An M_S4.9 earthquake occurred at 08:17 on the December 27, 2016 in Rongchang District, Chongqing, and the epicenter is located in the north central section of Huayingshan basement fault system on the eastern margin of Sichuan Basin. The seismicity shown in the historical earthquake catalogue was originally very weak in this area. Since the late 1980s, due to the impact of waste water reinjection in the natural gas field, earthquakes of magnitude ≥4.0 occurred frequently and 14 earthquakes with M_S≥4.0 have occurred, the largest of which was Rongchang M_S5.0 earthquake in 1997. In this paper, the fine three-dimensional P-wave velocity structures and relocation results of seismic events in Rongchang and its surrounding areas are inversed by double difference tomography method, based on the P-wave and S-wave arrival time data of 1786 seismic events recorded by Chongqing regional fixed network, mobile network and Zigong local network from January 2008 to June 2020.
The results show that: 1)The distribution of high-velocity and low-velocity zones within 4km depth is significantly different from that below 7~13km depth. The P-wave high-velocity zone at 4km depth is mainly distributed in Renyi-Rongchang area, where there are four water injection wells, a major concentration area of continuous water injection in Rongchang since 2008. The range of Renyi-Rongchang high velocity zone significantly gets narrowed at the 7km depth and is obviously different from that at the shallow surface. The velocity structures on the east and west sides of Huayingshan basement fault vary greatly from 7 to 13km. The P-wave velocity structures of different sections across Huayingshan basement fault all indicate that the depth of the interface between the sedimentary cover and crystalline basement is 12km in Rongchang area, which is basically consistent with the previous research results and the characteristics of seismic reflection profiles in Rongchang area. The inversed velocity structures well mirror the shape of Luoguanshan fold, and further confirm the reliability of our results. 2)The lateral difference of P-wave velocity structure in the shallow layer of Rongchang area varies greatly. There is a high-velocity zone near the Luo2^# water injection well at the axis of Luoguanshan anticline and the depth distribution is 3~7km. The hidden fault in the north wing of Luoguanshan anticline with buried depth of 1.7km is developed near well Luo2^#, and the high velocity zone is distributed along the dip of the hidden fault, which may indicate that the hidden fault may be the main channel for wastewater infiltration. The depth of wastewater infiltration is up to 7km, resulting in a large velocity difference between the two sides of the fault. The M_S4.9 earthquake on December 27, 2016 and the M_S4.0 earthquake on December 28, 2016 are just distributed in the velocity transition zone. Obvious high-velocity body was not found below 3km in Luo4^# water injection well, which may be related to the cessation of water injection in Luo4^# well in February 2001. 3)The results of seismic relocation indicate that earthquakes are mainly distributed in the axis of the strongly deformed Luoguanshan anticline, showing obvious stripe distribution in NE direction, and the focal dominant depth is 0~6km. Based on the focal mechanism solution and the regional seismotectonic environment, it is believed that the seismogenic fault of earthquakes above M_S4.0 on the south side of Guangshun transverse fault should be the hidden fault on the south wing of Luoguanshan, while the seismicity on the north side of Guangshun transverse fault may be related to the hidden fault on the north wing of Luoguanshan.

The elevation evolution history of the southeastern Tibet Plateau is of great significance for examining the deformation mechanism of the plateau boundary and understanding the interior geodynamic mechanics. It provides an important window to inspect the uplift and deformation processes of the Tibet Plateau, and also an important way to test two controversial dynamic end-element models of the Plateau boundary. In recent years, some breakthroughs have been made in the study of paleoaltitudes in the southeastern Tibet Plateau, which allows us to have a clearer understanding of its evolution process and dynamic mechanism. By reviewing and recalculation of the latest achievements of paleo-altitude studies of the basins in the southeastern Tibet Plateau from north to south, including the Nangqian Basin, Gongjue Basin, Mangkang Basin, Liming-Jianchuan-Lanping Basin, Eryuan Basin, Nuhe Basin and Chake-Xiaolongtan Basin, we discuss the surface elevation evolution framework of the Cenozoic geomorphology and dynamics in the southeastern Tibet Plateau. The results show as follows:
(1)There was an early Eocene-Oligocene quasi plateau with an altitude of at least 2.5km from the north to middle of the southeastern Tibet Plateau(north of Dali), while the surface elevation in the south(south of Dali to Yunnan-Guizhou Plateau)was relatively low, even close to sea level. Until Miocene, the north to middle of the southeastern Tibet Plateau reached the present altitude, while the southern part of the Tibet Plateau showed a differential surface uplift trend, which established the present geomorphologic pattern. But it cannot be completely ruled out that this trend was probably caused by the accuracy of the calculation results.
