地震放大效应与地下地质结构——龙门山山前玉皇观区域观测数据分析

通过布置于龙门山断裂带中段、龙门山山前玉皇观区域的地震观测台站阵列接收地震数据,研究该区域的地震动放大效应和地下地质结构.观测阵列共10台宽频带地震仪,分布在玉皇观河口冲积扇区域.分别采用参考场址谱比法(RSSR)和HV谱比法(HVSR)计算64个高信噪比近震数据的振幅谱比函数,结果显示在玉皇观区域具有较明显的地震动放大效应,并且局部场址效应显著.以S06场址为例,建立近地表地震地质模型,通过SH波放大效应正演模拟研究该场址的地震动放大模式.RSSR与HVSR的结果表明,两者所计算的场址放大效应主频一致,但是HVSR的放大峰值却比RSSR的放大峰值大一倍左右,表明HVSR的结果可能包含了波场在近地表低速层之下传播路径的改造作用.另外,采用27个远震P波的接收函数计算了该区域地壳上地幔S波速度结构.接收函数研究结果显示玉皇观地区的莫霍面深度为44 km,沉积盖层、结晶地壳和上地幔的S波速度分别为2.5 km·s~(-1)、3.5 km·s~(-1)和4.5 km·s~(-1).观测阵列台站之间的接收函数反演结果一致性较好,说明本研究区域范围内地形地貌等近地表结构因素的相对变化对接收函数的影响不大.

Seismic amplification effect and geological structure:observation and analysis of seismic waves for the Yuhuangguan area in front of the Longmenshan

Using earthquake data recorded by a seismic array deployed in the Yuhuangguan area, which is located at the middle section of the Longmenshan fault in front of the range, we have studied the seismic amplification effect and the underground geological structures of this area. Totally 10 seismic observation stations were deployed in the Yuhuangguan delta area. The reference site spectral ratio method (RSSR) and horizontal to vertical spectral ratio method (HVSR) were used separately to calculate the amplitude spectral ratios of the seismic data of 64 local-seismic events with high SNR. The results show significant ground motion amplification in the Yuhuangguan delta area. A near-surface seismic model was built for site S06 to analyze seismic amplification using SH wave forward modeling. Peak frequencies calculated by RSSR and HVSR are consistent, but the values of peak seismic amplification from HVSR is twice of that from RSSR. This may reflect the additional passage change of the waves in the deep path below the local sediment. In addition, the shear wave velocity structure of the crust and upper mantle beneath Yuhuanguan was calculated by using P wave receiver function of 27 teleseismic events. The results show that the depth of Moho beneath Yuhuangguan is 44 km, and the shear wave velocity in the sedimentary layers, crystalline crust and upper mantle are 2.5 km·s^-1, 3.5 km·s^-1 and 4.5 km·s^-1, respectively. The high consistency of receiver function inversion results at all stations indicates that the receiver function method mainly reflects the deep structure, while the influence of the near surface structure, such as the topography and geomorphology within the research area, on the receiver function is relatively little.