近场强地震动模拟中对破裂的方向性效应和上盘效应的表达

RUPTURE DIRECTIVITY AND HANGING WALL EFFECT IN NEAR FIELD STRONG GROUND MOTION SIMULATION

为了模拟近场强地震动,采用了基于有限断层模型的一种随机合成方法。震源破裂面被剖分成一定数量的子源,总的地震矩分成数量更多的子震的矩。一给定子源中一次子震引起的场地地震动的傅氏谱,通过考虑点源的谱、随着距离的衰减、能量耗散及近地表效应等导出。据此幅值谱和一个随机相位谱,并与一个时程包络函数结合,合成一个子震时程。将各子源中各个子震引起的所有时程叠加,得出场地的地震动时程。叠加中,各子震时程之间的时滞,据子震发震时差和子源至场地的距离差别引起的时差确定。对一个设定地震,选用4个断层面倾角,计算了近场21个地点的地震动。结果表明,本文方法可以很好地表达破裂的方向性效应和上盘效应。为了验证方法的可靠性,对1994年美国北岭地震中3个近断层台站MCN,LV3和PCD模拟的地震动与实际记录的加速度反应谱和时程作了比较。

A random synthesis procedure based on finite fault model is adopted for near field strong ground motion simulation, in this paper. The fault plane of the source is divided into a number of sub-sources; the whole moment magnitude is also divided into more sub-events. The Fourier spectrum of ground motion caused by a sub-event in given sub-source, then can be derived by means of taking the point source spectrum, attenuation with distance, energy dissipation, and near surface effect, into account. A time history is synthesized from this amplitude spectrum and a random phase spectrum, combined with an envelope function. The ground motion is worked out by superposition of all time histories from each sub-event in each sub-source, with time lags that depend on the differences between the triggering times of sub-events and distances of the sub-sources. From the example of simulations at 21 near field points in a scenario earthquake with 4 dip angles of the fault plane, it is illustrated that the procedure can describe the rupture directivity and hanging wall effect very well. To validate the procedure, the response spectra and lime histories recorded at three near fault stations MCN, LV3 and PCD during the Northridge earthquake in 1994, are compared with the simulated ones.