8:30 AM - 8:45 AM
[J08-2-01] Tomographic imaging of the seismic velocity structure in southern Hokkaido, Japan: Implications for distributions of the crustal deep low-frequency earthquakes
At southern Hokkaido, Japan, the crustal deep low-frequency earthquakes (CDLFE) are often observed beneath both active volcanoes and non-active volcanic area, which corresponds to shallow swarm-like activity of the regular crustal earthquakes. It is considered that the CDLFEs relate to activity of volcanoes and regular crustal earthquakes (e.g., Takahashi and Miyamura, 2009). However, those relations have not been revealed in clearly. In order to improve understandings for the CDLFEs and those relating phenomena, we investigated seismic velocity structure beneath southern Hokkaido and discussed the relations based on the heterogeneous structure.
For estimating the seismic velocity structure in detailed, we adapted the double-difference tomography method (Zhang and Thurber, 2003; 2006). The number of earthquakes occurred in southern Hokkaido in the period from March 2003 to June 2016 were 15,645 (A magnitude range of 1.5-6.5), and travel times of 306,335 for P wave and 242,093 for S wave were adapted to the estimations.
The obtained results obviously show that the low-velocity and high-Vp/Vs zones are estimated in depths of about 20-40 km beneath active volcanoes. Moreover, it is confirmed extensions of the low-velocity and high-Vp/Vs zones to the CDLFEs within the non-active volcanic area. At a depth of 10 km, seismic velocity beneath the volcanoes tends to be decreased from that calculated in surroundings while high-velocity anomaly is widely detected. The crustal earthquakes involving swarm-like activity above the CDLFEs are correspondingly located with the high-velocity zones. Because it is considered that presence and migration of fluids or melts contribute to trigger the CDLFEs (e.g., Ukawa and Ohtake, 1987), these observations suggests that the CDLFEs closely relates to the crustal earthquakes and volcanoes though the fluids or melts, and the related phenomena may be linked to the crustal heterogeneity.
For estimating the seismic velocity structure in detailed, we adapted the double-difference tomography method (Zhang and Thurber, 2003; 2006). The number of earthquakes occurred in southern Hokkaido in the period from March 2003 to June 2016 were 15,645 (A magnitude range of 1.5-6.5), and travel times of 306,335 for P wave and 242,093 for S wave were adapted to the estimations.
The obtained results obviously show that the low-velocity and high-Vp/Vs zones are estimated in depths of about 20-40 km beneath active volcanoes. Moreover, it is confirmed extensions of the low-velocity and high-Vp/Vs zones to the CDLFEs within the non-active volcanic area. At a depth of 10 km, seismic velocity beneath the volcanoes tends to be decreased from that calculated in surroundings while high-velocity anomaly is widely detected. The crustal earthquakes involving swarm-like activity above the CDLFEs are correspondingly located with the high-velocity zones. Because it is considered that presence and migration of fluids or melts contribute to trigger the CDLFEs (e.g., Ukawa and Ohtake, 1987), these observations suggests that the CDLFEs closely relates to the crustal earthquakes and volcanoes though the fluids or melts, and the related phenomena may be linked to the crustal heterogeneity.