利用PGA快速确定汶川地震破裂特征

RAPID INVERSION OF FAULT RUPTURE CHARACTERISTICS OF WENCHUAN EARTHQUAKE WITH PGA RECORDS

快速确定断层破裂特征是烈度速报的一项重要技术,断层破裂特征可为烈度速报提供震源模型,提升烈度速报准确性。通过汶川地震加速度记录,提出一种快速计算震源破裂参数的方法。假定断层为线源模型,以一定间距将断层离散化为若干子源,以震中为不动点,通过旋转获得所有断层可能的走向,通过每次移动一个子源,获得断层所有可能的破裂方式,将二者结合即可给出断层所有可能的空间分布;计算每种断层空间分布与每个台站的断层距,利用加速度记录峰值和断层距统计回归衰减关系,分析每个衰减关系的拟合残差,残差最小拟合效果最好的衰减关系所对应断层参数,能够对该次地震的地震动场有最合理的解释,也最有可能是实际地震中的断层空间分布。

A magnitude 8.0 earthquake occurred in Wenchuan, Sichuan Province of China at 14：28 on May 12, 2008, with the epicenter at north latitude 31 degrees, east longitude 103.4 degrees, and the focal depth of 14km. Result of focal mechanism was given by a number of research institutes and academics, from a few hours to several days, or even months after the earthquake. Such the situation determines that some of the methods cannot be used to earthquake emergency and rescue services, or applicable to rapid reporting of seismic intensity. A lot of strong motion acceleration records are accumulated in Wenchuan earthquake. This paper puts forward a new method which uses adequate Wenchuan earthquake acceleration records and can be applied in the future to the intensity assessment in fast report to quickly estimate the fault strike, rupture mode and rupture length. Assuming the seismic source is a line source model, the fault is discretized into sub-sources with 10km each, the length of the initial fault rupture is determined using statistical relations; taking the initial rupture point as a fixed point, and for a fault of arbitrarily rupture direction, by moving sub- source at one side to the source at the other side, all possible rupture modes of fault along certain rupture direction can be obtained; Using the initial rupture point as the center and rotating the fault of any rupture modes, all possible rupture directions are acquired, then, combining the two together gets all the possibilities of rupture directions and modes of fault. The real earthquake must be one of the circumstances. Fault distance between each of the possible fault distributions and stations is calculated, and statistical regression method is used to analyze the peak ground acceleration attenuation relationships, then, variance analysis is conducted on the fitting results. The rupture direction and mode corresponding to the minimum variance of the attenuation law can most reasonably explain the spatial distribution of fault in the ground motion field, which, in another word, are the real rupture direction and mode of fault in the actual earthquake. The time it takes to fix intensity for quick report is less than 20 seconds by two-parameter method and within 2 minutes by three-parameter method, which can meet the time requirement for quick report. The calculation result is accurate, so this method is feasible.