2020 Volume 20 Issue 7 Pages 7_133-7_157
A new method for investigating the attenuation characteristics of strong ground motions is proposed. In the method, bedrock motions (BRM), which are obtained by dividing observed motions (OBS) by relative site factors (RSF), are examined in the study area. The RSF are evaluated as the ratios of site factors relative to a reference bedrock site by a regression analysis to satisfy all relative site factors of neighborhood site pairs involved in the area. This method was used to examine the attenuation characteristics of strong motions observed at K-NET and KiK-net sites in the Tohoku and Kanto regions caused by four plate boundary earthquakes that occurred in the Pacific subduction area. The attenuation distributions of BRM amplitudes obtained by this method for these earthquakes are characterized by smaller variations than those of OBS. A comparison of attenuation models evaluated by regression analyses of BRM and OBS using a polynomial formula of the logarithm of source distance shows that the difference of variations between the BRM and OBS attenuation models is consistent with the variations of the RSF. This indicates that the variations of attenuation model of BRM should be interpreted as variations of source radiation characteristics and/or wave propagation path characteristics.
