5:15 PM - 6:45 PM
[SSS08-P02] Deviation of station magnitude in northeastern Japan
Keywords:Local Magnitude
Local magnitude is typically estimated based on seismic wave amplitudes. To estimate local magnitude, the displacement or velocity amplitudes of an earthquake are first used for estimating the station magnitude. The magnitude of earthquake is then obtained by averaging these station magnitudes. This scheme, however, may introduce to an estimation bias if observed amplitudes are affected by local effects such as site amplification. This bias would be more significant as smaller magnitude because the smaller number of stations are used for magnitude estimation. In this study, we investigated the deviations between the station magnitudes and their average magnitudes, hereinafter referred to as station magnitude deviations, by estimating the observed station magnitudes from the velocity amplitudes of seismic waves observed in and near northeastern Japan.
First, we estimated the station magnitude based on the observed seismic wave amplitudes using cataloged values provided by the Japan Meteorological Agency. We analyzed earthquakes with original JMA magnitudes equal or less than 4.0 and depths equal or less than 50 km occurred in and around the Tohoku region between 2000 and 2023. We adopted the velocity magnitude formula proposed by Watanabe (1971) to estimate the station magnitude. Then, the station magnitudes for an earthquake were averaged over all stations to obtain the average magnitude. Finally, the station magnitude deviation was estimated by subtracting the average magnitude from the station magnitude.
We observed systematic variations in the station magnitude deviations, with some stations exhibiting consistently larger or smaller deviations. This observation suggests that the station magnitude deviation is site-specific; however, the correlation between these deviations and values that characterize site amplification in strong ground motion such as AVS30 and ARV, was generally low. Additionally, other indices including microtopographic classification, relation to the locations of active volcanoes, average epicentral distances used to our estimation, and borehole depths of Hi-net stations, did not show clear correlation with the estimated station magnitude deviation. We also note that the distribution of estimated station magnitude deviation at some stations was deviated from a normal distribution. The causes of these deviations remain unclear. Further study, including direct estimation of site amplification factor based on the strong motion records, would be necessary to understand the cause of the station magnitude deviations.
First, we estimated the station magnitude based on the observed seismic wave amplitudes using cataloged values provided by the Japan Meteorological Agency. We analyzed earthquakes with original JMA magnitudes equal or less than 4.0 and depths equal or less than 50 km occurred in and around the Tohoku region between 2000 and 2023. We adopted the velocity magnitude formula proposed by Watanabe (1971) to estimate the station magnitude. Then, the station magnitudes for an earthquake were averaged over all stations to obtain the average magnitude. Finally, the station magnitude deviation was estimated by subtracting the average magnitude from the station magnitude.
We observed systematic variations in the station magnitude deviations, with some stations exhibiting consistently larger or smaller deviations. This observation suggests that the station magnitude deviation is site-specific; however, the correlation between these deviations and values that characterize site amplification in strong ground motion such as AVS30 and ARV, was generally low. Additionally, other indices including microtopographic classification, relation to the locations of active volcanoes, average epicentral distances used to our estimation, and borehole depths of Hi-net stations, did not show clear correlation with the estimated station magnitude deviation. We also note that the distribution of estimated station magnitude deviation at some stations was deviated from a normal distribution. The causes of these deviations remain unclear. Further study, including direct estimation of site amplification factor based on the strong motion records, would be necessary to understand the cause of the station magnitude deviations.