A total of 17 low-cost single-frequency L1 global positioning system (GPS) receivers with real-time internet transmission have been set up to intensify the pre-existing network of continuously operating reference stations (CORS) in southeastern Taiwan since 2008. The main objective of this study is to investigate the validity and uncertainty of the L1 stations in southeastern Taiwan. It is well known that the main error source of single-frequency GPS relative positioning in low latitude areas comes from an atmospheric delay, even if the relative distance is only a few kilometres. In this study, two methods of correction algorithms, including adopting local ionospheric models and applying correction terms from local CORS, are tested to estimate the long-period accuracy of station positioning. Our results indicate that the standard deviation of calibrated relative positioning is in a linear trend with respect to the baseline length. The derived positioning accuracies from applying correction terms from CORS provide satisfactory results with the linear ratios of standard deviation/baseline of 0.11 +/- 0.02, 0.12 +/- 0.02, 0.44 +/- 0.06 mm km(-1) in the north, east and up component, respectively for relative distances under 30 km. The corresponding positioning scatterings amount to 3, 3 and 13 mm, in the north, east and up component, respectively. Although the use of a local ionospheric model algorithm can significantly reduce positioning variation, especially in the north component, the use of the correction terms method yields the best positioning results for three components, horizontal and vertical.

The Longitudinal Valley Fault (LVF) in eastern Taiwan is a high slip rate fault (about 5 cm/yr), which exhibits both seismic and aseismic slip. Deformation of anthropogenic features shows that aseismic creep accounts for a significant fraction of fault slip near the surface, whereas a fraction of the slip is also seismic, since this fault has produced large earthquakes with five M w >6.8 events in 1951 and 2003. In this study, we analyze a dense set of geodetic and seismological data around the LVF, including campaign mode Global Positioning System(GPS) measurements, time series of daily solutions for continuous GPS stations (cGPS), leveling data, and accelerometric records of the 2003 Chenkung earthquake. To enhance the spatial resolution provided by these data, we complement them with interferometric synthetic aperture radar (InSAR) measurements produced from a series of Advanced Land Observing Satellite images processed using a persistent scatterer technique. The combined data set covers the entire LVF and spans the period from 1992 to 2010. We invert this data to infer the temporal evolution of fault slip at depth using the Principal Component Analysis‐based Inversion Method. This technique allows the joint inversion of diverse data, taking the advantage of the spatial resolution given by the InSAR measurements and the temporal resolution afforded by the cGPS data. We find that (1) seismic slip during the 2003 Chengkung earthquake occurred on a fault patch which had remained partially locked in the interseismic period, (2) the seismic rupture propagated partially into a zone of shallow aseismic interseismic creep but failed to reach the surface, and (3) that aseismic afterslip occurred around the area that ruptured seismically. We find consistency between geodetic and seismological constraints on the partitioning between seismic and aseismic creep. About 80–90% of slip on the southern section of LVF in the 0–26 km, seismogenic depth range, is actually aseismic. We infer that the clay‐rich Lichi Mélange is the key factor promoting aseismic creep at shallow depth.

We analyzed continuous GPS (CGPS) data recorded on 15 stations from 2002 to 2009 and examined the CGPS-derived strain along with local seismicity to characterize the current crustal deformation at the plate junction around the Hualien area in Taiwan. By examining the CGPS time series in detail, we discovered abnormal variations in the CGPS horizontal displacements and an annual cycle with a peak-to-peak difference of more than 20mm. Most stations move in the ESE direction during May to October, and move in the opposite direction during November to April every year. We found that the average semi-annual velocity of each CGPS station is generally parallel to the direction of convergence between the Eurasian and Philippine Sea plates, and that the CGPS temporal areal strain is strongly related to the occurrence of larger local earthquakes, while the strain reverses from contraction to extension. The CGPS displacement is well known to have been influenced by seasonal changes or loadings from several environmental factors. We tested these perceptions with the newly acquired CGPS data and seismicity, and found that the environmental factors are unlikely to explain the patterns of surface motion in the study area. We also compared our results with previously reported cases and found distinctive patterns in the temporal and spatial distributions of the CGPS data and seismic behavior. The geodetic and seismic observations should provide motion constraints for further studies of the plate junction kinematics from collision to subduction around the Hualien area in Taiwan.

