汶川 MS8．0地震震前近震源区地壳形变机制探究

摘要

Various deformation data showed that the deformation characteristics of near-source area were not obvious and a short-time quiet state existed before the Wenchuan MS 8.0 earthquake.In order to study the deep dynamic factors and deformation mechanism of the phenomenon mentioned above,based on the 13-issue repeated gravity data of Chengdu area from 1996 to 2007,which have been dealt with by adjustment processing,we obtained the spatio-temporal evolution characteristics of crustal density variation in near-source area along horizontal and vertical directions ten years before the Wenchuan earthquake by 3-D inversion of crustal density.The density variation distribution is ordered in space and mainly concentrates on the Longmenshan fault zone and its vicinity. Additionally,the range of density variation in deep is more obvious than sha-llow,which may mean the fault activity in deep is more significant.From the view of temporal distribution,the density variation rate is uneven,that is,it is severe 3--8 years before Wenchuan earthquake and unobvious in short term before the event.Based on the deformation characteristics and the dynamic evolution process of density variation in different depth before the Wenchuan earthquake,we deduced that the layered crust movement of Longmenshan fault zone results in small deformation in shallow and obvious density variation in deep from the dynamics perspective.In addition,we selected and improved the combination-hardening model as the appropriate seismogenic model for the Wen-chuan earthquake according to the density variation characteristics of Longmen-shan fault zone and its adjacent areas.Finally,we combined the dynamic process with generating mechanism of earthquakes so as to reveal the deforma-tion mechanism in near-source area before Wenchuan earthquake.%多种形变资料表明，汶川地震震前越靠近震源区，其形变特征越不明显，且在近震源区震前呈现短期平静状态。为研究这种小变形现象的深部动力学因素及形变机制，本文基于成都地区1996—2007年13期重复重力观测数据，经平差处理后进行密度的三维反演，得到了汶川地震近震源区震前10年的地壳深部密度变化水平向和垂直向的时空分布特征。结果表明：密度变化在空间上呈有序分布，主要集中在龙门山断裂带及其附近区域，且深部变化幅度显著大于浅部，表明近震源区断裂带深部活动较为显著；从时间上来看，密度变化速率并不均衡，在震前3—8年介质密度变化剧烈，而在震前短期变化却不明显。根据震前形变特征和不同深度密度变化的动态演化过程，本文认为龙门山断裂带的地壳分层运动，导致了浅层地壳的小形变和深部显著的密度变化。此外，根据该断裂带及其附近地区的密度变化特征，本文选取和改进了适合汶川地震的孕震模型，即改进的组合-硬化模型，将动力学过程与孕育机制结合起来，以期对汶川地震震前近震源区的形变机制作出合理解释。