Estimates of source parameters for the Mw7.9 Wenchuan earthquake of 12 May 2008 from near-field seismograms

There are many important near-field accelerograms for wenchuan earthquake reacorded at short distances from the fault traces. We have processed and investigated most of the recorded near-field accelerograms produced by the main shock of the Mw 7.9 Wenchuan earthquake of 12 May 2008. According to our preliminary study, most of the displacements and velocity derived by direct and simple integration from of the accelerograms show significant drifts. After a simple baseline correction processing, some of the results still have certain drift, which maybe caused by the complex geological structure and site condition. Applying Brune’s model, we have determined important source parameters, such as rise time, stress drop, static offset and particle velocity from the processed and corrected records (displacements and velocities). The results of rise time show obvious directivity effect. Some static offset derived from the accelerograms are less than the coseismic offset from GPS data. 48 aftershocks with magnitude more than M4.0 have occurred in 12 May , the offset from GPS data maybe caused by mainshock and aftershocks together, other than only by the mainshock. Because of the complex geological structure in Wenchuan area, we use some slip distribution model inversed by others from far-field seismogram to investigate how the structure influence the seismograms.