摘要
地壳低速带是岩石圈中的力学软弱带,对地震的孕育有重要作用。人工探测和天然地震层析成像资料均表明,华北地区在不同深度上均存在低速带。为进一步对华北地区地壳低速带的特性进行约束,我们分析了在唐山地区进行大容量气枪震源探测所获得的高分辨率观测资料。分析中发现,该地区的地震记录具有如下主要特征:①直达波Pg视速度为6km/s,可以追踪到约70km;②Moho面反射波PmP从50km开始就以强势震相出现;③PmP震相在150km处比Pg震相约滞后2s。本文利用合成地震图,通过波形正演对观测数据进行拟合,结果表明,合成地震图不仅可以解释各种震相的走时,同时也能较好地反映PmP和Pg的相对振幅。模型结果显示:①该地区平均地壳厚度为33km;②在该地区上下地壳各存在一个低速层。通过分析热流和大地电磁测深资料,认为该地区上/下低速层可能分别是流体和热作用的结果。
The low velocity zone is a mechanical weak band in the crust,which plays an important role in breeding of earthquakes.Artifical detection and nature earthquake tomography both show that low velocity layers exist in different depths in North China.We analyzed the high-resolution data detected by large volume airgun source in North China in order to put further constraint on the properties of the low velocity layers there.We found the following main characteristics in airgun data:firstly,apparent velocity of the direct P-wave's is approximately 6km/s,which could be traced upto nearly 70km;secondly,the PmP phase which reflected from Moho begin to appear as strong phases from 50km;thirdly,the PmP phase at 150km is 2s lag of the Pg phase.All these features show that the low velocity layer exist in the crust in this area.We use synthetic seismograms to forward modeling waveform to fit the observed data.The results showed that synthetic seismograms can not only explain the travel time of various seismic phases,but also obtain the amplitude ratio of PmP and Pg.The results of our modeling include:① the average thickness of crust in this area is 33km;② there exists a low velocity layer in both the upper and lower crust in this region.By comparing the heat and magnetotelluric data,we conclude that the low velocity layers in the upper and lower crusts were caused by fluid and thermal effect respectively.