(2)The quantitative constraints on the uplift process of the southeastern Tibet Plateau during Cenozoic provide certain constraints for the dynamic mechanism of geomorphic evolution in the southeastern Tibet Plateau. The northern and central parts of the southeastern Tibet Plateau can be well explained by the plate extrusion model. In this model, the collision and convergence between India and Eurasia plate or Qiangtang block and Songpan-Ganzi block resulted in the shortening and thickening of the upper crust in the region, and making the early stage(early Eocene)surface uplift. Subsequently, due to delamination or the continuous convergence between the Qiangtang block and the Songpan-Ganzi block resulting in the shortening and thickening of the crust, the plateau continued to grow northward and rose to its present altitude around Miocene. In the Eocene, the area from the south of the southeastern Tibetan plateau to the Yunnan-Guizhou Plateau mainly showed a low altitude. It seems that it may be in the peripheral area not affected by the shortening and thickening of the upper crust during the early stage India-Eurasia plate collision or plate extrusion and escape. In addition, as proposed by the lower crustal channel flow model, the lower crust material made the low-relief upland surface extending thousands of kilometers in the region uplift gradually towards the southeast, which seems to explain the low elevation landform of the region in the early stage, but it could not explain the whole uplift process of the southeastern Tibet Plateau. Therefore, a single dynamic model may not be able to perfectly explain the Cenozoic complex uplift process of the southeastern Tibet Plateau, and its process may be controlled by various dynamic processes.
(3)According to the paleoaltitude reconstruction results, if most areas of the ancient southeastern Tibet Plateau, especially the area to the north of Jianchuan Basin, had been uplifted in a certain scale and became part of the early plateau in the early Cenozoic, and reached to the current surface altitude around Miocene, the widely rapid surface erosion in this area since Miocene probably would be a continuous lag response to the finished surface uplift process, and the lag time may correspond to the sequential response process of surface uplift, the decline of river erosion base level and the gradual enhancement of river erosion capacity. Therefore, it is not proper to regard the rapid denudation and rapid river undercutting as the starting time of plateau uplift, as proposed in the previous thermochronological study.

The Wulong M_S5.0 earthquake on 23 November 2017, located in the Wolong sap between Wenfu, Furong and Mawu faults, is the biggest instrumentally recorded earthquake in the southeastern Chongqing. It occurred unexpectedly in a weak earthquake background with no knowledge of dramatically active faults. The complete earthquake sequences offered a significant source information example for focal mechanism solution, seismotectonics and seismogenic mechanism, which is helpful for the estimation of potential seismic sources and level of the future seismic risk in the region. In this study, we firstly calculated the focal mechanism solutions of the main shock using CAP waveform inversion method and then relocated the main shock and aftershocks by the method of double-difference algorithm. Secondly, we determined the seismogenic fault responsible for the M_S5.0 Wulong earthquake based on these calculated results. Finally, we explored the seismogenic mechanism of the Wulong earthquake and future potential seismic risk level of the region.
