Abstract:An earthquake with a magnitude（ML） of 2.1 occurred at 16：40 UTC,October 12,2011,in the Shijingshan district in Beijing with an epicenter at 39.95° N,116.16° E.Eleven near-field strong motion stations,located less than 20 km from the epicenter,clearly recorded temporal variation of acceleration during the earthquake.The largest value of peak ground acceleration in the east-west（EW） and north-south（NS） components,22.528 gal and 30.989 gal,respectively,were both recorded by station LSG,which is situated approximately 5.6 km from epicenter.The largest value of peak ground acceleration in the vertical component was 12.797 gal,recorded by station CCS at an epicenter distance of 2.2 km.We synthesized the horizontal component by EW and NS records and used Generic Mapping Tools（GMT） software to mesh and illustrate the influence of horizontal and vertical acceleration on the field contour map.The results show that generally,the value of the NS component is larger than that of the EW component,and the horizontal component is larger than the vertical.Moreover,the acceleration distribution and influence field contour map of the horizontal component shows that peak ground acceleration in the southern area is a low value and that in the northern area is high.In addition,the two regions are divided by a transition zone that strikes near east-west（EW） or NEE.We obtained the average peak ground value of three components at 1 s intervals from station records after the earthquake occurred,which we synthesized into horizontal and vertical components to illustrate temporal variations in the map produced by GMT.After analyzing the average value records and temporal variation map,we detected that the decay in the NS direction is faster than that in the EW direction and that decay is faster in the vertical component than that in the horizontal component.Excepting the near-field strong motion stations,23 Capital Circle broadband digital seismic stations recorded digital waveform data during the earthquake.We interpreted initial P-wave polarity from these records and employed grid-point test method to obtain the Focal Mechanism Solution of the event.Results indicate normal faulting dominant in the Beijing-Tianjin area.The best double couple solution of the event is 295°,39° and-74° for strike,dip and rake angel,respectively;those of the other nodal plane are 96°,53°,and-102° with a 13% inconsistency ratio.By combining these results with the acceleration analysis results,we determined that the strike of the second nodal plane with a larger dip angel is near the EW direction,which is consistent with the strike of acceleration transition zone.In addition,the microepicenter of the event does not coincide with the macroepicenter.These findings indicate that the nodal plane of strike 96°,dip 53°,and rake-102°may coincide with the seismogenic fault.Although the work in this study is a preliminary analysis for acceleration records and seismic digital waveform data,the results can provide a reference for quick determination of earthquake influence field and hazard assessment following a large earthquake and are highly significant for earthquake emergency response and disaster relief.
