汶川地震前龙门山断裂带闭锁程度和滑动亏损分布研究

Study on fault locking and fault slip deficit of the Longmenshan fault zone before the Wenchuan earthquake

利用1999—2007期GPS水平速度场数据,采用Defnode负位错反演程序估算了龙门山断裂在汶川地震前的闭锁程度和滑动亏损分布,结合龙门山断裂带附近地表水平应变率场结果,综合分析了震前地壳变形特征.反演结果表明,震前龙门山断裂中北段处于完全闭锁状态,闭锁深度达到21km(闭锁比例0.99)左右,垂直断层方向的挤压滑动亏损速率约为2.2mm/a,平行断层方向的右旋滑动亏损速率约为4.6mm/a.龙门山断裂南段只有地表以下12km闭锁程度较高(闭锁比例0.99),垂直断层方向滑动亏损速率约为1.4mm/a,平行断层方向滑动亏损速率约为4.6mm/a;在12～16km处闭锁比例约为0.83,垂直断层方向滑动亏损速率约为1.2mm/a,平行断层方向滑动亏损速率约为3.8mm/a;在16～21km处闭锁比例约为0.75,垂直断层方向滑动亏损速率约为1.1mm/a,平行断层方向滑动亏损速率约为3.5mm/a.在21～24km处整条断裂均逐步转变为蠕滑.上述反演结果与区域应变计算获得的龙门山断裂带中北段整体应变积累速率较低、南段应变积累速率较高相一致,均表明中北段闭锁程度高、南段闭锁程度稍低,该特征可以较好地解释汶川地震时从震中向北东向单向破裂现象.

By using the GPS horizontal velocity field of 1999—2007 and the negative dislocation model of Defnode,we inverted for fault locking and fault slip deficit of the Longmenshan fault before the Wenchuan earthquake,and analyzed crustal deformation before earthquake combining with GPS horizontal strain rate around the Longmenshan fault zone.The inversion results show that the middle-northern segment of the Longmenshan fault is completely locked before the earthquake and the locking depth is about 21 km（locking fraction is 0.99）,in which normal compressional slip deficit rate is about 2.2 mm/a and parallel dextral slip deficit rate is about 4.6 mm/a.Meanwhile the southern segment of the fault is tightly locked from the surface to 12 km depth（locking fraction is 0.99）,in which normal slip deficit rate is about 1.4 mm/a and parallel slip deficit rate is about 4.6 mm/a.The locking fraction decreases to about 0.83 at 12～16 km depth,in which normal slip deficit rate is about 1.2 mm/a and parallel slip deficit rate is about 3.8 mm/a.The locking fraction is about 0.75 at 16～21 km depth,in which normal slip deficit rate is about 1.1 mm/a and parallel slip deficit rate is about 3.5 mm/a.The Longmenshan fault turns to creeping gradually between 21 km and 24 km.The inversion results are consistent with the regional strain results that strain accumulation rate of the middle-northern segment of the Longmenshan fault zone is low while the southern segment of it is high.They both show that the middle-northern segment of the Longmenshan fault is very highly locked while the southern segment of it is a little less locked and this characteristic can explain well the rupturing process which propagated from the epicenter to the northeast in one direction.