Structural Deformation Style And Study Of The Paleoearthquake At The Beichuan-Dengjia Segment Along The Central Fault, Longmen Mountain

At 14:28, May 12, 2008, a startled earthquake of Ms 8.0 occurred in Wenchuan County, Sichuan Province, which has become the most disastrous earthquake of casualties property damage since 1976 Tangshan Ms7.8 Earthquake.The theme of this thesis is selected as"Structural deformation style and study of the paleoearthquake at the Beichuan-Dengjia segment along the central fault, Longmen Mountain"because of the following reasons:⑴After Wenchuan Earthquake, China Earthquake Administration organized the expert emergent investigation, and has accumulated lots of basic data, which aimed at surface ruptures and paleoearthquake of Longmen Mountain Fault zone.⑵The surface ruptures showed: The co-seismic vertical slip of central fault of Longmen Mountain was major, and characterized by right-slip. It gradually translated into the fault which is the main right co-seismic strike slip. The segment of Beichuan-Dengjia is the very transition segment where the co-seismic vertical slip translated into the right co-seismic strike slip, so that the studying of surface ruptures style is great meaning.⑶The surface ruptures of Beichuan-Dengjia showed it was a normal fault, but the rupture style of Beichuan-Dengjia north and south segment both were reverse fault on the center fault zone, and the whole Longmen Mountain Center Fault is in the extrusion environment, so that the fault of the ruptures was very special, and the studying of the special deformation style is very important for understanding the Longmen Mountain Fault.⑷After the earthquake, the author followed the work team and studied paleoearthquake in the Longmen Mountain Center and Front Fault, who has excavated trenches in Yingxiu, Leigu, Bailu, Hanwang and Xiaoyudong. Other scholars also has excavated trenches for paleoearthquake in Guixi, Pingtong and Nanba in the north of the Longmen Mountain Center Fault. So far, there isn't any elaborate study of the paleoearthquake in the segment, so that the studying of the paleoearthquake in the important segment is very helpful to understand the in-situ recurrence of large earthquake in the whole Longmen Mountain Fault zone.The study contents and the used technique of this work are summarized below.⑴Collect and interpret the data of the whole Longmen Mountain Fault zone, especially Longmen Mountain Center and Front Fault zone, explore in the studying segment, and contrastively study regional geology, geomorphology and the background of seismology and geology.⑵On the surface ruptures, there might be activities before and more fine grain size Quaternary sediments, so that the paleoearthquake events and collect samples can be clearly analyzed and estimated. Choose the appropriate place to study paleoearthquake of the segment by synthesizing the excavated trenches and nearby erosion of fault scarp.⑶Surveyed the height distribution of all tectonic geomorphic planes near the surface ruptures in Beichuan-Dengjia segment, vertical and horizontal displacements of the segments which had reliable markers, and large-scale topographic map of important segment, in order to study displacement distribution of the fault, kinematics characteristics and segmentation of these faults.⑷Base on analyzing and contrasting the measured distribution of surface ruptures and collected data (including geology and geomorphology characteristics of surface ruptures, kinematics environment, image data, GPS data, deep seismic exploration data and cross-over fault level data), study and discuss the special structural deformation style of the segment.This work has obtained the following insights.⑴The surface rupture of Longmen Mountain Center Fault in Beichuan-Dengjia segment has a special rupture process: it is the largest co-seismic vertical slip displacement of the fault in Shaba Village in the north of Beichuan, which reduced rapidly along the 55°and reduced to about 2.1m. The fault kept right co-seismic strike slip, which is only about 2m with the largest co-seismic vertical slip displacement in Shaba Village in the north of Beichuan, but the right co-seismic strike slip displacement increased rapidly along the 55°and approximated the co-seismic vertical slip displacement in 5Th Group of Haiguang Village.(the right co-seismic strike slip displacement is 3.0m and the co-seismic vertical slip displacement is 3.2m).⑵Beichuan-Dengjia is just between the Longmen Mountain Center Fault Yingxiu-Beichuan and north segment in the north of Beichuan, which is the transition where the segment main of the co-seismic vertical slip translate into the co-seismic strike slip.⑶The fault in Beichuan-Dengjia segment is different of the other segments, which puts up a normal fault inclining east, but the fact NW wall rose is consonant with other segments of Longmen Mountain Fault.⑷The co-seismic vertical slip displacement in Shaba Village absolutely is the the co-seismic vertical slip displacement of Longmen Mountain Fault. So far, 9±0.5m is the largest co-seismic vertical slip displacement of Wenchuan Earthquake.⑸The three trenches all show that the segment occurred 2 events(including 5.12 earthquake) at least, which were excavated at 1st Group of Shaba Village, 5th Group of Haiguang Village and 6th Group of Haiguang Village. Considering the observation of the scarp of the surface rupture and the analysis of geomorphology, we can judge that the magnitude of the last event is similar to the 5.12 event. In contrast with the paleoearthquake results of other revealed segments of Longmen Center and Front Fault, it shows that the result of the segment is consistent with the other revealed segments.⑹The surface rupture of Longmen Mountains Center Fault extended through Beichuan and reached Nanba in 5.12 Earthquake, and the result of paleoearthquake shows that Yingxiu-Beichuan, Beichuan-Dengjia and North of Beichuan all occurred 2 events at least, which means the surface rupture of last event also extended through Beichuan, so that Beichuan isn't the dividing segment between Longmen Mountains Center Fault middle and north part to the surface rupture.The main evolvement of the work:⑴Base on the work of the former scholar, investigated in detail the structural deformation style at the Beichuan-Dengjia segment along the Longmen Mountain Central Fault, obtained the translatable characteristics of the co-seismic vertical and stike slip displacements of the surface ruptures of the transition segment, and explained and discussed the special normal fault style.⑵The three trenches all show that the segment occurred 2 events(including 5.12 earthquake) at least, which were excavated at 1st Group of Shaba Village, 5th Group of Haiguang Village and 6th Group of Haiguang Village. Considering the observation of the scarp of the surface rupture and the analysis of geomorphology, we can judge that the magnitude of the last event is similar to the 5.12 event. In contrast with the paleoearthquake results of other revealed segments of Longmen Center and Front Fault, it shows that the result of the segment is consistent with the other revealed segments.The main problem and lack of the work:⑴A problem hasn't been studied and explained that surface ruptures strikingness changed from NE to N at 6th Group of Haiguang Village and changed back to N after crossing the Jian River.⑵Because of the lack of the time, there isn't any result of the collected samples of the trenches, and the paleoearthquake periods of the segment aren't able to judged either.