Double Difference Earthquake Location Method And Its Application In The Location Of Wenchuan Earthquake Aftershocks

In this thesis, we introduce the double difference earthquake location method and its application in the relocation of Wenchuan earthquake aftershocks. The credible hypocentral location is the basis of analysis for seismogenic structure and mechanism. The double difference earthquake location method is the most accurate method of micro-earthquake location. More than 60000 aftershocks, including 4751 earthquakes with the magnitude above 3.0, occurred in the central-northern section of Longmen Shan tectonic belt in the past one and a half year after the occurrence of Wenchuan earthquake. Accurately locating these aftershocks has great significance for us to analyze seismogenic mechanism of Wenchuan earthquake and to study deep structure of Longmen Shan.The main research content and results of this paper are as follows:1. The double difference earthquake location algorithm, application method of the computer program-HYPODD and data preparation process were introduced in detail.2. Wenchuan earthquake and its 2957 aftershocks occurred between 12 May 2008 and 31 December 2008 were relocated by using the double difference earthquake location algorithm. After? relocating, we obtained 2678 new hypocenters, the mean square root residual error reduced from 1.01 s to 0.20 s. The horizontal and vertical errors of relocated hypocenters are±0.85km and±1.3km respectively.3. The location results of Wenchuan earthquake aftershocks show that the aftershocks spread along the 350 km-long rupture belt in the central-northern section of Longmen Shan and the width of aftershocks distribution is about 15-30km. The hypocenter depth of aftershocks concentrated in the 5-20km in the upper crust, the average focal depth is 11.4km. The depth profile indicates that the aftershocks activity converges at the front- range fault, the central fault and the back- range fault. These three faults are all inclined to northwest; the dip seems to become gentler gradually from west to east. They merge into the shear zone at the depth of 20-22km and form into"imbricate structure"eventually.4. The location results of Wenchuan earthquake aftershocks suggest that there are two northwest aftershock zonations along the directions of Xiao-Yu Dong-Mianchi -Lixian and Qinghcuan-Wenxian with the length of about 90km and 70km respectively . It can be inferred that two north-west tear faults extend along the two northwest aftershocks and they are approximately vertical to Longmen Shan tectonic belt. For north part aftershocks (north to ShuiGuan town),? the tendency presents concentrating distribution and gradually deviates from the Beihuan-Yingxiu fault and Qingchuan - Pingwu fault toward northwest. So we can infer that a blind fault lies aslant Beichuan-Yingxiu fault and Qingchuan-Pingwu fault in the north section of the Longmen Shan.