汶川8.0级地震前GPS与跨断层资料反映的运动与变形演化特征

Movement and Deformation Evolution Characteristics Reflected by GPS and Cross-fault Data before the M_S8.0 Wenchuan Earthquake

为了研究与总结2008年5月12日汶川8.0级地震前GPS与跨断层资料反映的龙门山断裂带及其周边地区的运动、构造变形、应变积累演化过程,以及汶川地震临震阶段可能的物理机制,本文综合1999~2007期GPS速度场、1999~2008年大尺度GPS基线时间序列、1985~2008年跨断层短水准等资料进行了相关分析与讨论。结果表明:(1)GPS速度剖面结果显示,宽达500km的川西高原在震前有明显的连续变形,而四川盆地一侧和跨龙门山断裂带基本没有变形趋势,表明震前川西高原在持续不断地为已经处于闭锁状态的龙门山断裂带提供能量积累。(2)GPS应变率结果显示,震前龙门山断裂带中北段的NW侧EW向挤压变形明显,变形幅度从远离断裂带较大到靠近断裂带逐渐减小,而断裂带变形微弱;龙门山断裂带西南段周边形成了显著的EW向挤压应变集中区,应变积累速率明显大于中北段。(3)断层闭锁程度反演结果显示,除了汶川地震的震源位置闭锁相对较弱,且西南段有大概20km宽度断层在12~22.5km深度为蠕滑状态以外,震前整条龙门山断裂基本处于强闭锁状态。(4)大尺度GPS基线结果显示,跨南北地震带区域的NE向基线从2005年开始普遍出现压缩转折,反映NE向地壳缩短的相对运动增强。(5)跨断层短水准场地结果显示,震前年均垂直变化速率和形变累积率很低,表明断层近场垂向活动很弱、闭锁较强。通过以上分析认为,在相对小尺度的地壳变形中,震前龙门山断裂带深浅部均处于强闭锁状态,断裂带水平与垂直变形都很微弱,这可能经历了一个缓慢的过程,而且越是临近地震的发生,微弱变形的范围可能越大;在相对大尺度的地壳变形中,震前龙门山断裂带西侧的巴颜喀拉块体东部地区经历了地壳缓慢且持续的缩短挤压变形,为龙门山断裂带应变积累持续提供了动力支持。

In order to study and summarize movement,tectonic deformation and strain accumulation evolution characteristics of the Longmenshan fault and its surrounding area reflected by GPS and cross-fault data before the MS8.0 Wenchuan earthquake,and study the possible physical mechanism late in the seismic cycle of the Wenchuan earthquake,we use GPS velocity during 1999 ~ 2007,GPS baseline time series on large scale during 1999 ~ 2008 and cross-fault leveling data during 1985 ~ 2008 to make analysis and discussion.（ 1） GPS velocity profiles show that obvious continuous deformation across the eastern Tibet before the earthquake distributed across a zone at least 500 km wide,while there is little deformation in the Sichuan Basin and Longmenshan fault zone,which means that the eastern Tibet provides energy accumulation for the locked Longmenshan fault zone continuously.（ 2） GPS strain rates show that the east-west compression deformation is large northwest of the mid-northern segment of the Longmenshan fault zone,and deformation amplitude decreases gradually from far field to near fault zones,and there is little deformation in fault zones. The east-west compression deformation is significant surrounding the southwestern segment of the Longmenshan fault zone,and strain accumulation rate is larger than that of the mid-northern segment.（ 3） Fault locking indicates nearly whole Longmenshan fault is locked before the earthquake except that the hypocenter of the earthquake is weaker locked,and a 20 km width patch in the southwestern segment between 12 km to 22.5 km depth is in creeping state.（ 4） GPS baseline time series in the northeast direction on large scale become compressive generally from 2005 in the north-south seismic zone,which reflects that relative compression deformation enhances.（ 5） The cross-fault leveling data show that annual vertical change rate and deformation trend accumulation rate in the Longmenshan fault zone are little,which indicates that vertical activity near the fault is very weak and the fault is tightly locked. By analyzing GPS and cross-fault data before the Wenchuan earthquake,we consider that the Longmenshan fault is tightly locked from the surface to the deep,and the horizontal and vertical deformation are weak surrounding the fault in relative small scale crustal deformation. The process of weaken deformation may be slow,and when close to large earthquake,the weaken deformation area may be larger.Continuous and slow compression deformation across the eastern Tibet before the earthquake provides dynamic support for strain accumulation in the Longmenshan fault zone in relative large scale crustal deformation.