10:45 AM - 12:15 PM
[SSS08-P03] Deep crustal structure in the north Ibaraki area inferred from a distribution of the crustal reflector
Keywords:Crustal reflector, North Ibaraki area, S-wave reflected wave
Seismicity around the north Ibaraki and Fukushima Hamadori areas has been active since the 2011 Tohoku-oki Earthquake. The active seismicity has been observed at depths of 15-20 km, in addition to depths of shallower than 10 km. Recently, Usuda et al. [2022, Zisin] reported the presence of the S-wave reflector at depths of 15-25 km beneath the north Ibaraki areas. These observations suggest that the crustal structure relates to the distribution characteristics of the crustal seismicity in the areas. However, the relations between the crustal structure and seismicity are still unclear.
The Geological Survey of Japan, AIST, has maintained the seismograph array (GSJ array) in the north Ibaraki area since June 2016. Later phases are often observed for the crustal earthquakes at the GSJ array and surrounding permanent stations. These later phases are interpreted as the reflected S-wave generated by the crustal reflector [Usuda et al., 2022]. In our previous presentation [Shiina et al., 2022, SSJ], we investigated the distribution of the crustal reflector by using the arrival times of the reflected S-waves. The obtained reflector locates at depths of 15-25 km beneath the north Ibaraki area, which support the presence of the crustal reflector indicated by Usuda et al. [2022]. Additionally, it was revealed that the crustal reflector inclined about 26° to the northwest.
In this presentation, we discuss the detailed distribution of the crustal reflector and its relations to the crustal structure and seismicity in the north Ibaraki and Fukushima Hamadori areas. The obtained crustal reflector locates more southward than the one imaged by Usuda et al. [2022]. The apparent difference comes from the difference in methods; Shiina et al. [2022] handled the strike and dip of the reflector as unknown parameters, while Usuda et al. [2022] assumed a horizontal reflector. Moreover, Usuda et al. [2022] analyzed the earthquakes that occurred in 2012, while Shiina et al. [2022] used ones that occurred from June 2016 to 2021, suggesting that the crustal reflector stably existed beneath the north Ibaraki area for at least 10 years.
In and below the lower crust of the north Ibaraki area, the low-velocity [e.g., Tong et al., 2012, Solid Earth] and low-resistivity [Umeda et al., 2015; JGR] zone was detected. The presence of geofluid was inferred from the low-velocity and low-resistivity zone [e.g., Umeda et al., 2015]. The crustal reflector is distributed above this low-velocity and low-resistivity zone, indicating that the reflector corresponds to the top of the fluid-rich zone beneath the north Ibaraki area. The top of the low-velocity zone is deepened to the north (in the Fukushima Hamadori area) compared to the north Ibaraki area [e.g., Tong et al., 2012]. This trend coincides with the dip direction of the crustal reflector. Usuda et al. [2022] also reported the reflector around the Moho discontinuity beneath the Fukushima Hamadori area. The reflectors within the crust and around the Moho are detected below the depth where the crustal earthquakes occur, suggesting that the distributions of the geofluid contribute to the lateral variations in the bottom of the seismogenic layer in the target areas.
Acknowledgment:
We used Hi-net data from NIED and the JMA Unified Earthquake Catalog. This study was supported by MEXT Project for Seismology toward Research Innovation with Data of Earthquake (STAR-E) [grant number JPJ010217], and partially supported by the JST CREST [grant number JPMJCR1763].
The Geological Survey of Japan, AIST, has maintained the seismograph array (GSJ array) in the north Ibaraki area since June 2016. Later phases are often observed for the crustal earthquakes at the GSJ array and surrounding permanent stations. These later phases are interpreted as the reflected S-wave generated by the crustal reflector [Usuda et al., 2022]. In our previous presentation [Shiina et al., 2022, SSJ], we investigated the distribution of the crustal reflector by using the arrival times of the reflected S-waves. The obtained reflector locates at depths of 15-25 km beneath the north Ibaraki area, which support the presence of the crustal reflector indicated by Usuda et al. [2022]. Additionally, it was revealed that the crustal reflector inclined about 26° to the northwest.
In this presentation, we discuss the detailed distribution of the crustal reflector and its relations to the crustal structure and seismicity in the north Ibaraki and Fukushima Hamadori areas. The obtained crustal reflector locates more southward than the one imaged by Usuda et al. [2022]. The apparent difference comes from the difference in methods; Shiina et al. [2022] handled the strike and dip of the reflector as unknown parameters, while Usuda et al. [2022] assumed a horizontal reflector. Moreover, Usuda et al. [2022] analyzed the earthquakes that occurred in 2012, while Shiina et al. [2022] used ones that occurred from June 2016 to 2021, suggesting that the crustal reflector stably existed beneath the north Ibaraki area for at least 10 years.
In and below the lower crust of the north Ibaraki area, the low-velocity [e.g., Tong et al., 2012, Solid Earth] and low-resistivity [Umeda et al., 2015; JGR] zone was detected. The presence of geofluid was inferred from the low-velocity and low-resistivity zone [e.g., Umeda et al., 2015]. The crustal reflector is distributed above this low-velocity and low-resistivity zone, indicating that the reflector corresponds to the top of the fluid-rich zone beneath the north Ibaraki area. The top of the low-velocity zone is deepened to the north (in the Fukushima Hamadori area) compared to the north Ibaraki area [e.g., Tong et al., 2012]. This trend coincides with the dip direction of the crustal reflector. Usuda et al. [2022] also reported the reflector around the Moho discontinuity beneath the Fukushima Hamadori area. The reflectors within the crust and around the Moho are detected below the depth where the crustal earthquakes occur, suggesting that the distributions of the geofluid contribute to the lateral variations in the bottom of the seismogenic layer in the target areas.
Acknowledgment:
We used Hi-net data from NIED and the JMA Unified Earthquake Catalog. This study was supported by MEXT Project for Seismology toward Research Innovation with Data of Earthquake (STAR-E) [grant number JPJ010217], and partially supported by the JST CREST [grant number JPMJCR1763].