Rapid exhumation of 3-10mm/yr of the Taiwan metamorphic range is often explained as the unroofing of the retrowedge of a doubly vergent mountain belt. Yet, to date, the Central Range fault forming the boundary of the retrowedge has displayed no definitive evidence for recent seismic activity and no unambiguous geomorphic expression over much of the fault. The 2013M6.4 Rueisuei reverse-faulting earthquake nucleated at the eastern boundary of the retrowedge and appears to illuminate the west dipping Central Range fault. We estimate the fault geometry and coseismic slip distribution using a uniform stress drop slip inversion and surface displacements derived from GPS and strong-motion data. We identify a similar to 42 degrees dipping blind reverse fault, consistent with the previously proposed buried Central Range fault beneath the highly active Longitudinal Valley fault. This earthquake may be the first indication that rapid exhumation and uplift occur along a distinct fault structure bounding the eastern margin of the Taiwan retrowedge.

Taiwan's active mountain belt is a spotlight for orogenic studies and was first used to test the critical-taper wedge mechanics. The concept of an orogenic wedge above a shallow detachment surface has been highly influential on current understanding of orogenic processes in Taiwan. However, the recent M-L 6.2 and M-L 6.5 2013 Nantou reverse-faulting earthquakes in central Taiwan have nucleated below the proposed detachment, indicating that active mountain building is occurring below the orogenic wedge. We estimate the coseismic slip distributions and fault geometry using the uniform stress drop slip inversions. The earthquakes occur on essentially the same 30 degrees dipping fault plane ramping up from similar to 20km depth near a cluster of 1999 Chi-Chi earthquake aftershocks to the shallow detachment and the Chi-Chi fault plane. The fault could be a deep extension of a mature shallow fault or a newly developed deep ramp fault that is not reflected in the surface geology.

The time history and spatial dependence of seismic-wave propagation on the ground surface and through the ionosphere following the 2011 off the Pacific coast of Tohoku Earthquake were reconstructed from dense seismic networks and from Global Positioning System (GPS) array observations, respectively. Using total electron content (TEC) data recorded by a dense GPS receiver network, the near-source ionosphere perturbations induced by this giant earthquake were analyzed and high-resolution images of seismic-wave propagation in the ionosphere are presented. Similar spatial images of ground motions were reconstructed from observations by a dense seismic array. Observations of this event provide, for the first time, the opportunity to compare near-source ground motions with the near-field seismo-traveling ionosphere disturbance (STID) excited by the ground motions. Based on the results, the nature of the source rupture and seismic-wave propagation are discussed. Both seismic and ionosphere observations indicate that seismic energy propagated radially outward initially from the hypocenter, but that the circular shape of the propagation front became gradually distorted as the source rupture became extended. Coherent wavefronts from the two analyses show contrasting patterns during the later stage of propagation, possibly due to different patterns of spatial variations in the physical properties of the solid earth and of the ionosphere.

We invert measurements of coseismic displacements from 139 continuously recorded GPS sites from the 2010, Jiashian, Taiwan earthquake to solve for fault geometry and slip distribution using an elastic uniform stress drop inversion. The earthquake occurred at a depth of ~23km in an area between the Western Foothills fold-and-thrust belt and the crystalline high mountains of the Central Range, providing an opportunity to examine the deep fault structure under Taiwan. The inferred rupture plane is oblique to the prominent orientation of thrust faults and parallel to several previously recognized NW-striking transfer zones that appear to connect stepping thrusts. We find that a fault striking 318°–344° with dip of 26°–41° fits the observations well with oblique reverse-sinistral slip under a low stress drop of about 0.5MPa. The derived geodetic moment of 2.92×1018N-m is equivalent to a Mw=6.24 earthquake. Coseismic slip is largely concentrated within a circular patch with a 10-km radius at the depth between 10 and 24km and maximum slip of 190mm. We suggest this earthquake ruptured the NW-striking Chishan transfer fault zone, which we interpret as a listric NE-dipping lateral ramp with oblique slip connecting stepping thrust faults (ramps). The inferred slip on the lateral ramp is considerably deeper than the 7–15km deep detachment identified in previous studies of western Taiwan. We infer an active basal detachment under western Taiwan at a depth of at least ~20–23km based on these inversion results. The earthquake may have nucleated at the base of the lateral ramp near the intersection with the basal detachment. Coulomb stress change calculations suggest that this earthquake moved several NE-striking active thrust faults in western Taiwan nearer to failure.

