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

2003年十胜冲大地震是日本部署K-NET和KiK-NET等密集强震台网以来发生的第一次M8级地震。我们研究的速度响应的空间分布（阻尼因子h=0.05）与不同的自然周期的地震，使用密集的强震台网数据。短周期（T= 0.1秒; T=自然周期）速度响应分布显示了俯冲带地震的典型特征，日本北部太平洋沿岸沿着观测到了高响应值。这一特征可以用日本北部的横波衰减结构来解释，即路径效应。另一方面，长周期（T= 20秒）的速度响应分布具有特殊的功能：：在可比距离上，震中北方的响应值高于西南部。这一特性主要是由长周期瑞利波的辐射方向图和震源效应来解释的。一个显著的例子是在具有厚沉积层的大平原上激发长周期盆地面波。另一个值得注意的是非线性场地反应。我们发现轻度的非线性现场响应在位于洪泛平原沉积物的软土场地，通过比较弱和强运动的传递函数，使用井下阵列数据的非线性确认。在最大的余震期间，在位于填海土地上的不那么软的土壤场地也观察到了独特的非线性;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.