5:15 PM - 6:45 PM
[U15-P17] Relation between earthquake swarm activity and tides in the Noto region, Ishikawa Prefecture: From January 1, 2023 to January 1 2024, M7.6
Keywords:Tide, Noto Peninsula, Earthquake swarm
1. Introduction
The earthquake swarm activity before 2023 in the Noto region can be spatially divided into four clusters in the northern, southern, eastern, and western areas (regions N, S, E, and W, respectively). Furthermore, in region S, activity can be divided two depth clusters: Ss (hypocenters shallower than 14 km) and Sd (deeper than 14 km).
Hirose et al. [2024, EPS] analyzed the relationship between earthquake swarm activity and tides from 1 January 2018 to 31 December 2022 in the Noto region, and reported a tidal correlation only at greater depths in the southern part of the analyzed area ('region Sd'). There, they inferred that an increase in pore fluid pressure caused by the inflow of deep fluids may have led to a decrease in fault fracture strength, making the local seismicity relatively susceptible to the effects of tidal forces.
After that, on 5 May 2023, a larger earthquake of M 6.5 occurred in region E, and the subsequent expansion of the active region in the ocean area has blurred the boundary between regions N and E. Then, on January 1, 2024, M 7.6 earthquake occurred with the rupture starting point at 37.5°N near the boundary between regions N and E, being about 150-km fault length. Therefore, as same as the previous study [Hirose et al. 2024], we investigated tidal correlations of earthquakes from January 1, 2023 to January 1, 2024, M7.6.
2. Data
From the JMA unified hypocenter catalog, we extracted high-precision hypocenters of events with magnitude M >= 1.3 and flags K, k, or A occurring between January 1, 2023 and January 1, 2024 16:10 (time when M 7.6 occurred) at hypocenter depths of less than 20 km. We set aftershocks area of M 6.5 as the analysis area because the division of aftershocks area of M 6.5 was unclear and the seismicity before M 7.6 have occurred in the area while the previous study [Hirose et al. 2024] examined five clusters of earthquake swarm activity: regions N, Ss, Sd, E, and W. In this study, we analyzed data obtained by using a general declustering method [e.g., De Natale and Zollo 1986]. There were 23 events before M 6.5 and 68 events before M 7.6 (note that 44 events excluding M 6.5 and before that).
3. Method
We calculated theoretical tidal responses on the faults using the "TidalStrain.2" Fortran software [Hirose et al. 2019]. Calculation of theoretical tidal responses requires the hypocentral location, occurrence time, and fault parameters. For the hypocentral location and occurrence time, we used catalog information. For the fault parameters, we assumed that reverse faults with a northeasterly strike and southeasterly dip predominated (i.e., we assigned to all events a strike of 45°, a dip of 45°, and a rake of 90°), as in the previous study [Hirose et al. 2024].
To investigate the dependence of an earthquake on the tidal level (amplitude), we investigated the case that the tidal level is divided into two bins, positive or negative. The tidal sensitivity can be simply defined as the absolute value of the difference between the Nobs/Nexp ratios in the two bins, where Nobs is the number of earthquakes observed during the period when the tidal force takes the value within the strain/stress bin (i.e., positive or negative), and Nexp is the expected number of earthquakes in the same period if there were no correlation to tides (here Nexp is based on the tidal amplitude sampled at 15-minute intervals during four days before and after each earthquake). To test whether a correlation was statistically significant, we used a chi-square test at a significance level of 5% to test the null hypothesis that the event occurrence rate and the tidal occurrence rate (background distribution) belong to the same population.
4. Result and Discussion
Period 1: From January 1 to May 5, 2023 14:41 (before M 6.5)
Period 2: From July 1, 2023 to January 1, 2024 16:09 (before M 7.6)
Period 3: From January 1, 2023 to January 1, 2024 16:09 (before M 7.6)
In above all cases, there was no case in which the null hypothesis that the event occurrence rate and the tidal occurrence rate (background distribution) belong to the same population could be rejected at a significance level of 5% using a chi-square test.
The main areas of seismicity during this analysis period (from 2023) are regions N, E, and ocean areas. In the previous study [Hirose et al. 2024] that analyzed data from 2018 to 2022, no significant tidal correlation was observed in regions N and E. Therefore, it is thought that the fault strength in regions N and E did not decrease enough to allow tidal stress to affect the fault, as before 2022.
