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Groundwater has been over-pumped and excessive use during the past decades due to the lack of sufficient surface water caused by rapid economic developments and growing population, especially in the central part of Taiwan. The alluvial fan of the Cho-Shui River in Western Taiwan suffers the most serious land subsidence hazard with a maximum subsidence rate in excess of 3 cm/yr which is affecting the transportation infrastructures across the land subsidence area. The long-term spatial land subsidence variation from 1995 to 2020 reveals that the center of land subsidence area changed significantly with time from the coastal area to inland area. The decreasing of land subsidence could obviously be detected by the velocity profile along the Taiwan High Speed Rail during different time periods from geodetic measurements. Not only the vertical displacement increases the risk on potential damage on transportation infrastructures across the land subsidence bowls but also the induced additional horizontal displacement owing to the vertical subsidence could result in the unexpected risk for the infrastructure. In this study, we used the technique of multi-temporal InSAR to calculate the vertical deformation and the east-west deformation combined with the velocity field of the ascending and descending orbits. Three large-scale subsidence bowls are detected and accompanied by maximum additional horizontal deformation of ~8 mm/yr than that predicted by tectonic movement outside of the subsidence bowl. This additional E-W displacement is the major risk concerns of the N-S trending Taiwan High Speed Rail.

Slow-moving landslides can transform into catastrophic landslides at certain conditions. In recent decades, catastrophic landslides around the world have caused widespread damage to buildings and threatened human lives. Thus, the detection of surface displacement variation and the definition of precursory motion for extensive slow-moving landslides become a primary goal of current researches. This study dedicates to detect and monitor surface displacement of multiple slow-moving landslide areas in Taiwan by the multitemporal interferometric SAR (MTInSAR) technique. A new package was established to generate interferograms of multiple slow-moving landslide areas at the same time. Thus, the surface displacement information in large spatial coverage and a long period of time can be obtained efficiently. The landslide characteristics in space and time domain of those slow-moving landslides were investigated and analyzed. Especially, the significant acceleration movements of certain slow-moving landslides were observed and agreed with the in situ monitoring data. The results of this study provide important information of landslide hazard assessment and show the opportunity of detecting the landslide precursory motions by the MTInSAR technique in the future.

SUMMARYAn earthquake with a magnitude of Ms 6.0 and shallow focal depth of ∼4 km struck the Changning county, Sichuan province, China on 2019 June 17. The hypocentre is located in the fold-and-thrust belt with plentiful shale gas and salt mine resources. One hypothesis is that the shallow fault could be affected by the artificial pressure water injection including the disposal of wastewater, fracking shale gas extraction and salt mining in Changning area. In this study, SAR (Synthetic Aperture Radar) images, historical earthquakes, aftershocks and seismic reflection data were collected to jointly investigate the characteristics of the 2019 Changning earthquake. The source model inferred from the InSAR coseismic deformation observation reveals that the 2019 Changning earthquake is attributed to a blind fault dipping to southwest with dominant thrust and sinistral strike slip. Moreover, a small shallow fault developing within the Changning anticline was triggered by the main shock, which contributed to the surface displacements as observed in the north of the epicentre. The estimated maximum slip of 0.49 m is located at the depth of ∼1.9 km, ∼9 km northwest of the epicentre. The Coulomb failure stress change caused by the previous two large earthquakes, which occurred in the hydraulic fracturing area, suggesting that they have little effect on the initial rupture of the 2019 Changning earthquake. Despite this, they have a positive triggering effect on the fault rupture in the northwest of the seismogenic fault. In addition, the analysis on the relation between the positive Coulomb failure stress change and the aftershocks suggests that the aftershocks may have different motion patterns from the main shock. The analysis also shows the earthquakes occurrence in the seismogenic zone may be affected by the high pore pressure due to the long-term injection of salt mining for more than three decades.

We found some obvious abnormal strain induced by heavy rainfall from borehole strainmeters deployed in Western Taiwan. The strain induced by rainfall can be divided into two parts, one is the quick response for extra loads of rainwater on the ground, and another one is the slow response for rainwater infiltrating into the strata. The quick and slow rainfall responses of areal strain data are analyzed using the technique of recursive digital filtering. Moreover, the rainfall impact functions of the studied stations are calculated using deconvolution. We found, in most cases, the response strain will reach the maximum in half an hour after heavy rainfall, and then show an exponential decay, it might persist more than 200 h depending on the hydrogeological condition around the station. Whereas the river flowing beside the station will help accelerating the runoff dispersion and reducing rainfall decay time in the hill or mountain region. We also compare the results after calibration in term of isotropic and vertical coupling individually. We found that the response strains are smaller in vertical coupling rather than isotropic coupling. The effects of debris avalanches caused by intensive rainfall in the mountain areas can be viewed as two types of rock deformation: generated only under the influence of rainfall and generated by the increased load in the river channels due to rainfall-induced landslides or debris flow. When the cumulative rainfall exceeds a certain threshold, the strain response curves show a noticeable anomaly likely due to the effects of the debris flow events in places prone to landslides.

The ascending and descending InSAR deformations derived from ALOS-2 and Sentinel-1 satellite SAR images and GPS displacements are used to estimate the fault model of the 2018 Mw 6.4 Hualien earthquake. The sinistral strike-slip fault dipping to the west with a high dip angle of 89.4° and a rake angle of 201.7° is considered as the seismogenic fault of this event. This seismogenic fault also triggered the ruptures of the Milun fault, which dips to the east with a dip angle of ~72°, and an unknown west-dipping fault with a dip angle of 85.2°. Two predicted faulting models indicate that the InSAR deformation fields include more postseismic slip than those of the GPS data. The north segment of the Milun fault and west-dipping fault have been triggered by the rupture of the seismogenic fault, but the postseismic slip occurred only in the south segment of the Milun fault. The InSAR-derived co-seismic and postseismic faulting model suggests that the significant slip concentrates at depths of 2.4–15.0 km of the main fault, 0.0–14.0 km of the Milun fault. Only minor slip is detected on the west-dipping fault. The maximum fault slip of ca. 2.1 m is located at the depth of ca. 2.4 km under the Meilun Tableland. The Coulomb failure stress (CFS) change calculated by the co-seismic and postseismic faulting model shows that there is a significant CFS increase in the east of the south segment of the Milun fault, but few of the aftershocks occur in this area, which indicates a high risk of future seismic hazard.

The Shanchiao fault, located to the west of the Taipei Basin in northern Taiwan, is a highly active normal fault that has a left-slip component and fault length of over 40 km. We suggest that the Shanchiao fault still has the ability to induce coseismic subsidence in the Taipei Basin under present extensional regime of northern Taiwan. In order to characterize the coseismic ground deformation and assess the potential inundation hazards in Taipei Basin, we estimate surface displacements using elastic dislocation models. The largest slip is assumed to be located underneath the Kuandu area due to the observation of deepest Tertiary basement in this area. Based on the topography changes due to coseismic deformation in a potential magnitude 7 event induced by reactivation of the Shanchiao fault, a 2D inundation model was adopted to simulate several inundation scenarios, including potential flood inundation below high tide condition and under various return-period design rainfall events. The predicted inundation maps based on various return-period flood events can provide information to assess potential earthquake-induced inundation hazards.