5:15 PM - 7:15 PM
[SSS08-P04] Three-dimensional Attenuation Structure in the Noto Peninsula, Japan(2)
- A Study Using the Twofold Spectral Ratio Method -
Keywords:Twofold Spectral Ratio Method, Attenuation Structure, Noto Peninsula
1. Introduction
In our recent presentation (Nakamura et al., 2024, JpGU), we determined a three-dimensional S-wave attenuation (Qs) structure in the region surrounding the source area of the 2024 Noto Peninsula Earthquake (Mj7.6). The tomographic results identified the low-Qs at depths of 0-20 km in the source area. At the northeastern tip of the peninsula, the seismic swarm has been active since the end of 2020. It is thought that the crustal fluid facilitated this seismic activity in this area (e.g., Nakajima, 2022; Amezawa et al., 2023; Kato, 2023). The low-Qs structure detected in our previous study seems to support the presence of fluid around the source area of the Noto Peninsula Earthquake. However, there are tradeoffs between the Qs structure and the other parameters in the tomographic analysis, making it difficult to extract a fine-scale Qs structure. To overcome this, we here employed the Twofold Spectral Ratio (TSR) method (e.g., Matsuzawa et al., 2003). The TSR method can cancel out the effects of source and site characteristics by taking the spectral ratio for pairs of two earthquakes and two stations and derives Qs values along those unwrapped ray paths.
2. Data and Methods
S-wave spectral amplitudes were calculated from acceleration records recorded at stations of the K-NET and KiK-net at surface ground. We collected earthquakes of which Mj was less 7.5 and the F-net focal mechanism solutions were available. The period for collecting earthquakes was from January to July 2024. To avoid contaminations of nonlinear responses, records with the maximum acceleration amplitudes exceeding 100 cm/s^2 were excluded.
When the spectral amplitudes, F, is given at two stations, 1 and 2, for two earthquakes, A and B, the Qs values are obtained as follows (Matsuzawa et al., 2003):
-πf *Qs-1 = ln-CTSR / DDTT ,
where
ln-CTSR = ln(FA*r1A ) + ln(F2B *r2B) - ln(F1 B *r1 B) - ln(F2A *r2 A),
DDTT = r1 A /Vs + r2B /Vs - r1 B /Vs - r2 A /Vs,
and, r is hypocentral distance, and Vs is S-wave velocity (3.5km/s). The ln-CTSR values represent the spectral amplitude ratio corrected for geometric spreading factor. The Qs values are evaluated as slope of the ln-CTSR for the DDTT by using the least-squares method (Fig. 1).
In this study, we focused on the two regions around the source area of the Noto Peninsula Earthquake, and two station pairs considered for each region (Fig. 2):
Region 1:
1-1: ISK003 (Wajima) - ISK006 (Togi)
1-2: ISKH06 (Shika) - ISK015 (Omachi)
Region 2:
2-1: ISK001 (Otani) - ISK003 (Wajima)
2-2: ISK015 (Omachi) - ISKH03 (Uchiura)
The station pairs of 1-1 and 2-1 located just above the source area and the other pairs distributed south.
3. Results and Discussion
We obtained following characteristics in the Qs values by the TSR method (Fig. 2):
1. The Q values for the station pairs (1-1 and 2-1) were generally smaller than those for the southern pairs (1-2, 2-2).
2. Compared to the tomography results for depths of 0-20 km, the small Qs values were identified for all station pairs.
Most earthquakes analyzed in this study are the aftershocks of the Noto Peninsula Earthquake, implying that they are distributed along the fault plane. Thus, their ray paths pass through the vicinity of the fault plane before arriving to the northern pairs. Consequently, the smaller Qs values, compared to the tomographic results, suggest strong S-wave attenuation concentrated near the fault plane, as observed in other fault zones (e.g., Blakeslee, 1989; Yamada and Oda, 2018). In the peninsula, the crustal fluid is thought to exist deep portions of the crust (e.g., Nishimura et al., 2023). Our results support that the crustal fluid is localized around the fault plane, potentially contributing to the activity of the seismic swarm and the occurrence of the Noto Peninsula earthquake.
