Active tectonic characteristics of river terraces along the Tianquan River, southern Longmenshan fault zone, China

The Longmenshan fault zone at the western edge of the Sichuan Basin has long been identified as an active tectonic belt. This has been clearly illustrated by the disastrous Wenchuan and Lushan earthquakes in the recent decade. The two earthquakes, however, have distinctive characters. In the north, the Wenchuan event occurred on major fault zones identified previously. But in the south, the Lushan event was not accompanied by surface ruptures. Before the Lushan event, there were not many active tectonic studies in this area, either. As a result, the seismogenic structure of the Lushan event is still under debate. In order to further understand the neotectonic characteristics of the Lushan earthquake region, we analyzed fluvial terraces first by satellite images, in the hope that such geomorphic features would provide information of active structures of the area. Along the Tianquan River, river terraces are particularly well developed near two cities, Tianquan and Shiyang. After detailed field investigations, we found that underneath these terraces, early Tertiary bedrocks crop out below river sediments that are only 1-3 m thick. This indicates that the Tianquan River has incised into bedrocks. The slope of the terrace surfaces is similar to that of the present-day riverbed, and the river sediments in the terrace outcrops have similar grain size distribution as current riverbed sediments. Therefore, we suggest that the terraces along the Tianquan River were result of tectonic uplift. Using the Real Time Kinematic-Global Positioning System (RTK-GPS) and Digital Elevation Model (DEM) results, we defined Tianquan terraces with 4 levels, and Shiyang terraces with 11 levels. With the ages of the terraces and the amount of incision, we obtained the incision rate of the Tianquan River. The incision rate of the Tianquan River is about 1-2 mm/year, very similar to the structural activity rate of the northern Longmenshan fault zone. We also found the structural activity rate in Tianquan area increased recently, and this trend is very similar to the results obtained from studies of Qingyi River terraces, in the southern region Longmenshan fault zone. Therefore, we speculated that the structures became more active recently in the entire southern Longmenshan fault zone. Based on our results, the tectonic activity poses potential threatening to this area. Therefore, we hope the results of this study would provide more information of neotectonic characteristics of the southwestern Sichuan Basin, as well as future earthquake hazards in this densely populated region.