The March 04, 2010, Jiashian, Taiwan earthquake (M-w 6.4) ruptured an unknown fault at depth in southwestern Taiwan. The main shock initiated near the town of Liuquei at 23 km depth and the rupture propagated westward. Measurements of coseismic displacements from Taiwan Continuous GPS Array indicate horizontal displacements of 5-27 mm in the NW-SW directions to the west of the epicenter; while horizontal movements to the east of the epicenter are absent. The GPS vertical displacements show an uplift motion of about 5-25 mm near the epicenter, in contrast to a small movement of about 5-10 mm observed in the far-field GPS sites. We use coseismic GPS displacements and an elastic half-space dislocation model to invert for fault geometries and coseismic slip distribution associated with the Jiashian earthquake. Our preferred model exhibits 0.05-0.1 m of reverse slip and similar to 0.04 m of left-lateral slip on a N324 degrees -trending fault with dip of 40 degrees to NE, consistent with the earthquake focal mechanisms from BATS, USGS/NEIC, and Global CMT. The highest slip of 0.12 m mainly occurs to the west of the epicenter at a depth range of 15-20 km. Given the rigidity modulus of 60 GPa, the geodetic moment is 4.95 x 10(18) N-m, equivalent to a M-w 6.4 earthquake and consistent with the seismic moment estimated from seismic waveform inversion. Additionally, we notice that the mainshock rupture area is surrounded by high seismicity between 1991 and 2007, suggesting that the Jiashian earthquake may be triggered by the high stress concentration in the vicinity. The calculated Coulomb stress changes on nearby fault systems imparted by the coseismic slip suggest that the Jiashian earthquake may encourage failures on the Chukou fault and inhibit ruptures on the Hsinhua fault. However, the Coulomb stress changes are more complicated on the Chaochou fault and Chishan fault with both positive and negative stress changes. (C) 2011 Elsevier B.V. All rights reserved.

One of the prominent features observed in the groundwater-overdraft area is the land subsidence that creates a permanent damage to the land surface. Severe land subsidence has occurred in the western Taiwan due to excessive extraction of groundwater in the past decades. The areas of land subsidence have been gradually expanded and caused many problems such as flooding and building damage. This study examines the elevation changes on surface, which is the vertical component from the Global Positioning System in the Choshuichi Alluvial Fan of central Taiwan and describes the land-subsidence phenomena quantitatively. The Choshuichi Alluvial Fan has a high density array of wells monitoring the groundwater level variations of various aquifers automatically in an hourly basis. Through an assumed linear relationship, the inferred results show that the Global Positioning System data are closely correlated to the groundwater level variations of deep aquifers. The highest correlations between elevation changes and groundwater level variations have been found in the land subsidence area, even if a drought period is present. The responses on the surface vertical displacements from the groundwater level changes of deep aquifers can be quantitatively estimated. Furthermore, the long term subsidence trends can be derived and be used as useful reference for land and water resources management.

We use GPS-derived surface velocities, seismicity, as well as estimates of earthquake focal mechanisms from the time period before the 1999 Chi-Chi earthquake to evaluate spatial variations of surface strain rate and crustal stress regime in the Taiwan plate boundary zone. We estimate strain rates with a new but simple approach that solves for surface velocity on a rectangular grid while accounting for the distance between observations and each grid node and the impact of a spatially variable density of observations. This approach provides stable and interpretable strain-rate estimates. In addition, we perform a stress tensor inversion using earthquake focal mechanisms determined by P waves first-motion polarities. Our estimates of the principal orientations of two-dimensional surface strain rate tensor generally agree with the inferred orientations of the stress axes. This agreement suggests that a large scale variation of stress orientations from the surface to the base of the crust is insignificant and the predicted faulting style is consistent with stress buildup during the interseismic loading. We find that the geometric configuration of the Chinese continental margin alone cannot fully explain the distribution of maximum contraction and compressive axes in Taiwan. Distribution of seismicity and focal mechanisms before and after the Chi-Chi mainshock suggest that the maximum principal stress axis is vertically-oriented in the Central Range; in contrast to the horizontal maximum principal stress axis in western Taiwan and the Longitudinal Valley. Extension in the Central Range reflects the consequence of exhumation and crustal thickening.

