Activity Analysis Of Active Faults In Chengdu Basin

Chengdu Basin is located in the eastern margin of the Qinghai-Tibet Plateau,between the Longmen Mountain Fault zone and Longquan Mountain Fault zone,the active fault develops,the strong earthquake is frequent,many earthquakes in history have influenced the city of Chengdu in different degrees.The Quaternary fracture structure in the study area is relatively developed.The paper focuses on the activity characteristics of the Quaternary faults in the Chengdu Basin,including the Luoshui-Dujiangyan fault,the Dagu fault,the Datang fault,the Pujiang-Xinjin fault,and the Longquan slope on the west slope.In this paper,the fracture characteristics and activity of Chengdu Basin are studied by collating the previous data and analysis,combining with field Geological Survey,trough exploration and shallow seismic prospecting.Based on the analysis of Quaternary strata and drilling results,the fault activity of Chengdu Basin was explained and verified.It provides scientific basis for earthquake-proof design,land-City construction planning and earthquake-prevention plan of Chengdu City.It is found that the Luohe-Dujiangyan fault has no obvious sectional features,and it is the geological and geomorphic dividing line between Longmen Mountain and Chengdu Basin,most of which have the characteristics of North tilting and inversion,and the overall appearance is concealed and partially concealed.Shallow seismic exploration reveals that the two ends of the fault are buried shallowly,the middle section is buried deeper,and the latest active fault period is Holocene,the newest activity period of fault is Holocene.Combined with historical data and the results of this study,it is considered that it has the ability to generate 6.5±0.5 magnitude earthquakes.Dayi Fault is a part of the front of Longmen Mountain,which is concealed under the Quaternary flood accumulation layer of Chengdu plain.Some sections show continuous folds and thrusting phenomena on the upper plate of fracture.Drilling data show that the Dagu fault is a late Pleistocene active fault with a breaking distance of about 11m and a potential seismic capability of about 6.5±0.5.Shallow seismic prospecting reveals that Pengzhou fault is an inverse fault with a moving NE,And has the characteristics of piecewise activity,broken discontinuities.Xu Du lines indicate that Pengzhou fault has not been active since the Quaternary of the Jinma River,and the line of Yinfengzhen indicates that the extension of Pengzhou fault to the north-east terminates in the south of the Duck River.The drilling results show that the Pengzhou fault is a late Pleistocene active fault with a fault distance of about 7.5m.There is also the ability to generate 5.5 ± 0.5 magnitude earthquakes.The Da Tang Fault is located in the west of Qionglai and Pujiang,toward the north and south,the topography of the basin is the cutting of the large middle Pleistocene alluvial.It is observed in the fault outcrop area that faults are often developed in the sandy Mudstone and mudstone of the Cretaceous Irrigation Group,and the Late Pleistocene gravel layer is overlying.The fracture is still active in the Late Pleistocene,and the average vertical slip rate is about 0.04mm/a.Combined with this study,we estimate the potential ability of the Datang fault to have a magnitude 5.5 earthquake.The Pujiang-Xinjin fault is the south-east boundary of Chengdu Basin.According to the fault profile near the South East slope of Pujiang County City and Pujiang Shou ' an,It is inferred that the fracture has activity of late Quaternary.Further confirmed as having late Pleistocene activity according to drilling data.Combined with this study,the author believes that the Pujiang-Xinjin fault may have the potential for a magnitude of 6.0 ± 0.5 earthquake.The west slope fault of Longquan Mountain is the east boundary of Chengdu Plain,which can be divided into three sections of North,middle and south,and no obvious fault landform traces are found in these three sections of the fault.The Quaternary activity in the northern and southern segments is weak,the fault is not found,the middle part is Late Pleistocene early activity fracture.The drilling results verified that the Shanxipo fault in Longquan was a late Pleistocene active fault,Its vertical slide rate is 0.05 to 0.13 mm/a.Based on previous achievements and combined with the study of this paper,the author believes that the Shanxi Slope fault in Longquan may have the capability of earthquake of magnitude 6 or so.The buried depth of Quaternary sediments in Chengdu Plain is generally high in the west and low in the east,it can be divided into three secondary tectonic units,Western margin belt,Eastern edge belt and central sag belt.The results show that the basement structure has obvious control effect on the sedimentary of the fourth system.Pengzhou Fault and Pujiang-Xinjin fault are the new center of Quaternary sediments in Chengdu Plain.In summary:the fractures in the Chengdu basin have unique geometric features,but they also have similarities to a certain extent.They have many features such as right-sliding and sliding,and late-Quaternary activities.Fracture activity has a certain control over the depth of sediment accumulation in Chengdu Plain and the gestation of earthquakes around the fault.There have also been many earthquakes of various sizes in history.It is speculated that the intra-basin faults have potential seismic capabilities with a minimum of 5.5 and a maximum of 7 or less.