The earthquake swarm activity before 2023 in the Noto region can be spatially divided into four clusters in the northern, southern, eastern, and western areas (regions N, S, E, and W, respectively). Furthermore, in region S, activity can be divided two depth clusters: Ss (hypocenters shallower than 14 km) and Sd (deeper than 14 km).
Hirose et al. [2024, EPS] analyzed the relationship between earthquake swarm activity and tides from 1 January 2018 to 31 December 2022 in the Noto region, and reported a tidal correlation only at greater depths in the southern part of the analyzed area ('region Sd'). There, they inferred that an increase in pore fluid pressure caused by the inflow of deep fluids may have led to a decrease in fault fracture strength, making the local seismicity relatively susceptible to the effects of tidal forces.
After that, on 5 May 2023, a larger earthquake of M 6.5 occurred in region E, and the subsequent expansion of the active region in the ocean area has blurred the boundary between regions N and E. Then, on January 1, 2024, M 7.6 earthquake occurred with the rupture starting point at 37.5°N near the boundary between regions N and E, being about 150-km fault length. Therefore, as same as the previous study [Hirose et al. 2024], we investigated tidal correlations of earthquakes from January 1, 2023 to January 1, 2024, M7.6.
2. Data
From the JMA unified hypocenter catalog, we extracted high-precision hypocenters of events with magnitude M >= 1.3 and flags K, k, or A occurring between January 1, 2023 and January 1, 2024 16:10 (time when M 7.6 occurred) at hypocenter depths of less than 20 km. We set aftershocks area of M 6.5 as the analysis area because the division of aftershocks area of M 6.5 was unclear and the seismicity before M 7.6 have occurred in the area while the previous study [Hirose et al. 2024] examined five clusters of earthquake swarm activity: regions N, Ss, Sd, E, and W. In this study, we analyzed data obtained by using a general declustering method [e.g., De Natale and Zollo 1986]. There were 23 events before M 6.5 and 68 events before M 7.6 (note that 44 events excluding M 6.5 and before that).
3. Method
We calculated theoretical tidal responses on the faults using the "TidalStrain.2" Fortran software [Hirose et al. 2019]. Calculation of theoretical tidal responses requires the hypocentral location, occurrence time, and fault parameters. For the hypocentral location and occurrence time, we used catalog information. For the fault parameters, we assumed that reverse faults with a northeasterly strike and southeasterly dip predominated (i.e., we assigned to all events a strike of 45°, a dip of 45°, and a rake of 90°), as in the previous study [Hirose et al. 2024].
To investigate the dependence of an earthquake on the tidal level (amplitude), we investigated the case that the tidal level is divided into two bins, positive or negative. The tidal sensitivity can be simply defined as the absolute value of the difference between the Nobs/Nexp ratios in the two bins, where Nobs is the number of earthquakes observed during the period when the tidal force takes the value within the strain/stress bin (i.e., positive or negative), and Nexp is the expected number of earthquakes in the same period if there were no correlation to tides (here Nexp is based on the tidal amplitude sampled at 15-minute intervals during four days before and after each earthquake). To test whether a correlation was statistically significant, we used a chi-square test at a significance level of 5% to test the null hypothesis that the event occurrence rate and the tidal occurrence rate (background distribution) belong to the same population.
4. Result and Discussion
Period 1: From January 1 to May 5, 2023 14:41 (before M 6.5)
Period 2: From July 1, 2023 to January 1, 2024 16:09 (before M 7.6)
Period 3: From January 1, 2023 to January 1, 2024 16:09 (before M 7.6)
In above all cases, there was no case in which the null hypothesis that the event occurrence rate and the tidal occurrence rate (background distribution) belong to the same population could be rejected at a significance level of 5% using a chi-square test.
The main areas of seismicity during this analysis period (from 2023) are regions N, E, and ocean areas. In the previous study [Hirose et al. 2024] that analyzed data from 2018 to 2022, no significant tidal correlation was observed in regions N and E. Therefore, it is thought that the fault strength in regions N and E did not decrease enough to allow tidal stress to affect the fault, as before 2022.