Geomorphic indices and relative tectonic uplift in the Guerrero sector of the Mexican forearc

Tectonically active areas,such as forearc regions,commonly show contrasting relief,differential tectonic uplift,variations in erosion rates,in river incision,and in channel gradient produced by ongoing tectonic deformation.Thus,information on the tectonic activity of a defined area could be derived via landscape analysis.This study uses topography and geomorphic indices to extract signals of ongoing tectonic deformation along the Mexican subduction forearc within the Guerrero sector.For this purpose,we use field data,topographical data,knickpoints,the ratio of volume to area（Rva）.the stream-length gradient index（St）,and the normalized channel steepness index（k_（sn））.The results of the applied landscape analysis reveal considerable variations in relief,topography and geomorphic indices values along the Guerrero sector of the Mexican subduction zone.We argue that the reported differences are indicative of tectonic deformation and of variations in relative tectonic uplift along the studied forearc.A significant drop from central and eastern parts of the study area towards the west in values of R_（VA）（from ~500 to^300）,St（from ~500 to ca.400）,maximum St（from ~1500-2500 to ~ 1000） and k_（sn）（from ~150 to ~100） denotes a decrease in relative tectonic uplift in the same direction.We suggest that applied geomorphic indices values and forearc topography are independent of climate and lithology.Actual mechanisms responsible for the observed variations and inferred changes in relative forearc tectonic uplift call for further studies that explain the physical processes that control the forearc along strike uplift variations and that determine the rates of uplift.The proposed methodology and results obtained through this study could prove useful to scientists who study the geomorphology of forearc regions and active subduction zones.

Gravity variation in the Tibet area before the Nepal Ms8.1 earthquake

This research utilized two periods of gravity monitoring results from 2010 to 2013 from the Continental Tectonics Environmental Monitoring Network of China, analyzed the corre- lation between gravity variation in the Tibet area and the Nepal Ms8.1 earthquake, and investigated the gravity variation mechanism in combination with the crust vertical movement and horizontal strain field observed by Global Positioning System （GPS）. The research results indicated that （1） the gravity variation exhibited apparent characteristics of a positive anomaly and high gradient zone in the Himalayan frontier. This observation is consistent with the existing recognition of the gravity anomaly and occurrence regularity of a strong earthquake; （2） the gravity variation exhibited apparent consistence with the spacious distribution of the vertical movement and the horizontal deformation field in that area. The crustal vertical movement was not the direct cause leading to the gravity vari- ation. It is assumed that the crust stress-strain accumulation in the Qinghai-Tibetan Plateau and its adjacent areas is the important factor that resulted in the variation of gravity.

Gravity variation before the Akto Ms6.7 earthquake, Xinjiang

The relationship between gravity variation and the Akto Ms6.7 earthquake on November 11, 2016, was studied by use of mobile gravity observation data from the China continental structural environmental monitoring network. The result revealed that before the Akto earthquake, a high positive gravity variation was observed in the Pamir tectonic knots region （within a maximum magnitude of approximately ＋80 microgal）, which was consistent with the existing knowledge of gravity abnormality and the locations of strong earthquakes. In view of the recent strong seismic activities in the Pamir tectonic knots region, as well as the strong upward crust movement and compressive strain, it is believed that gravity change in the Pamir tectonic knots region reflects the recent strong seismic activities and crust movement.