Since 2001, we have set up a dense geodetic network with 52 campaign-mode GPS sites and seven continuously recording GPS stations as well as six leveling routes in the Taitung area, Taiwan. Our aim was to better characterize near-fault crustal deformation of active faults at the plate suture of the Philippine Sea plate and Eurasia in southeastern Taiwan. On 1 April 2006, a moderate shallow earthquake (Mw 6.1, depth 10.8 km) occurred within this network. This earthquake resulted from rupturing of a geologically unknown or suspected fault (called the Y fault) located underneath the eastern margin of the Central Range. After removing the impacts of secular motions and postseismic slip, we estimated the coseismic displacements of the Peinan earthquake from the GPS and leveling measurements before and after the main shock. Three deformation types with distinct slip behaviors were revealed in three different regions: (1) near the epicenter—around 45 mm movement in the S-SSW direction with +20 to −20 mm vertical motion, in the northern part of the Y fault; (2) south of the epicenter across the southern part of the Y fault—approximately 35 mm in a westward movement with −60 mm subsidence (footwall side) and 40 mm in a SSW movement with at least 50 mm uplift (hanging-wall side), in the southern part of the Y fault; (3) northeast away from the epicenter—about 10 mm in a northward displacement with +15 to −10 mm vertical motions, in the Longitudinal Valley and on the western flank of the Coastal Range. This unique coseismic deformation pattern sheds new light on the characteristics of the suture zone between the Eurasian and Philippine Sea plates at the southernmost Longitudinal Valley. We used GPS and leveling measurements to invert for the fault geometry and the coseismic slip distribution. The optimal modeled fault is an 80° west-dipping fault at a depth of 0.5–20 km. The highest slip of about 0.33 m is located to the south of the hypocenter at a depth of 9–16 km. The total geodetic moment in our optimal model is 2.3 × 1018 Nt-m, which is equivalent to an earthquake of Mw 6.2. The surface coseismic displacements as well as the inferred coseismic slip distribution indicate a drastic change of slip behaviors in the middle of the Y fault. The left-lateral slippage near the hypocenter turned dramatically to reverse faulting with left-lateral component as rupturing propagated to the southern portion of the fault, suggesting that a possible right-lateral faulting occurred that coseismically cross cut the northern middle Peinanshan massif in the Longitudinal Valley.

Two successive large earthquakes (ML = 6.96 and 6.99) occurred in southern Taiwan offshore of the Hengchun town, Pingtung county, with the two main shocks separated by an interval of only 8 minutes. Based on a dense network of continuously recording GPS stations (CORS) in Taiwan and adopting two different post-processing methods, we estimate the coseismic displacements and characterize their ground motions. Daily solution algorithm is used to determine the total coseismic displacements of the dual main shocks from 30-second sampling rate data; however the coseismic displacement for the individual main shock cannot be resolved. We thus adopt the kinematic positioning technique using 1-second sampling rate (1 Hz) data to determine the individual coseismic displacements for each main shock as well as the evolution of the ground shaking. The results show only three stations near the epicenters with significant total coseismic displacements of 3 - 5 centimeters. We find that the stations farther north of the epicenters area indeed reveal significant coseismic displacements but moving in the opposite direction between the two main shocks. The coseismic displacement of the first main shock is consistent with a NNE-trending normal faulting in the lower crust offshore of southern Taiwan, while that of the second main shock likely agrees with an ENE-trending right-lateral strike-slip faulting, although the possibility of a NNW-trending left-lateral strike-slip faulting cannot be ruled out. The 1 Hz GPS data can record the coseismic ground shaking in great detail, including the first motion direction and the amplitude and arrival time, which are comparable to the seismometer data. By applying an exponential attenuation behavior with hypocenter distance we observe that four stations in the coastal plain exhibit relatively larger amplitudes of the ground shaking, implying a significant influence of thick unconsolidated deposits in that area.

