11:30 AM - 11:45 AM
[SCG56-04] Estimation of subsurface structures in the Iwaki-Joban region based on microtremor array surveys for the assessment of low-to-medium temperature geothermal resources
★Invited Papers
Keywords:Microtremor array survey, Seismic observation, Groundwater, Geothermal resources
Iwaki City, Fukushima, hosts abundant low-to-medium temperature geothermal resources, particularly in the Joban region, where approximately 5 tons per minute of hot spring water is discharged from the source. The region contains the Itozawa and Yunodake faults, which were the source faults of the 2011 Hamadori earthquake (Mj 7.0), along with numerous other faults that likely influence the local geothermal system. In fact, a rise in groundwater levels and new groundwater discharge were observed in the eastern area of the Yunodake fault after the earthquake.
At greater depths, fluid behavior has also been investigated. Togo et al. (2014) suggested that some of the discharged water originates from the subducting oceanic plate, while Sato et al. (2020) proposed the existence of a high-pressure hydrothermal reservoir at approximately 40 km depth. Moreover, a seismic swarm was observed at around 10 km depth after the earthquake, suggesting some form of fluid activity in the deep subsurface.
However, large-scale geophysical surveys, such as reflection seismic surveys, have not been conducted in urban areas, and no studies have directly clarified the relationship between deep (~10 km) and shallow fluid activity based on geophysical exploration. In this study, we deployed a temporary local seismic network and conducted a large-scale microtremor array survey (~30 km in total length) to elucidate the connection between deep and shallow fluid activity and to advance the understanding of the low-to-medium temperature geothermal system in the Joban region.
Our local seismic observations revealed low levels of microseismic activity near the depth where the seismic swarm occurred after the Hamadori earthquake, potentially indicating fluid-related activity at depth. The microtremor array survey provided S-wave velocity structures and basement depths, identifying multiple basement depth discontinuities that likely correspond to faults. One of these estimated faults is located near the site where groundwater discharge began after the Hamadori earthquake, suggesting it may serve as a fluid pathway.
This study successfully identified fault displacements that could act as fluid pathways through a large-scale microtremor survey. Moving forward, we will continue local seismic observations to refine our understanding of seismic event distributions and achieve a more comprehensive understanding of fluid behavior from deep to shallow depths, ultimately improving our knowledge of the low-to-medium temperature geothermal system in the Joban region.
Acknowledgement: This research was performed by the commissioned research fund provided by F-REI (JPFR23070103, JPFR24070103). We thank the consortium members of the project for their help and advice on this study.
At greater depths, fluid behavior has also been investigated. Togo et al. (2014) suggested that some of the discharged water originates from the subducting oceanic plate, while Sato et al. (2020) proposed the existence of a high-pressure hydrothermal reservoir at approximately 40 km depth. Moreover, a seismic swarm was observed at around 10 km depth after the earthquake, suggesting some form of fluid activity in the deep subsurface.
However, large-scale geophysical surveys, such as reflection seismic surveys, have not been conducted in urban areas, and no studies have directly clarified the relationship between deep (~10 km) and shallow fluid activity based on geophysical exploration. In this study, we deployed a temporary local seismic network and conducted a large-scale microtremor array survey (~30 km in total length) to elucidate the connection between deep and shallow fluid activity and to advance the understanding of the low-to-medium temperature geothermal system in the Joban region.
Our local seismic observations revealed low levels of microseismic activity near the depth where the seismic swarm occurred after the Hamadori earthquake, potentially indicating fluid-related activity at depth. The microtremor array survey provided S-wave velocity structures and basement depths, identifying multiple basement depth discontinuities that likely correspond to faults. One of these estimated faults is located near the site where groundwater discharge began after the Hamadori earthquake, suggesting it may serve as a fluid pathway.
This study successfully identified fault displacements that could act as fluid pathways through a large-scale microtremor survey. Moving forward, we will continue local seismic observations to refine our understanding of seismic event distributions and achieve a more comprehensive understanding of fluid behavior from deep to shallow depths, ultimately improving our knowledge of the low-to-medium temperature geothermal system in the Joban region.
Acknowledgement: This research was performed by the commissioned research fund provided by F-REI (JPFR23070103, JPFR24070103). We thank the consortium members of the project for their help and advice on this study.