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[U15-P02] Seismic activity of the 2024 Noto Peninsula Earthquake from December 2020, including the M7.6 earthquake on January 1, 2024.
Keywords:the 2024 Noto Peninsula Earthquake, Double-Difference hypocenter relocation method using waveform cross-correlation, nonstationary ETAS model, earthquake probability
Introduction
An earthquake of magnitude 7.6 occurred on January 1, 2024, after about three years of active earthquake swarm in the northeastern part of the Noto Peninsula from December 2020. In this presentation, we report these seismic activities and the results of analysis using seismic activity models.
Methods and Data
To investigate the seismic activity, relocated hypocenter data by double-difference method using waveform cross-correlation was used. For the M6.5 and M7.6 earthquakes, source fault inversion analysis using seismic waveforms was performed. In the seismic activity model analysis, we analyzed the background seismicity rate μ(t) of the nonstationary ETAS model (Kumazawa and Ogata, 2013) and calculated the earthquake probability.
Seismic activity
(1) From 2018 to 2023
The earthquake swarm was distributed in an area of about 30 km square in the northeastern part of the Noto Peninsula. It is roughly divided into four areas: north, south, east, and west. At the end of November 2020, crustal movements began as the μ(t) began to rise as the hypocenters rapidly moved to the deeper part at the south area. It was then active in the west, north, and east areas.
After the M6.5 earthquake on May 5, 2023, the distribution of the hypocenter dipped to the southeast expanded to shallow areas. The main fault slip of M6.5 is distributed on the shallow area of the north and east areas. After the M6.5 earthquake, the μ(t) continued to be high in the north and east areas. The fact that μ(t) is high even after the M6.5 earthquake suggests that the fluid may continue to be involved in the activity after the occurrence of M6.5.
The details of this section described in Iwakiri et al. (2023).
(2) After the M7.6 earthquake on January 1, 2024
The source fault slip of the M7.6 earthquake started in the earthquake swarm area just before the M7.6 earthquake and spread bilaterally, during for about 40 seconds. After the occurrence of the M7.6, the seismic activity area expanded to an area of about 150 km extending from northeast to southwest, and the hypocenter distribution was mainly dipped to southeast, but dipped to northwest in the northeastern part of the active area. These arrangements and shapes are generally consistent with the fault models shown in the previous other report. The hypocenter distribution in the southwestern part of the active area shows clusters in various directions.
On the other hand, seismic activity with M3~4 temporarily increased after the occurrence of M7.6 at three areas more than 40 km away from the active area of M7.6. One of them is near the Atotsugawa fault zone, where seismic activity has been observed. The spatial distribution of coulomb stress changes ΔCFF at a depth of 10 km, assuming the M7.6 fault slip model as the source and the nodal plane of the M7.6 CMT solution as the receiver, shows more than +10 kPa in the northeast and southwest around the active area.
Probability-based seismic activity prospect
After the M7.6 earthquake, the Japan Meteorological Agency (JMA) called for precautions for disaster prevention. For one week after the occurrence of M7.6, JMA called for the occurrence of an earthquake of the same magnitude as the first M7.6 based on past cases. After one week, JMA called for the prospect based on the probability of an earthquake occurrence after and before M7.6.
The probability after M7.6 was calculated using an evaluation method that combined the Omori-Utsu formula and the Gutenberg-Richter law. The probability before M7.6 was calculated by the Poisson process using the number of earthquakes in 1919~2023.
Probability-based prospects were made approximately every week during periods when the probability of an earthquake with a seismic intensity 5 or higher exceeded 10%, which is equivalent to about one occurrence in a month. At the end of February 2024, when the probability value fell below 10%, the regular call was terminated.
An earthquake of magnitude 7.6 occurred on January 1, 2024, after about three years of active earthquake swarm in the northeastern part of the Noto Peninsula from December 2020. In this presentation, we report these seismic activities and the results of analysis using seismic activity models.
Methods and Data
To investigate the seismic activity, relocated hypocenter data by double-difference method using waveform cross-correlation was used. For the M6.5 and M7.6 earthquakes, source fault inversion analysis using seismic waveforms was performed. In the seismic activity model analysis, we analyzed the background seismicity rate μ(t) of the nonstationary ETAS model (Kumazawa and Ogata, 2013) and calculated the earthquake probability.
Seismic activity
(1) From 2018 to 2023
The earthquake swarm was distributed in an area of about 30 km square in the northeastern part of the Noto Peninsula. It is roughly divided into four areas: north, south, east, and west. At the end of November 2020, crustal movements began as the μ(t) began to rise as the hypocenters rapidly moved to the deeper part at the south area. It was then active in the west, north, and east areas.
After the M6.5 earthquake on May 5, 2023, the distribution of the hypocenter dipped to the southeast expanded to shallow areas. The main fault slip of M6.5 is distributed on the shallow area of the north and east areas. After the M6.5 earthquake, the μ(t) continued to be high in the north and east areas. The fact that μ(t) is high even after the M6.5 earthquake suggests that the fluid may continue to be involved in the activity after the occurrence of M6.5.
The details of this section described in Iwakiri et al. (2023).
(2) After the M7.6 earthquake on January 1, 2024
The source fault slip of the M7.6 earthquake started in the earthquake swarm area just before the M7.6 earthquake and spread bilaterally, during for about 40 seconds. After the occurrence of the M7.6, the seismic activity area expanded to an area of about 150 km extending from northeast to southwest, and the hypocenter distribution was mainly dipped to southeast, but dipped to northwest in the northeastern part of the active area. These arrangements and shapes are generally consistent with the fault models shown in the previous other report. The hypocenter distribution in the southwestern part of the active area shows clusters in various directions.
On the other hand, seismic activity with M3~4 temporarily increased after the occurrence of M7.6 at three areas more than 40 km away from the active area of M7.6. One of them is near the Atotsugawa fault zone, where seismic activity has been observed. The spatial distribution of coulomb stress changes ΔCFF at a depth of 10 km, assuming the M7.6 fault slip model as the source and the nodal plane of the M7.6 CMT solution as the receiver, shows more than +10 kPa in the northeast and southwest around the active area.
Probability-based seismic activity prospect
After the M7.6 earthquake, the Japan Meteorological Agency (JMA) called for precautions for disaster prevention. For one week after the occurrence of M7.6, JMA called for the occurrence of an earthquake of the same magnitude as the first M7.6 based on past cases. After one week, JMA called for the prospect based on the probability of an earthquake occurrence after and before M7.6.
The probability after M7.6 was calculated using an evaluation method that combined the Omori-Utsu formula and the Gutenberg-Richter law. The probability before M7.6 was calculated by the Poisson process using the number of earthquakes in 1919~2023.
Probability-based prospects were made approximately every week during periods when the probability of an earthquake with a seismic intensity 5 or higher exceeded 10%, which is equivalent to about one occurrence in a month. At the end of February 2024, when the probability value fell below 10%, the regular call was terminated.