfall, due to low level westward flow and upper level eastward flow, caused it to cover a larger area. The mixing ratio of rainfall formed in the upper mountainous areas in northeastern Taiwan if the upstream moisture content was reduced significantly. A temperature inversion at low levels resulted in a decrease in relative humidity and an increase in stability, requiring that the mixing ratio of rainfall should develop closer to the mountainous areas. If a low level wind blew parallel to the orientation of the mountains (NNE-SSW), a higher mixing ratio of rainfall could occur in the mountainous areas of western Taiwan.
In the epicentral areas of major recent earthquakes, landslide density scales with peak ground acceleration. Topographic site effects on seismic waves are known to cause important gradients in ground acceleration in individual mountain ridges. Using landslide maps from the epicentral areas of earthquakes near Northridge, California, Chi-Chi, Taiwan, and the Finisterre Mountains of Papua New Guinea, we have investigated the control of these site effects over the location of earthquake induced slope failure. In our examples, earthquake-triggered landslides clustered near ridge crests, where the susceptibility to landsliding was greatest. This pattern is strongest in the Northridge epicentral area, and secondary landslide clusters were found in colluvial slope toes in western Taiwan and above inner gorges in the Finisterre Mountains. In contrast, rainfall-triggered landslides in the western Southern Alps of New Zealand were evenly distributed over all slope segments, and the landslide susceptibility was lowest near ridge crests. Observed patterns of earthquake induced landsliding are consistent in a diverse geological substrate. They correlate with the distribution of very steep slopes in the epicentral areas, but we demonstrate that topographic site effects can also be a strong control. Using the impedance operator method, we have modeled the propagation of seismic waves in a generic ridge-and-valley topography with and without inner gorge. This topography has little effect on incoming P waves, but a strong effect on S waves, giving rise to a significant amplification of peak ground accelerations at or near ridge crests, and at convex knickpoints within ridge flanks. The preferential orientation of landslides away from earthquake epicenters in the Finisterre Mountains and central west Taiwan is likely caused by asymmetric amplification of oblique incoming seismic waves across mountain ridges, and indicates that topographic site effects have dominated over topographic controls on landslide location in these areas. Although orientation of landslides in the Northridge area does not conform with this interpretation, our results suggest that knowledge of topographic site effects and the attenuation of seismic waves can be an important tool in the prediction of spatial patterns of earthquake induced landsliding.
One way to probe earthquake nucleation processes and the relation between stress buildup and seismicity is to analyze the sensitivity of seismicity to stress perturbations. Here, we report evidence for seasonal strain and stress (~ 2–4 kPa) variations in the Nepal Himalaya, induced by water storage variations which correlate with seasonal variations of seismicity. The seismicity rate is twice as high in the winter as in the summer, and correlates with stress rate variations. We infer ~ 10–20 kPa/yr interseismic stress buildup within the seismicity cluster along the high Himalaya front. Given that Earth tides exert little influence on Himalayan seismicity, the correlated seasonal variation of stress and seismicity indicates that the duration of earthquake nucleation in the Himalaya is of the order of days to month, placing constraints on faults friction laws. The unusual sensitivity of seismicity to small stress changes in the Himalaya might be due to high pore pressure at seismogenic depth.
The deep‐seated landslide located at the intersection of the Gradenbach and Möll valleys near Döllach, Austria, has been the target of many previous investigations [Kronfellner‐Kraus, G., 1980. Neue Untersuchungsergebnisse in Wildbächen — Der Talzuschub in Abhängigkeit von Niederschlägen. Int. Symp. Interpraevent Bad Ischl 1: 179–192; Weidner, S., Moser, M., Lang, E., 1998. Influence of hydrology on sagging of mountain slopes (“Talzuschübe”) — New results of time series analysis. 8th International IAEG Congress, Vancouver, Canada, Balkema, Rotterdam, 1259–1266; Weidner, S., 2000. Kinematik und Mechanismus tiefgreifender alpiner Hangdeformationen unter besonderer Berücksichtigung der hydrologischen Verhältnisse. Dissertation, Friedrich–Alexander‐Universität Erlangen–Nürnberg, 246.]. In this paper, photogrammetric, GPS and geophysical data have been utilized to derive a constraint on the kinematics of the sagging process. The photogrammetric models have been based on aerial photographs from 1962 and 1996. Displacement vectors of about 50 individual characteristic points have been determined; these clearly show the area of the sagging slope. From 1999 to 2004, eleven GPS campaigns have been carried out, yielding very accurate displacement vectors at four monitoring points. Information about the internal structure of the slope was determined using seismic surveys.
Study of slope displacement for management from landslide monitoring cases in Taiwan
- Liao
Documentation for the GAMIT analysis software
- Herring
Precipitation in Taiwan
- Water Resources Agency
Preliminary results on the study of landslides in Taiwan
- Lin
Geological characteristics and deformation mechanisms of the large deep-seated rockslide in Lushan, Nontou county
- Lin