N‐S trending rifts are widely distributed in southern Tibet, suggesting that this region is under E‐W extension, behind the N‐S collision between the Eurasia and India plates. Geophysical anomalies and Miocene magma extrusions indicate the presence of dispersed weak zones in the middle to lower crust in southern Tibet. These weak zones are partially located underneath the N‐S rifting systems. In order to study the formation of rifts in collision zones, we have developed a high‐resolution 3‐D thermomechanical model of continental lithosphere with bidirectional compressional‐extensional deformation, and spatially localized weak and low‐density zones in the middle to lower crust. Our numerical experiments systematically reproduce the development of N‐S trending rifts. Model results reveal that the weak middle to lower crust triggers the development of normal faults in the upper crust and surface uplift, whereas regions without such weak layer or with small‐scale weak zones are characterized by strike‐slip faulting. Geodynamic properties (density, depth, and geometry) of the weak middle to lower crust and Moho temperature notably influence the rifting pattern. In addition, rifting formation is critically controlled by large E‐W extension, with the ratio of extensional to compressional strain rate larger than 1.5 in the model with continuous weak middle crust. Our simulated rifting patterns correlate well with the observations in southern Tibet; we conclude that a combination of the bidirectional compression‐extension and the presence of locally weak middle to lower crust triggered the development of the rifting systems in southern Tibet.

The Tibetan Plateau is characterized by high elevation and complex fault systems. High‐precision vertical movement data can provide important constraints for understanding the growth and expansion of the Tibetan Plateau. We collected approximately 116,000 km of leveling data, 21 continuous GNSS data sets, and their connecting surveying data; we jointly processed these data using a Helmert joint adjustment method to acquire the high‐precision vertical velocity field of the Tibetan Plateau and its surrounding areas. The primary results are as follows: (a) Vertical uplift is dominant on the southern, northeastern, and southeastern margins of the plateau, with uplift rate ranges of 2.0–3.0, 1.0–3.8, and 1.0–2.0 mm/yr, respectively; (b) Conspicuous subsidence is located along the southern portion of the Ganzi fault, with vertical rates ranging from −3.3 to −0.5 mm/yr; (c) Velocity profiles show that vertical deformation varies in different parts of the Tibetan Plateau, which is mostly accommodated by large strike‐slip and thrust faults, such as the Kunlun, Ganzi, and Longmenshan faults. Most of the surface uplift is accommodated by crustal shortening in the interior of the Tibetan Plateau; abrupt changes in vertical rates in eastern Tibet and the widely distributed surface subsidence of southeastern Tibet are consequences of crustal flow and gravitational collapse. Overall, the Tibetan Plateau is characterized by continuous deformation, with large spatial variations accommodated by complicated tectonic processes.

On December 16, 2013, right after the Three Gorges Reservoir (TGR) reached its highest annual water level, a powerful M5.1 earthquake occurred in Badong County, China's Hubei Province. The epicenter is 5.5km away from the upstream boundary and 100km from the dam. Was this earthquake triggered by the impoundment of the TGR, and what are itssubsequences? To answer these questions, we constructed a coupled three-dimensional poroelastic finite element model to examine the ground surface deformation, the Coulomb Failure Stress change (∆CFS) due to the variation of elastic stress and pore pressure, and the elastic strain energy potential accumulation in the TGR region upon the occurrence of this event.Our calculated maximum surface deformation values beneath the TGR compare well with GPS observations, which validates our numerical model. At the hypocenter of the earthquake, ∆CFS is around 8.0~11.0kPa, revealing that it may be eventually triggered by the impoundment. We also discovered that the total elastic strain energy potential accumulation due to the impounded water load is around 1.7×10¹²J, merely equivalent to 0.01% of the total energy released by this event, indicating that the this earthquake is predominately controlled by the typical regional 2 tectonic settings as well as the weak fault zones, and the reservoir impoundment might only facilitate its procedure or occurrence. Furthermore, the stress level in this region remains high after this earthquake and the subsequent reservoir-triggered micro-seismicity or even bigger event are highly possible.

Abstract:The spatiotemporal deformation response of a seismogenic fault to a large earthquake is of great significance to understanding the nucleation and occurrence of the next strong earthquake.