09:00 〜 09:15
[SCG58-01] Slab-derived fluid storage in the crust elucidated by earthquake swarm
★Invited Papers
キーワード：群発地震、誘発地震、スラブ起源流体
Slab-derived fluids and their migration play a critical role in broad crustal dynamics in the subduction zone, such as catastrophic earthquakes and relevant phenomena. However, no quantification of slab-derived fluid budget in higher spatiotemporal resolution has not been achieved beyond the spatiotemporal resolutions of current geophysical and geological studies. Seismic swarms are often the most clear phenomena to correlate with the dynamic behavior of slab-derived fluid. This is because they typically show the apparent spatial migration of seismic activity with time, which has often been interpreted as caused by fluid migration. Seismic swarm activity in the Yamagata-Fukushima border was activated after the Tohoku-Oki earthquake, and this sequence shows clear upward migration in their hypocenters (Yoshida and Hasegawa, 2018). Also, various features of seismic activity have been investigated, showing very similar characteristics to injection-induced seismicity. Therefore, we utilized the insight from injection-induced seismicity, where the fluid seismicity coupling phenomena have been investigated intensively in the last decades, and investigate the fluid volume involved in the swarm sequence.
We set our target on an intense earthquake swarm associated with the M9 Tohoku earthquake. We inversely estimated the fluid volume involved in this seismic swarm sequence with McGarr’s theory (McGarr, 2014), Seismogenic index model (Shapiro et al., 2010), which estimated the maximum magnitude associated with injected fluid volume. In addition, we utilized the Darcy law and cubic law models as hydrological approaches. We modeled diffusivity from seismic swarm migration and then converted it to permeability. The flow region was modeled as the seismic zone. As a result, all four models quantified the fluid volume in a range of 106−109 m3 for various scenarios; then, we could narrow the range to 106−108 m3 as a plausible range considering the consistency between the estimates from the four models.
We estimated the duration of fluid storage or recharged with geological proxies. Assuming that all the fluid that caused the seismic swarm was supplied by the melt or partially melting zone of the Bandai-Azuma-Adatara volcanic clusters below the Miocene Otoge volcanic caldera, the duration of fluid recharge is estimated as 10-104 years, using the background H2O flux under the NE Japan arc (~13 t/yr/m of the arc length) and seismic swam region (10 x 10 m2). This duration is further constrained by the fact of seismic silence in this area by modern seismic monitoring to 102-104 years. This duration range includes the recurrence time of the M9class earthquake of 103 years, suggesting a possible link between periodic earthquake swarms accompanied by M9 class earthquakes. The amount of estimated fluid volume also suggests the possible link to the quartz vein precipitation considering the geothermal gradient at the seismic swarm region. In general, the quartz vein was considered to be formed on a million years scale, but this study suggests the possibility of mineral vein and ore deposits formation in such a short term of the seismic swarm (2 years). As such, our new interdisciplinary approach was able to provide new quantification of the dynamic behavior of slab-derived fluid and simultaneously led the various further discussion related to this topic.
We set our target on an intense earthquake swarm associated with the M9 Tohoku earthquake. We inversely estimated the fluid volume involved in this seismic swarm sequence with McGarr’s theory (McGarr, 2014), Seismogenic index model (Shapiro et al., 2010), which estimated the maximum magnitude associated with injected fluid volume. In addition, we utilized the Darcy law and cubic law models as hydrological approaches. We modeled diffusivity from seismic swarm migration and then converted it to permeability. The flow region was modeled as the seismic zone. As a result, all four models quantified the fluid volume in a range of 106−109 m3 for various scenarios; then, we could narrow the range to 106−108 m3 as a plausible range considering the consistency between the estimates from the four models.
We estimated the duration of fluid storage or recharged with geological proxies. Assuming that all the fluid that caused the seismic swarm was supplied by the melt or partially melting zone of the Bandai-Azuma-Adatara volcanic clusters below the Miocene Otoge volcanic caldera, the duration of fluid recharge is estimated as 10-104 years, using the background H2O flux under the NE Japan arc (~13 t/yr/m of the arc length) and seismic swam region (10 x 10 m2). This duration is further constrained by the fact of seismic silence in this area by modern seismic monitoring to 102-104 years. This duration range includes the recurrence time of the M9class earthquake of 103 years, suggesting a possible link between periodic earthquake swarms accompanied by M9 class earthquakes. The amount of estimated fluid volume also suggests the possible link to the quartz vein precipitation considering the geothermal gradient at the seismic swarm region. In general, the quartz vein was considered to be formed on a million years scale, but this study suggests the possibility of mineral vein and ore deposits formation in such a short term of the seismic swarm (2 years). As such, our new interdisciplinary approach was able to provide new quantification of the dynamic behavior of slab-derived fluid and simultaneously led the various further discussion related to this topic.