Investigation for the mechanism of the seismic damage band on Nobi earthquake

本文对1891年野比地震中观测到的地震损伤带的形成机制进行了研究。我们分析了野比平原地下结构的大量资料，并将其与爱知县的深部结构调查结果进行了汇总。它们被用来创建Nobi平原的三维深层地下结构。在野比平原边缘的约罗断层附近部署了一个地震阵列。该阵列的记录表明，在基岩边缘观测到表面波的激发，并在基岩边缘附近几公里处与直波发生干扰，导致地震地面运动放大。对Nobi平原的二维结构进行有限元分析，也观察到相同的放大效应。这种放大可能造成地震损伤带。研究了久府-一宫线的来源模型。能解释野比地震观测到的水平变化的模型是倾角为75度的断层，而不是目前所考虑的垂直断层。然而，根据余震震源机制估计的应力场没有显示出久府-一宫线发生的证据。对灾区进行了重新调查，发现Nobi平原的重灾区不是带状的，而是大面积的。结果表明，破坏是由表层土结构的特性引起的。

We investigated the mechanism of the seismic damage band that have been observed on the 1891 Nobi earthquake. We have obtained following resuts :We have analyzed many data of underground structure of Nobi plain, and compiled them with the results of investigation of deep structures by Aichi prefecture. They are used to create 3D deep underground structure of Nobi plain.A seismic array is deployed near Yoro fault that places on the edge of Nobi plain. The records obtained by this array show that surface waves that would be excited on the edge of basement rock are observed, and interfered with direct wave around few kilometers apart from the edge, resulting amplification of seismic ground motion. Also, the same amplification is observed by FEM analysis using 2D structure of Nobi Plain. This amplification may cause the seismic damage band.The source model of Gifu-Ichinomiya line is investigated. The model that can explain the level cahange observed on Nobi earthquake is the fault with dip angle of 75 degree, rather than vertical fault that have been considered so far. The stress field estimated from source mechanism of aftershocks, however, shows no evidence for the occurrence of Gifu-Ichinomiya line.The area of disasters are re-surveyed and the highly damaged region is not band-shaped but wide area in Nobi plain. This result shows the damage is due to the characteristics of surface soil structures.