17:15 〜 19:15
[SSS12-P17] Systematic Differences Between Moment Magnitude and JMA Magnitude Beneath Choshi City in Eastern Japan
キーワード：moment magnitude、JMA magnitude
The magnitude of earthquakes is one of the most fundamental parameters in seismology and is included in most earthquake catalogs. This parameter is crucial for understanding seismicity characteristics, such as the b-value in the Gutenberg-Richter relation [Gutenberg and Richter, 1944], and for evaluating interplate coupling through repeating earthquake analysis [e.g., Igarashi, 2020]. However, since methods for estimating magnitudes vary across catalogs, it is essential to understand their differences and how they depend on factors such as the size, location, and time.
The National Research Institute for Earth Science and Disaster Resilience (NIED) has been conducting moment tensor (MT) analysis of earthquakes in and around Japan since 1997 [Fukuyama et al., 1998]. The NIED provides an MT catalog (F-net MT catalog) of earthquakes with a Japan Meteorological Agency (JMA) magnitude (MJ) of 3.5 or larger, resulting in a catalog of over 45,000 earthquakes as of December 2024. The MT inversion of observed waveforms with periods longer than 20 sec provides the moment magnitude (Mw) along with the MT, and centroid depth while the horizontal location is fixed at the epicenter estimated by JMA. In contrast, MJ is estimated based on the maximum amplitude of observed ground displacement [Katsumata, 2004] and velocity [Funasaki, 2004]. When comparing Mw and MJ, certain regions exhibit relatively smaller MJ values compared to Mw. Note that we analyzed earthquakes occurring after September 25, 2003, when JMA revised its magnitude estimation method. In addition, from the F-net MT catalog, we extracted high-quality MT solutions with variance reduction exceeding 70% and at least three stations used in the MT analysis.
In this study, we focus on the seismicity beneath Choshi City in Eastern Japan, a region characterized by abundance of data from a relatively large number of earthquakes and surrounding observatories. In this region, earthquakes with significantly negative MJ-Mw values are located at depths of 40-60 km. The number of earthquakes in this depth range is 154, and MJ-Mw value are systematically distributed around -0.40 ± 0.56, whereas the nationwide distribution is approximately 0.01 ± 0.53. This discrepancy is more pronounced for moderate-sized earthquakes with Mw less than 5.0. The difference between Mw and an alternative magnitude estimated by the NIED following Watanabe [1971] exhibits a similar trend, although the magnitude differences are smaller than those of MJ-Mw.
The F-net MT catalog indicates that the earthquakes located in this region and at depths of 40-60 km predominantly exhibit thrust faulting with an east-west compression axis. Taking the focal mechanisms and locations into account, most of the earthquakes are interpreted to have occurred at the plate boundary between the Pacific and Philippine Sea plates. To further investigate the observed magnitude discrepancies, we plan to analyze waveform characteristics, which may provide insights into the underlying causes of these variations, such as source and path effects.
The National Research Institute for Earth Science and Disaster Resilience (NIED) has been conducting moment tensor (MT) analysis of earthquakes in and around Japan since 1997 [Fukuyama et al., 1998]. The NIED provides an MT catalog (F-net MT catalog) of earthquakes with a Japan Meteorological Agency (JMA) magnitude (MJ) of 3.5 or larger, resulting in a catalog of over 45,000 earthquakes as of December 2024. The MT inversion of observed waveforms with periods longer than 20 sec provides the moment magnitude (Mw) along with the MT, and centroid depth while the horizontal location is fixed at the epicenter estimated by JMA. In contrast, MJ is estimated based on the maximum amplitude of observed ground displacement [Katsumata, 2004] and velocity [Funasaki, 2004]. When comparing Mw and MJ, certain regions exhibit relatively smaller MJ values compared to Mw. Note that we analyzed earthquakes occurring after September 25, 2003, when JMA revised its magnitude estimation method. In addition, from the F-net MT catalog, we extracted high-quality MT solutions with variance reduction exceeding 70% and at least three stations used in the MT analysis.
In this study, we focus on the seismicity beneath Choshi City in Eastern Japan, a region characterized by abundance of data from a relatively large number of earthquakes and surrounding observatories. In this region, earthquakes with significantly negative MJ-Mw values are located at depths of 40-60 km. The number of earthquakes in this depth range is 154, and MJ-Mw value are systematically distributed around -0.40 ± 0.56, whereas the nationwide distribution is approximately 0.01 ± 0.53. This discrepancy is more pronounced for moderate-sized earthquakes with Mw less than 5.0. The difference between Mw and an alternative magnitude estimated by the NIED following Watanabe [1971] exhibits a similar trend, although the magnitude differences are smaller than those of MJ-Mw.
The F-net MT catalog indicates that the earthquakes located in this region and at depths of 40-60 km predominantly exhibit thrust faulting with an east-west compression axis. Taking the focal mechanisms and locations into account, most of the earthquakes are interpreted to have occurred at the plate boundary between the Pacific and Philippine Sea plates. To further investigate the observed magnitude discrepancies, we plan to analyze waveform characteristics, which may provide insights into the underlying causes of these variations, such as source and path effects.