5:15 PM - 7:15 PM
[SSS12-P09] Changes in b-values before and after the 2024 Noto Peninsula Earthquake
Keywords:seismicity, the 2024 Noto Peninsula Earthquake, b-value, earthquake swarm
This study focuses on the changes in b-values in the Noto Peninsula, where seismic activity has become active since 2020. The b-value (i.e., the slope of the frequency-magnitude distribution of earthquakes in a log-linear plot) has been suggested to be negatively correlated with differential stresses based on laboratory studies, numerical modeling, and analysis of natural seismicity. In the case of the 2011 Tohoku-Oki earthquake, b-values have been reported to decrease for several years before the mainshock and afterwards increased rapidly, which has been interpreted to be due to changes in the stress field.
The Noto Peninsula area has experienced an earthquake swarm since 2020, which culminated with an M7.6 earthquake occurred on January 1, 2024. Since this earthquake (referred below as mainshock), seismic activity around the Noto Peninsula has greatly expanded from near the Sado Island to off the western coast of the Noto Peninsula. The occurrence of a major earthquake during an earthquake swarm is unprecedented, making the seismic activity in the Noto Peninsula unique in this respect.
In this study, using the Japan Meteorological Agency (JMA) catalog, we estimated the temporal changes in b-values for earthquakes that occurred around the Noto Peninsula from 2020 to 2024. We calculated the b-values by using the “b-positive” method as well as the conventional maximum likelihood method. The results revealed that the b-values in the vicinity of the Suzu city (Ishikawa prefecture), where the earthquake swarm activity had been active, experienced b-value changes that were differently from those in other areas. In the southwestern Noto Peninsula and off the northeastern coast, b-values showed an increasing trend after the mainshock, while in the vicinity of the Suzu city, b-values increased once after the mainshock but decreased to the values before the mainshock by the end of 2024. Considering the correlation between b-values and differential stress, it is possible that the areas where the b-values increased were affected by stress release due to aftershock activity, and that the stress in the vicinity of the Suzu city began to increase again after some time had passed since the mainshock.
In the future, we plan to examine how the changes in the b-values are related to the fault slip during the mainshock. As shown in previous studies, in the case of the 2011 Tohoku-Oki earthquake, areas of large slip, where large stress release is considered to have occurred, experienced a significant increase in b-value after the large earthquake. We plan a similar comparison for the 2024 Noto Peninsula earthquake and discuss the possible physical mechanisms responsible for the b-value changes.
The Noto Peninsula area has experienced an earthquake swarm since 2020, which culminated with an M7.6 earthquake occurred on January 1, 2024. Since this earthquake (referred below as mainshock), seismic activity around the Noto Peninsula has greatly expanded from near the Sado Island to off the western coast of the Noto Peninsula. The occurrence of a major earthquake during an earthquake swarm is unprecedented, making the seismic activity in the Noto Peninsula unique in this respect.
In this study, using the Japan Meteorological Agency (JMA) catalog, we estimated the temporal changes in b-values for earthquakes that occurred around the Noto Peninsula from 2020 to 2024. We calculated the b-values by using the “b-positive” method as well as the conventional maximum likelihood method. The results revealed that the b-values in the vicinity of the Suzu city (Ishikawa prefecture), where the earthquake swarm activity had been active, experienced b-value changes that were differently from those in other areas. In the southwestern Noto Peninsula and off the northeastern coast, b-values showed an increasing trend after the mainshock, while in the vicinity of the Suzu city, b-values increased once after the mainshock but decreased to the values before the mainshock by the end of 2024. Considering the correlation between b-values and differential stress, it is possible that the areas where the b-values increased were affected by stress release due to aftershock activity, and that the stress in the vicinity of the Suzu city began to increase again after some time had passed since the mainshock.
In the future, we plan to examine how the changes in the b-values are related to the fault slip during the mainshock. As shown in previous studies, in the case of the 2011 Tohoku-Oki earthquake, areas of large slip, where large stress release is considered to have occurred, experienced a significant increase in b-value after the large earthquake. We plan a similar comparison for the 2024 Noto Peninsula earthquake and discuss the possible physical mechanisms responsible for the b-value changes.