Study on effective methods of combined analysis of microseisms and gravity data to reveal bedrock structure

本研究的目的是发展一种合适的方法来阐明三维基底结构的沉积盆地与厚的土壤沉积，利用微震和重力异常观测资料，旨在小区划的城市地区，预防和减轻地震灾害。研究基岩结构的基本思路是：（1）利用阵列观测的微地震，从瑞利波频散估计速度结构;（2）利用水平-垂直频谱比（H/V）分析，估计不同地点基岩的深度。在盆地内，在多个站点进行了阵列观测，并使用盆地内多个站点的H/V数据对所得的速度结构进行了三维模型插值。对于台阵分析，我们开发了一种仅使用两套地震仪的简单技术，2sSPAC方法。布格重力异常提供了密度分布。这样，我们就可以讨论不同物理参数，即地震波阻抗和密度的结构模型，得到物理上可靠的结构模型。在本研究项目中，我们研究了遭受地震严重破坏的各种构造盆地：京都盆地、近江盆地、广岛平原、由美滨半岛、鸟取平原、奈良盆地、中国云南丽江盆地和土耳其阿达帕扎里。结果表明，严重破坏集中发生在基岩形态较陡的区域。除奈良盆地外，所有分析的盆地都是如此，因为没有关于奈良盆地损害分布的信息。总之，我们的方法被认为是有用的微分区沉积盆地。

The object of this research is to develop a suitable method to elucidate 3D basement structure of a sedimentary basin with thick soil deposits, using observational data of microseisms and gravity anomaly, aiming at microzonation of urban area for prevention and mitigation of earthquake disaster. Basic idea for studying the configuration of bedrock is (1) estimation of velocity structure from the dispersion of Rayleigh waves with array observation of microseisms, and (2) estimation of depth to bedrock at various sites with horizontal-to-vertical spectral ratio (H/V) analysis. In the basin, array observations are made at several sites, and the resultant velocity structures are interpolated for 3D model using H/V data at many sites in the basin. For array analysis, we have developed a simple technique using only 2 set of seismometers, 2sSPAC method. Bouguer gravity anomaly provides distribution of densities. Thus, we can discuss the structure model with different physical parameters, namely, seismic impedance and density, and can obtain physically reliable structural model. In this research project, we studied various structural basin which had been seriously damaged by earthquakes: Kyoto basin, Ohmi basin, Hiroshima Plain, Yumigahama Peninsula, Tottori Plain, Nara basin, Lijiang basin in Yunnan, China, and Adapazari, Turkey. As a result, it is pointed out that, concentration of severe damage took place in the region where the configuration of bedrock is steep. This is true for all the basins analyzed but Nara basin, because there is no information on damage distribution in Nara basin. In conclusion, our method is considered useful for microzoning sedimentary basin.