The results show the parameters of the focal mechanism solution, which are:strike24°, dip 16°, and rake -108° for the nodal plane Ⅰ, and strike223°, dip 75°, and rake -85° for the nodal plane Ⅱ. The calculations are supported by the results of different agencies and other methods. Additionally, the relocated results show that the Wulong M_S5.0 earthquake sequence is within a rectangular strip with 4.7km in length and 2.4km in width, which is approximately consistent with the scales by empirical relationship of Wells and Coppersmith(1994). Most of the relocated aftershocks are distributed in the southwest of the mainshock. The NW-SE cross sections show that the predominant focal depth is 5~8km. The earthquake sequences suggest the occurrence features of the fault that dips northwest with dip angle of 63° by the least square method, which is largely consistent with nodal planeⅡof the focal mechanism solution. Coincidentally, the field outcrop survey results show that the Wenfu Fault is a normal fault striking southwest and dipping 60°~73° by previous studies. According to the above data, we infer that the Wenfu Fault is the seismogenic structure responsible for Wulong M_S5.0 earthquake.
We also propose two preliminary genetic mechanisms of "local stress adjustment" and "fluid activation effect". The "local stress adjustment" model is that several strong earthquakes in Sichuan, such as M8.0 Wenchuan earthquake, M7.0 Luzhou earthquake and M7.0 Jiuzhaigou earthquake, have changed the stress regime of the eastern margin of the Sichuan Basin by stress transference. Within the changed stress regime, a minor local stress adjustment has the possibility of making a notable earthquake event. In contract, the "fluid activation effect" model is mainly supported by the three evidences as follows:1)the maximum principle stress axial azimuth is against the regional stress field, which reflects NWW-SEE direction thrusting type; 2)the Wujiang River crosscuts the pre-existing Wenfu normal fault and offers the fluid source; and 3)fractures along the Wenfu Fault formed by karst dissolution offer the important fluid flow channels.

Statistical study of earthquakes in the past, due to the small-medium size magnitude earthquake associated with surface rupture are rare, considers that only the earthquakes beyond magnitude 6 1/2 could produce surface ruptures in the most cases. Identification of paleoseismic events is also often based on this assumption. In this paper, we summarized 56 historical moderate size earthquakes worldwide, which have clearly documented about surface ruptures from 1950 to 2014.Results show that the magnitude lowest limit of the earthquake associated with surface rupture may be lower than the 6 1/2 , probably is about 5, even can be as low as 3.6 under extreme conditions. Additionally, from the view of theory and practice, this paper explored the effect of control factors on surface rupture, so as to indicate that the shallow focal depth is one of the most important factors for small-medium size earthquake associated with surface rupture, also included are the high heat flow values, tensile tectonic environment and active fault with weak friction strength. Although the probability that small magnitude earthquake produces surface rupture is low, it is not impossible. In the interpretation of paleoearthquake events, it also cannot overgeneralize that the corresponding earthquake magnitude must be 6.5 or greater as long as the fracture appeared, while ignoring the possibility of some moderate size earthquakes.

There are many episodes of multiple-level lacustrine terraces along the entrance of the Yarlung Zangbo Great Canyon. Besides, very thick fluvio-lacustrine sediments are buried beneath the cover of the riverbed. Optically stimulated luminescence and radiocarbon dating provide an approximate timeline of upper valley deposits and reveal at least two glacially dammed lake events (Ⅰ and Ⅱ) which have deposition ages of 7～9ka (Ⅰ) and 20～30ka(Ⅱ), respectively. The recent two episodes of glacially dammed lakes produced two steps of lacustrine terraces (T₁, T₂) correspondingly, which are of elevations 2906～2 956m and 3100～3 060m. The formation of paleo-dammed lakes reflects that the Zelunglung Glacier in the west slope of Mt. Namche Barwa had progressively advanced to block the Yarlung Tsangpo River during the early Holocene and the Last Glacial Maximum. The glacially dammed lake I has a relatively smaller extent. Its lacustrine sediments are distributed mainly from Datuoka to Mirui with maximum thickness about 5～8m. Its end is roughly at the south of Milin County. The glacially dammed lake Ⅱ occupies a large area with the end roughly nearby Lang County. Its sediments are exposed from Datuoka to Wolong with maximum thickness about 100m. After the later fluvial erosion, the lacustrine sediments of this lake formed 1～3 levels of secondary terraces.