高频GPS和强震仪数据在汶川地震参数快速确定中的初步应用

Rapid determination of source parameters for the 2008 Wenchuan earthquake constrained by high-rate GPS and strong motion data

高频GPS可以实时获取地表位移数据,在地震学中有十分重要的现实应用,比如快速获取震中、震级、地震烈度甚至震源破裂过程.本文以汶川地震为例,首先利用近场7个GPS台站数据反演震中位置,由于高频GPS和测震学确认的震相不一致,两种震中结果相距约15.7km.然后对高频GPS和强震动数据进行了比较分析,我们的统计结果表明,尽管由于工作原理不同,高频GPS数据中的地震动峰值与强震记录相比存在明显差异,但是高频GPS记录的PGA、PGV和PGD同样可以作为计算地震烈度的指标.进而,使用SMBLOC程序对强震记录进行事后的基线偏移校正,得到与实时高频GPS精度相当的地表位移序列.最后,采用移动平均窗口对这些位移数据作平滑,基于最速下降法和OKADA模型,对汶川地震断层破裂的过程进行了回溯性准实时反演.结果表明,汶川地震主断层由西南向东北方向破裂,以14∶28∶04为基准,在震后20s提供初始震级MW7.0,震后70s震级稳定在MW7.8,但断层仍在破裂,在震后159s根据位移波形判断事件基本结束.研究表明,实时地表位移数据可以快速准确获取强震震级和破裂方向,从而使得高频GPS将对现有地震预警系统提供很好的补充.

High-rate GPS facilities can detect real-time surface displacement,which has many seismic applications,such as rapid determination of the epicenter locations,earthquake magnitude and intensity,and even earthquake rupture process.In this study,taking the 2008 Wenchuanearthquake as an example,we use data from 7 GPS stations to calculate the epicenter location.We find that the two epicenters determined by high-rate GPS and seismic data are ~15.7 km apart from each other,due to the difference between GPS and seismic waveforms.Comparing the highrate GPS and accelerograms statistically shows that although GPS data and strong motion record have their own physical meanings,respectively and the peak ground motion recorded by two datasets are obviously different,the PGA,PGV and PGD in GPS data can also be used to calculate seismic intensity.In addition,we correct the baseline bias using the SMBLOC program to obtain surface displacement time series,which can be deemed as equivalent to real-time high-rate GPS data.Based on Okada dislocation and a finite fault segment model,we finally invert the static offset displacement data smoothed from the corrected and integrated strong motion displacement data by a moving window.Our inversion results show that the main seismogenic fault of the Wenchuan earthquake ruptured from southwest to northeast,which is consistent with most of other studies.Furthermore,we try to conduct it in a quasi-real-time way and to rapidly determine the earthquake rupture process of the Wenchuan earthquake.Thus,our result also shows that the first estimated magnitude can be MW7.0 as fast as 20 seconds after the origin time,that is 14∶28∶04.Besides,70 seconds after the origin time the magnitude estimation of MW7.8 is stable,with the condition that the fault rupture is still going.At last,159 seconds after the origin time,the Wenchuan event stops according to the displacement time series.The results prove that real-time surface displacement can be used to determine the magnitude and rupture direction of a major earthquake quickly and accurately.High-rate GPS will provide a good complement to the existing earthquake early warning system.