基于复合方法的天水盆地宽频带地震动模拟

本文采用有限差分和随机振动合成结合的复合方法,模拟了当礼县—罗家堡断裂发生矩震级Mw7.7级大地震时,在天水盆地产生的宽频带地震动场,分析了在设定地震条件下盆地内的地震动分布特征,为该区黄土地震滑坡分析提供了地震动参数结果。结果显示:(1)有限差分法和随机振动合成法可以很好地互补,得到盆地内地表宽频带地震动;(2)地震在盆地区域产生了强烈地震动,PGA(峰值加速度)介于150~900 gal,离断层较近的区域东南角的PGA最大,随着断层距的增加,PGA逐渐减小。河谷南侧的PGA值相比北侧较大,具备诱发滑坡的强大动力条件;(3)盆地区域PGV(峰值速度)最大为120 cm/s。受第四系覆盖层放大效应和地形放大效应共同影响,水平向地震动在盆地区域东侧和中部具有较大PGV,而西侧PGV相对较小。竖向地震动在盆地区域东侧较弱,而在中部和西侧较强,特别是最西侧陡峭的山坡上,PGV达到了最大值。此外,竖向地震动明显受到覆盖层厚度的影响,譬如在盆地区域南侧的中间部位,也具有较大的PGV。

BROADBAND GROUND MOTION SIMULATION IN TIANSHUI BASIN BASED ON A HYBRID METHOD

Taking Lixian-Luojiabao fault as a causative fault of an Mw 7.7 scenario earthquake, a hybrid numerical method combining finite difference method with random vibration synthesis method is adopted to simulate broadband ground motion field in Tianshui basin, and the ground motion characteristics in the basin are analyzed, providing ground motion parameters for analyzing seismic loess landslides. The following conclusions are acquired:(1)Finite difference method and random vibration synthesis method are complementary to get broadband ground motion in the basin. (2)The scenario earthquake causes intensive ground motion in the basin, with PGA (peak ground acceleration) of 150~900 gal. The largest PGA is located in the southeastern corner of the basin where is nearest to causative fault, and it decreases with the distance from causative fault. Compared to the north side of river valley, the south side has larger PGA to induce landslides. (3)The largest PGV is 120cm/s in the basin. Affected by Quaternary covering layer amplification effect and terrain amplification effect, PGV in the east and the central parts of the basin are larger than those in the west part. (4)The vertical ground motion shows smaller PGV in the east part of the basin but larger ones in the west and central parts. Especially at the western most steep mountains, PGV reaches the maximum. In addition, the vertical ground motion can also reflect the influence of loess layer, for example, the PGV is also larger in the central part of the south side of the basin.