近地表速度结构对场地强地震动特征的影响

Influence of Near-surface Velocity Structure on Site Characteristics of Strong Ground Motion

近地表松散沉积层对地震波具有强烈的改造作用。为了揭示近地表速度结构对场地强地震动特征的影响机理,采用横波勘探和地脉动测试方法研究两个地震台站的场地条件,并对比分析场地测试结果与汶川地震记录的频谱特征。正如浅层地震勘探和地脉动数据所揭示的那样,MXT地震台的强震记录中,对应横波图像0.10s处强反射波的地震动分量（频率5 Hz左右）在该场地占明显优势,而对0.22s的弱反射界面（对应的基频为2.3Hz）的反映并不明显,分析认为这一现象归因于地下10-11m处粉土与卵石层间具有较大的波阻抗差异,因而造成地震反射图像上0.10s附近的强振幅掩盖了0.22s的反射波组,岷县强震记录频谱也显示为单峰值形态。横波勘探在WUD台获得的地震图像显示分别在0.22s和0.50s处有两组比较明显的反射波,其所对应的场地潜在响应频率分别为2.3Hz和1.0Hz,WUD台的强震记录频谱的形态也明显受这两个波阻抗界面的影响;通过对比地脉动与地震记录的频谱图,认为场地结构对地震信号与一般地脉动信号的影响是有差别的。研究表明：（1）剪切波的强阻抗界面深度及其平均波速是控制场地频谱主频的主要因素,近地表地层中阻抗强的界面对地震波的改造作用占主导地位;（2）近地表松散覆盖层的波速结构影响着场地地脉动频谱与地震动频谱的谱形变化;（3）一般情况下,浅部强波阻抗界面对地脉动频谱特征的影响可能大于深部（百米以内）,强震作用下深部波阻抗界面会使场地的响应主频向比其脉动基频更低的频段发展。

Strong ground motion can cause significant changes in unconsolidated layers;therefore,the influence of near surface velocity structure on the characteristics of strong ground motion should be clarified.A combination of S-wave seismic exploration and microtremor testing is conducted near two seismic stations in which a series of aftershock records was recorded during the Wenchuan earthquake sequence.At the MXT station,both field tests and seismic records show a prominent resonance of 5Hz caused by a high impedance contrast at the boundary between silty soil and a pebble bed.This boundary occurs at a depth range from 10 mto 11mand is imaged as a strong reflection with a two-way time approximately 0.1s.Borehole data from this site verifies that the velocity increases from approximately 200 m/s to 400 m/s at a depth of 10 m depth.However,the 20 mdepth boundary shows a contrast between a pebble bed and slate and is imaged as a very slight reflection with less significance in the seismic spectrum.This phenomenon may have occurred because an excessively high amplitude at 0.1scovered the reflection of the deeper boundary.At the WUD station,the SH-wave reflection profiles with two-way travel times of approximately 0.22 sand 0.50 smark the boundaries of silty soil,roundstone,and sandstone.The reflectors respectively caused 2.3 Hz and 1.45 Hz resonances observed in the records of earthquakes and microtremors.Two or more boundaries of impedance contrast could have caused different actions with various focal mechanisms at the WUD station.The results of this study are summarized as follows：（1）A comparison of the test results and the strong ground motion characteristics revealed that the depth of the strong impedance boundary and the average shear wave velocity are the main factors in controlling the dominant frequency at the site;stronger impedance is predominant in transforming the characteristics of the seismic spectrum.（2）Whether microtremors or earthquakes,site spectrum characteristics are always influenced by the surface velocity structure.In particular,the corresponding frequency range of the highest amplitude or amplification is near the dominant frequency.Therefore,the spectral shape of the surface should be affected by the velocity structure of unconsolidated layers.（3）When the microtremor records are controlled by a surface wave,the effect of the shallow impedance boundary may stronger than deep boundary.Moreover,earthquakes can make the main frequency of the seismic response lower than that by microtremors because of the deeper impedance boundary and the nonlinear nature of soil layer.Furthermore,the characteristics of the seismic spectrum also show diversity due to the focal mechanism and the propagation path.