The genetic mechanism of Wenchuan Earthquake

Abstract

The genetic mechanism of the 5.12 Wenchuan Earthquake is still being debated and there is still no convincing general explanation for most of the phenomena. This is because researchers have ignored the important role of the Minshan block in the seismogenic process. The authors present a new opinion based on geological survey and comprehensive analyses. The Minshan block is a key tectonic element of the earthquake occurrence in the northwest triangle faulty block of Sichuan Province. The Minshan block is bordered by Longmen Mountain Range fractures in the south, the Huya fracture in the east, the Tazang fracture in the north and the Mounigou Valley fracture in the west. The rigidity of the block is relatively larger than those of the adjacent regions. The block’s eastward movement pushed by regional maximum main geo-stress is limited when it suddenly tapers off near the east triangle end with a bottle-neck effect, and this causes geo-stress concentration around it. The shape of the block is coffin-like, wide in the upper part and narrow in the lower part. When a strong earthquake occurs along the block margins, the lock-up effect temporarily released, resulting in geo-stress transmitted to the Pingwu-Qingchuan (Motianling block) region. This transmission caused the Wenchuan earthquake’s aftershocks to be concentrated in Qingchuan region. As the block moved eastward, the back of the block, i.e. the south segment of Mounigou Valley fracture, became active after the Wenchuan Earthquake. Therefore the aftershocks were concentrated along the south segment of Mounigou Valley fracture. Because the south margin is composed of the front range fracture, the geo-stress gradually released, causing many aftershocks along the Guanxian-Anxian fracture. The geological survey made after the Wenchuan Earthquake reveals that the surface ruptures in the south margin of Minshan block occur not along the Beichuan-Yingxiu fracture (central fracture) also along the front range fracture. The length of the surface rupture in the south margin ranges from several meters to several kilometers and it is distributed in en echelon (closely-spaced, parallel or subparallel, step-like surface ruptures). The vertical and horizontal displacements range from place to place and the thrusting component is dominant in the middle segment of Longmen Mountain Range structure belt. Nevertheless, the strike slip of the surface ruptures is dominant in the north segment of Longmen Mountain Range structure belt. Therefore the south margin is the original seismic structure. The sudden thrusting of the south margin of the Minshan block is the source event for the Wenchuan Earthquake.