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

本研究的目的是针对城市防震减灾的小区划，利用微震和重力异常的观测资料，研究一种适用于厚土层沉积盆地三维基底结构的解释方法。研究基岩形态的基本思路是：(1)用微震台阵观测瑞雷波的频散来估计速度结构；(2)用水平/垂直频谱比(H/V)分析估计不同场地的基岩深度。在盆地中，在多个站点进行了阵列观测，并利用盆地内多个站点的H/V数据对合成的速度结构进行了内插以建立三维模型。对于阵列分析，我们发展了一种仅用两台地震仪的简单技术，即2sSPAC方法。布格重力异常提供了密度分布。这样，我们就可以讨论具有不同物理参数的结构模型，即地震阻抗和密度，并可以得到物理上可靠的结构模型。在本研究项目中，我们研究了地震破坏严重的各种构造盆地：京都盆地、大美盆地、广岛平原、横滨半岛、鸟取平原、奈良盆地、云南丽江盆地、中国和土耳其的阿塔扎里。结果表明，基岩形态陡峭的区域是破坏最严重的区域。除了奈良盆地外，所有分析的盆地都是如此，因为奈良盆地没有关于损害分布的信息。综上所述，我们的方法对于沉积盆地的微区划分是有用的。

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.