用地震资料估计的龙门山断裂深部形变及其对于汶川地震成因的意义

Present Deep Deformation along the Longmenshan Fault by Seismic Data and Implications for the Tectonic Context of the Wenchuan Earthquake

2008年5月12日汶川8.0级地震前龙门山断裂被"忽视"的原因之一是地质学证据和GPS测量证据均显示龙门山断裂长期以来形变速率很低。问题是,构造地质的结果是对一个较长的时间尺度的,而GPS结果反映的是较短时间尺度上的、地表上的、水平方向的形变,因此有理由怀疑由此得到的结论能否反映现今龙门山断裂的深部形变的全貌。我们采用类似于Kostrov方法的思路,利用最近30a的地震资料,试图研究龙门山断裂的深部形变,并与其相邻的断裂进行比较。利用ML2.5以上的微震资料,给出了沿龙门山断裂带的累积Benioff应变,并根据震级-频度关系计算了a值的空间分布。作为对照,同时计算了龙门山断裂邻区沿鲜水河、安宁河、则木河断裂各区域内的累积Benioff应变和a值。结果表明,在与地质学证据不同的时间尺度上,在与GPS证据不同的时间尺度、不同的深度上,并且不仅考虑到水平形变,与周边的断裂带相比,龙门山断裂带其实并不是一个"安静的"断裂带。在更短的时间尺度上,可以回溯性地观察到微震活动的一些异常变化,但这些变化似乎很难用于该地震的预测。我们讨论了相关的观测资料分析对于汶川地震的成因的意义。

One of the causes for the Longmenshan fault to be ＂neglected＂ before the May 12, 2008, Wenchuan, Ms 8.0 earthquake was that geological evidences and GPS measurements all indicated that this fault is the one with extremely slow deformation rate. Suspecting that whether this conclusion holds for the real situation at depth because geological estimation deals with a very long time scale, and GPS measurement deals with the horizontal deformation at the surface of the Earth only within the recent decade, we tried to take the approach similar to Kostrov method to investigate the deep deformation using data of microseismicity since the recent 3 decades. We used local earthquake catalogues complete clown to ME 2. 5 to calculate the Benioff strain along the Longmenshan fault and the distribution of a-value in the Gutenberg-Richter frequency-magnitude relation. We considered the nearby Xianshuihe, Anninghe, and Zemuhe fault as a comparison. What is observed is that at the time scale different from geological process and GPS-observed deformation, for the depths different from the surface of ground, considering both horizontal and vertical deformation, and comparing with its neighboring fault systems, Longmenshan fault is indeed not as＇quiet＇ as traditionally assessed. At shorter time scale, some anomalous pattern of seismicity can be observed retrospectively before the Wenchuan earthquake, but they are hard to use as an indication of the approaching of this catastrophic event. Implication of the observations to the study of the Wenchuan earthquake is discussed.