Relocation Of Earthquakes Before And After Wenchuan Ms8.0Earthquake And The Study Of Focal Mechanism

We re-located M>2.0earthquakes which have reliable seismic phase betweenJanuary1999and April2011around Longmenshan area by use of the Hypo2000.Re-locating study area for29o~34o,N100o~106oE. the seismic observation report in the9years before and in the3years after the Wenchuan earthquakes sorting and filtering,considering the difference of velocity structure in both sides of Longmenshan faultzone, it used the uniform velocity model from Jie-Shou Zhu and got7454;before theearthquake and26278after the earthquake;It solved for focal mechanisms of M>3.5before and after the Wenchuan earthquakes using initial P waveforms in theLongmenshan area, got30before the earthquake and114after the earthquake.Itanalyzed the relationship of seismicity and active faults.According joint analyses with epicenter distribution figure, M-T figure andvertical profiles, before the Wenchuan earthquake, seismicity is much stronger activityin the middle and south part of Longmenshan area than in the north. Especially,in thenorthern rarely earthquake occurred; the aftershocks extended along the direction ofthe Longmen Shan fault zone was north-south segmented distribution. There was aNorth West aftershocks belt near the main shock. Around the Qingchuan,aftershockswas intensive,and not matched the history. Before the earthquake focal mechanismwas mainly dominated by reverse fault, a small amount of strike-slip and normal faultsoccurred.In the southern of aftershocks belt the stress direction was NE, and in thenorth west of Yingxiu-Li County was NE and NW,It was main strike-slip and reversefault. Strike-slip ratio increased and reverse fault declined in the later time;In themiddle of aftershocks belt the main stress direction was NE, the time–varying of thetype of mechanism was not strong;In the northern of aftershocks belt the main stressdirection was NE. The reverse fault ratio went up from early to late.