In our recent presentation (Nakamura et al., 2024, JpGU), we determined a three-dimensional S-wave attenuation (Qs) structure in the region surrounding the source area of the 2024 Noto Peninsula Earthquake (Mj7.6). The tomographic results identified the low-Qs at depths of 0-20 km in the source area. At the northeastern tip of the peninsula, the seismic swarm has been active since the end of 2020. It is thought that the crustal fluid facilitated this seismic activity in this area (e.g., Nakajima, 2022; Amezawa et al., 2023; Kato, 2023). The low-Qs structure detected in our previous study seems to support the presence of fluid around the source area of the Noto Peninsula Earthquake. However, there are tradeoffs between the Qs structure and the other parameters in the tomographic analysis, making it difficult to extract a fine-scale Qs structure. To overcome this, we here employed the Twofold Spectral Ratio (TSR) method (e.g., Matsuzawa et al., 2003). The TSR method can cancel out the effects of source and site characteristics by taking the spectral ratio for pairs of two earthquakes and two stations and derives Qs values along those unwrapped ray paths.
2. Data and Methods
S-wave spectral amplitudes were calculated from acceleration records recorded at stations of the K-NET and KiK-net at surface ground. We collected earthquakes of which Mj was less 7.5 and the F-net focal mechanism solutions were available. The period for collecting earthquakes was from January to July 2024. To avoid contaminations of nonlinear responses, records with the maximum acceleration amplitudes exceeding 100 cm/s^2 were excluded.
When the spectral amplitudes, F, is given at two stations, 1 and 2, for two earthquakes, A and B, the Qs values are obtained as follows (Matsuzawa et al., 2003):
-πf *Qs-1 = ln-CTSR / DDTT ,
where
ln-CTSR = ln(FA*r1A ) + ln(F2B *r2B) - ln(F1 B *r1 B) - ln(F2A *r2 A),
DDTT = r1 A /Vs + r2B /Vs - r1 B /Vs - r2 A /Vs,
and, r is hypocentral distance, and Vs is S-wave velocity (3.5km/s). The ln-CTSR values represent the spectral amplitude ratio corrected for geometric spreading factor. The Qs values are evaluated as slope of the ln-CTSR for the DDTT by using the least-squares method (Fig. 1).
In this study, we focused on the two regions around the source area of the Noto Peninsula Earthquake, and two station pairs considered for each region (Fig. 2):
Region 1:
1-1: ISK003 (Wajima) - ISK006 (Togi)
1-2: ISKH06 (Shika) - ISK015 (Omachi)
Region 2:
2-1: ISK001 (Otani) - ISK003 (Wajima)
2-2: ISK015 (Omachi) - ISKH03 (Uchiura)
The station pairs of 1-1 and 2-1 located just above the source area and the other pairs distributed south.
3. Results and Discussion
We obtained following characteristics in the Qs values by the TSR method (Fig. 2):
1. The Q values for the station pairs (1-1 and 2-1) were generally smaller than those for the southern pairs (1-2, 2-2).
2. Compared to the tomography results for depths of 0-20 km, the small Qs values were identified for all station pairs.
Most earthquakes analyzed in this study are the aftershocks of the Noto Peninsula Earthquake, implying that they are distributed along the fault plane. Thus, their ray paths pass through the vicinity of the fault plane before arriving to the northern pairs. Consequently, the smaller Qs values, compared to the tomographic results, suggest strong S-wave attenuation concentrated near the fault plane, as observed in other fault zones (e.g., Blakeslee, 1989; Yamada and Oda, 2018). In the peninsula, the crustal fluid is thought to exist deep portions of the crust (e.g., Nishimura et al., 2023). Our results support that the crustal fluid is localized around the fault plane, potentially contributing to the activity of the seismic swarm and the occurrence of the Noto Peninsula earthquake.