[S-SS11] Strong Ground Motion and Earthquake Disaster

[SSS11-20] Azimuthal Characteristics of Source Radiations from Small Events in Ground Motion Spectral Models

*Tomonori Ikeura¹ (1.Kajima Technical Research Institute)

Keywords:Anisotropic Source Radiation, Spectral Inversion Analysis, Ground Motion Model

In study on earthquake hazards and strong ground motion prediction related to input earthquake motion, reducing the variance of empirical ground motion models (GMMs) is an important issue. In research on empirical models of earthquake motion strength, a simplification of isotropic radiation has been adopted for source characteristics. However, this simplification is not theoretically valid, as radiation pattern effects due to focal mechanisms and directivity effect due to moving source are widely known, and there is a possibility that it will hinder the improvement of evaluation accuracy. In this study, as an observation of the anisotropy of source radiation in actual small earthquakes, we took the main shock (M5.3) of the earthquake on 08/09/2024 in western Kanagawa Prefecture, where the condition for the distribution of observation sites are relatively good, and aftershock 1: 08/14 (M4.2), and aftershock 2: 08/15 (M4.3), and investigated the anisotropy of source radiation based on data from 60 surrounding K-NET and KiK-net sites. I focused on the fact that observed spectral ratio between earthquakes at the same site represents the ratio of source radiation characteristics, and used a regression analysis of the observed spectral ratio to determine the relative radiation intensity for each direction for each earthquake. According to the directions with particularly strong radiation in the azimuth distribution of relative radiation intensity obtained for eight directions, the main shock was stronger in the west at low frequencies and in the northwest and south at high frequencies, while aftershock 1 was stronger in the east, with a factor of about 1.5 times. On the other hand, aftershock 2 was slightly stronger in the east to southeast, but did not exhibit the same strong anisotropy as the main shock or aftershock 1. The fact that such complex anisotropy was observed for each earthquake and frequency band even in a study of only three events of magnitude 4 to 5 suggests that this is not an uncommon phenomenon. Considering error propagation in Generalized Inversion Technique, in order to further reduce the variance of GMM in the future, it is necessary to consider anisotropy of source radiation even for small earthquakes.