A Study on Characteristics of Strong Ground Motions from the 2003 Tokachi-oki Earthquake

2003年的东京纵横地震是日本部署K网、Kik网等密集强震台网以来发生的第一次8级大地震，为Mw8.3级。利用稠密的强震台网资料，研究了该次地震不同自然周期的速度响应(衰减系数h=0.05)的空间分布。俯冲带地震的短周期(T=0.1秒；T=自然周期)速度响应分布具有典型的特征，高响应值出现在日本北部的太平洋沿岸。这一特征可以用日本北部的S波衰减结构来解释，即路径效应。另一方面，长周期(T=20秒)的速度响应分布呈现出一个特殊的特征：在可比距离处，震中北部的响应值高于西南部。这一特殊特征主要由震源效应、长周期瑞利波的辐射图样等解释。我们发现了各种类型的场地效应。一个值得注意的问题是在具有厚沉积层的大平原上激发长周期的盆地面波。另一个值得注意的问题是场地非线性反应。在一个位于洪泛区沉积上的软土地基上，我们发现了轻微的非线性场地响应；通过比较弱运动和强运动的传递函数，利用井下阵列数据证实了非线性的存在。在最大的余震期间，位于填海造地上的一个不那么软的土壤场地也观察到了明显的非线性；加速度图显示出尖峰波形。

The 2003 Tokachi-oki earthquake, a great interplate earthquake (Mw8.3), is the first M8 class earthquake in Japan since the dense strong-motion networks such as the K-NET and KiK-net have been deployed. We examine spatial distributions of velocity responses (damping factor h=0.05) with various natural periods for this earthquake by using the dense strong-motion network data. The short-period (T=0.1sec; T=natural period) velocity response distribution shows a typical feature for subduction-zone earthquakes; the high response values are observed along the Pacific coast of north Japan. This feature is explained by the S-wave attenuation structure beneath north Japan; that is, the path effect. On the other hand, the long-period (T=20sec) velocity response distribution shows a peculiar feature: at comparable distance, the response values in the northern part of the epicenter are higher than those in the south-western part. This peculiar feature is mainly explained by the source effect; the radiation pattern of long-period Rayleigh waves.We find various types of the site effects. A remarkable one is excitation of long-period basin surface-waves at large plains with thick sedimentary layers. The other remarkable one is nonlinear site response. We find mild nonlinear site response at a soft soil site located on flood plain deposits; the nonlinearity is confirmed by comparing the transfer functions for weak and strong motions using the downhole array data. The distinctive nonlinearity is also observed at a not-so-soft soil site located on reclaimed land during the largest aftershock; the accelerogram shows a spiky waveform.