We use GPS displacements collected in the 15 months after the 1999 Chi-Chi, Taiwan earthquake (M-w 7.6) to evaluate whether post-seismic deformation is better explained by afterslip or viscoelastic relaxation of the lower crust and upper mantle. We find that all viscoelastic models tested fail to fit the general features in the post-seismic GPS displacements, in contrast to the satisfactory fit obtained with afterslip models. We conclude that afterslip is the dominant mechanism in the 15-month period, and invert for the space-time distribution of afterslip, using the Extended Network Inversion Filter. Our results show high slip rates surrounding the region of greatest coseismic slip. The slip-rate distribution remains roughly stationary over the 15-month period. In contrast to the limited coseismic slip on the decollement, afterslip is prominent there. Maximum afterslip of 0.57 m occurs downdip and to the east of the hypocentral region. Afterslip at hypocentral depths is limited to the southern part of the main shock rupture, with little or no slip on the northern section where coseismic slip was greatest. Whether this results from along strike variations in frictional properties or dynamic conditions that locally favour stable sliding is not clear. In general, afterslip surrounds the area of greatest coseismic slip, consistent with post-seismic slip driven by the main shock stress change. The total accumulated geodetic afterslip moment is 3.8 x 10(19) N m, significantly more than the seismic moment released by aftershocks, 6.6 x 10(18) N m. Afterslip and aftershocks appear to have different temporal evolutions and some spatial correlations, suggesting that aftershock rates may not be completely controlled by the rate of afterslip.

The MW 6.5 Chengkung earthquake occurred at 04:38 UTC on 10 December 2003. Thirty continuously recording GPS stations (CORS) have been set up at different geological sites and distributed throughout a 140 km by 140 km area in southern Taiwan beginning in 2000. The GPS data is recorded daily for the CORS in 30 seconds sampling rate. The GPS data is utilized to study the coseismic and postseismic deformation associated with the Chengkung earthquake. The coordinates of the daily solution for each station were extracted from SINEX (Software INdependent EXchange) files to establish time series in the topocentric north-east-up (NEU) coordinate system. The secular crustal deformation of the station during the one year period was removed by applying the 2000-2003 interseismic velocities. The CORS near the Chihshan fault, which are located at the southern segment of the Longitudinal Valley Fault, indicated the largest postseismic displacement in eighteen months approached 86 mm (station SHAN) and 91 mm (station TAPO) in the horizontal and vertical components, respectively. The results of the CORS provide detailed information for the temporal process of postseismic deformation.

The MW 6.5 Chengkung earthquake occurred in eastern Taiwan at 04:38 UTC on 10 December 2003. The GPS data from eighteen continuously recording stations (CORS) and 86 campaign-surveyed stations (CSS) collected 18 days to 9 months before and 6 days to 4 months after the main shock are utilized to analyze the coseismic and postseismic deformation associated with the Chengkung earthquake. The earthquake resulted from rupturing of the Chihshang fault, a 25-km-long segment of the Longitudinal Valley Fault (LVF). The coseismic horizontal displacements in the hanging wall showed a fan-shape distribution with vectors towards the west. On the other hand, the movements of the revealed a mirror fan-shape with relatively lesser amounts of displacement. The largest coseismic displacement, which reached 126 mm and 263 mm in the horizontal and vertical components, occurred near the epicenter area in the hanging wall. The largest postseismic displacements in 109 days, which approached 59 mm and 68 mm in the horizontal and vertical components, occurred near the surface trace of the Chihshang fault (TAPO) and near the epicenter area (CHEN), respectively. The stations near the Chihshang fault indicated a more significant postseismic displacement than coseismic one.

On 31 March 2002, a M L = 6.8 earthquake (called the “331 earthquake”) occurred in northeastern Taiwan off Hualien. The shock lasted over 30 seconds across the entire island of Taiwan. The earthquake caused a few centimeters of coseismic deformation, and a few centimeters of ground motion in the northern and eastern parts of Taiwan. All of these displacements have been recorded by the Taiwan Continuous GPS Array (TCGA), and estimated by two different post-processing methods, namely the daily solution and the kinematic positioning algorithm. Precise evaluation of the coseismic deformation and capturing instantaneous ground motion at a level of just millimeters requires rigorous computational procedures. In this paper, a completely regular algorithm to estimate the crustal deformation in the Taiwan area has been applied to acquire coseismic deformation as a result of the 331 earthquake. A set of high sampling rate (1 Hz) data from the TCGA has been used to study simultaneous ground motion during the 331 earthquake. Using over 18 months of data and 100 stations of the TCGA, the coseismic deformation due to the 331 earthquake can be precisely estimated, and instantaneous ground motion can be observed in that portion of the TCGA network where stations with high sampling rate observations are situated. Since the coseismic deformation can be acquired precisely and the instantaneous ground motion can be computed by continuous GPS observations and integrated with seismic data, these results can aid the study of seismology and earthquake geodesy.