Climatically driven displacement on the Eglington fault, Las Vegas, Nevada, USA

The Eglington fault is one of several intrabasinal faults in the Las Vegas Valley, Nevada, USA, and is the only one recognized as a source for significant earthquakes. Its broad warp displaces Late Pleistocene spring deposits of the Las Vegas Formation, which record hydrologic fluctuations that occurred in response to millennial- and submillennial-scale climate oscillations throughout the late Quaternary. The sediments allow us to constrain the timing of displacement on the Eglington fault and identify hydrologic changes that are temporally coincident with that event. The fault deforms deposits that represent widespread marshes that filled the valley between ca. 31.7 and 27.6 ka. These marshes desiccated abruptly in response to warming and groundwater lowering during Dansgaard-Oeschger (D-O) events 4 and 3, resulting in the formation of a pervasive, hard carbonate cap by 27.0 ka. Vertical offset by as much as 4.2 m occurred after the cap hardened, and most likely after younger marshes desiccated irreversibly due to a sudden depression of the water table during D-O event 2, beginning at 23.3 ka. The timing of displacement is further constrained to before 19.5 ka as evidenced by undeformed spring deposits that are inset into the incised topography of the warp. Coulomb stress calculations validate the hypothesis that the substantial groundwater decline during D-O event 2 unclamped the fault through unloading of vertical stress of the water column. The synchroneity of this abrupt hydrologic change and displacement of the Eglington fault suggests that climatically modulated tectonics operated in the Las Vegas Valley during the late Quaternary.

Combination with precise leveling and PSInSAR observations to quantify pumping-induced land subsidence in central Taiwan

Choushui River Fluvial Plain (CRFP) is located in the western central Taiwan, where the geomaterials are composed of alluvial deposits. Because the CRFP area receives highly variable rainfall in wet and dry seasons, the groundwater becomes the main resource of residential water. The precise leveling monitoring from 1970s indicated that the coastal areas of CRFP had been threatened by serious pumping-induced land subsidence. On the basis of relatively accurate measurements of precise leveling measurements, we used cokriging technique to incorporate a number of InSAR images to quantify the surface deformation in CRFP. More specifically, the well-developed Persistent Scatterer InSAR (PSI) was employed to process 34 Envisat images (2005–2008) and the results of PSI was then used for improving the spatial resolution of data from precise leveling. The results of cokriging estimation indicate whether the rate or the area of the land subsidence slows down gradually from 2005 to 2008. The subsidence in the northern part of CRFP was influenced by the groundwater decline in aquifer III, and the southern part was influenced by groundwater decline in aquifer II and III. The cokriging estimation was also comparable with continuous GPS data, and their correlation coefficient is 0.9603 and the root mean square is 10.56 mm yr−1.

Discussion on the origin of surface failures in the Valley of Aguascalientes, México

Surface failures have been observed in the valley of Aguascalientes since the early 1980's. Although, groundwater pumping began in the early 1950's but became intensive until the late 1970's, when many of the surface failures appeared. For this reason, surface deformation (surface failures and land subsidence) has been associated to groundwater withdrawal. Recent observations, however, suggest that some of these surface discontinuities are the result of natural geologic stresses rather than those associated groundwater decline, at least in its origin. Geologically, this valley is a tectonic graben flanked by two north-to-south trending normal faults, and seismicity of low intensity has been detected into the valley evidencing that the study area is seismically active. In this work, we present and discuss evidence showing tectonic discontinuities on the surface or at a shallow depth. Evidence presented in this investigation brings up uncertainty about the purely pumping-induced origin of fissuring in the valley, and allows addressing some key questions, for instance, are the surface failures caused by the combined stresses of pumping and tectonism? Is the current tectonism sufficient to cause some surface or deep failures?