天山地震带境内外主要断层滑动速率和地震矩亏损分布特征研究

本文搜集、整理1998—2013年境内外天山及周边地区(包括中国新疆、哈萨克斯坦、吉尔吉斯斯坦等)500余个GPS观测点数据,采用GAMIT/GLOBK软件对其进行解算和平差计算,并利用了弹性块体模型计算区域块体边界断层闭锁深度、块体运动参数和主要活动断层的滑动速率.研究结果表明,东、西昆仑地震带闭锁深度最大(19km),其次为南天山地区,闭锁深度达到17km,闭锁深度最小的为哈萨克斯坦(13km);各块体相对欧亚板块作顺(逆)时针旋转,旋转速率最大(-0.7208±0.0034°/Ma)为塔里木块体,其围绕欧拉极(38.295±0.019°N,95.078±0.077°E)顺时针方向转动,旋转速率最小为天山东段(0.108±0.1210°/Ma),而天山东、西两段无论是在旋转速率上还是在旋转方向上都有显著的区别.西昆仑断裂带的滑动速率(10.2±2.8mm·a-1)最大,南天山西段滑动速率为9.5±1.8mm·a-1,其东段为3.9±1.1mm·a-1;而北天山东段滑动速率(4.7±1.1mm·a-1)高于北天山西段(3.7±0.9mm·a-1);塔里木盆地南缘的阿尔金断裂带平均滑动速率为7.6±1.4mm·a-1,其结果与阿勒泰断裂带滑动速率(7.6±1.6mm·a-1)基本相当;天山断裂带运动方式主要以挤压为主,而阿尔金、昆仑、阿尔泰以及哈萨克斯坦断裂带均是以走滑运动方式为主,除阿勒泰断裂带走滑方式为右旋以外,其余几个断裂带均为左旋运动.最后,利用主要断裂带的滑动速率计算出各地震带的地震矩变化率以及1900年以来地震矩累计变化量,其结果与利用地震目录计算所得到的地震矩进行比较,判定出各地震带上地震矩均衡分布状态,研究结果显示阿尔金、西昆仑、东昆仑和北天山东段断裂带存在较大的地震矩亏损,均具有发生7级以上地震的可能性,南天山东段和哈萨克斯坦断裂带地震矩亏损相对较小,具有孕育6~7级地震的潜能,而天山西段、阿勒泰地震矩呈现出盈余状态,不具在1~3年内有发生强震的可能.

Slip rates and seismic moment deficits on major faults in the Tianshan region

Over 500 GPS stations observed from 1998 to 2013 in the Tianshan region (including Xinjiang, Kazakhstan and Kyrgyzstan) were collected and analyzed using the GAMIT/GLOBK software. The site velocities were then used in an elastic block model to calculate both the locking depths and slip rates of the major active faults. The maximum locking depth (19 km) were found in the Eastern and Western Kunlun seismic zones; the locking depth is about 17 km in the Southern Tianshan region, while the minimum locking depth is 13 km in the Kazakhstan region. All blocks show some rotation relative to the stable Eurasian Plate. The Tarim block has the highest rotation rate (-0.7208±0.0034°/Ma), around the Euler Pole at 38.295±0.019°N and 95.078±0.077°E. The eastern Tianshan block has the lowest rotation rate (0.108±0.1210°/Ma), and the eastern and western sections of the Tianshan differ in both rotation rates and directions. The Western Kunlun fault zone has the highest slip rate (10.2±2.8 mm·a^-1), while the western and eastern segments of the Southern Tianshan fault slip at 9.5±1.8 mm·a^-1 and 3.9±1.1 mm·a^-1, respectively. On the Northern Tianshan fault, the slip rate of its eastern segment is higher (4.7±1.1 mm·a^-1) than the west segment (3.7±0.9 mm·a^-1). The average slip rate on the Altyn Tagh fault is 7.6±1.4 mm·a^-1, comparable to that on the Altay fault (7.6±1.6 mm·a^-1). The Tianshan faults are dominantly compressive, while the Altyn Tagh, Kunlun, Altay and Kazakhstan faults were mainly strike-slip; all faults in this region are left lateral strike slip faults except the Altay fault, which is right lateral strike-slip. By comparing moment accumulation calculated by the slip rates with seismic moment release based on earthquake catalogue since 1900, the balance of seismic moment on each fault is estimated. The results show significant seismic moment deficits on the Altyn, the Western Kunlun, the East Kunlun, and the eastern segment of the Northern Tianshan faults, capable to produce magnitude 7.0 earthquakes on these faults. Lower moment deficits are found on the eastern segment of the Southern Tianshan and Kazakhstan fault zones, where earthquakes with magnitude 6.0~7.0 are possible. The western segments of the Tianshan and Altay faults shows surplus of seismic moment, thus are unlikely to have large earthquake in the new future.