11:00 〜 11:15
[SCG46-17] Trans-crustal fluid distribution in the central Tohoku region
キーワード：比抵抗、地殻、流体、メルト
In the central Tohoku region, Quaternary volcanoes, including Naruko volcano and Mt. Kurikoma, are distributed, and inland earthquakes such as the 2008 Iwate-Miyagi inland earthquake have occurred around them and on the forearc side. Resistivity structures have been analyzed for these volcanoes and earthquakes using the broadband MT method since 1990 (e.g., Ogawa et al., 2014; Ichihara et al., 2014). Long-period MT studies have also been conducted for mantle structures (Ichiki et al., 2015). However, the connection of resistivity structures between the crust and the uppermost mantle has yet to be analyzed in detail. In addition, a regional 3D resistivity structure analysis covering volcanic seismic regions has yet to be detailed due to computer memory limitations. This study aims to elucidate the regional and detailed crust and uppermost mantle structure in the central Tohoku region using broadband MT data observed over the past 30 years.
We used 410 broadband MT data in the central Tohoku region covering the 100 km × 100 km area. The period band extends from 0.003s to 2,000s. The dataset also includes the past data from Asamori et al. (2010), Ishizu et al. (2022), Mitsuhata et al. (2001), and Sato et al. (2006). We obtained the 3-D resistivity structure using the 3d inversion code WSINV3DMT (Siripunvaraporn & Egbert, 2009). In this analysis, the impedance and tipper error floors were set to 10% and 20%, respectively, and a final model with an RMS of 2.69 was obtained.
The final model showed a continuous low resistivity zone along the volcanic arc from the uppermost mantle (40 km depth) to the lower crust (20 km depth) in the NNE-SSW direction. The conductors rise further into the upper crust by splitting toward volcanos.
The low resistivity under each volcano can be discriminated between melt and crustal fluids using seismic velocity. The low velocity and high Vp/Vs regions are interpreted as melt regions, whereas the low velocity and low Vp/Vs regions are crustal fluid regions (Okada et al., 2014). Thus, the melt region is shared up to 20 km depth beneath Mt. Kurikoma and Takamatsu-dake, but at shallower depths, it rises to a depth of 10 km only in the direction of Takamatsu-dake. Similarly, beneath the Onikobe caldera, the melt rises to a depth of 15 km.
The InSAR study (Takada & Fukushima,2013) detected multiple volcanic subsidences around Mt. Kurikoma after the 2011 Tohoku earthquake. We have found that the long wave-length subsidence areas correspond to the melt area at 20 km depth and that the peaks subsidence areas at Mt. Takamatsu and Onikobe caldera correspond to the shallower melt area.
We also found isolated blocks of low resistivity on the forearc side at 10 to 20 km depths. Low resistivities are located around the hypocenters of the northern Miyagi earthquakes (1900 and 1962), supporting the fluid valve action of seismogenesis (Sibson, 2009).
We used 410 broadband MT data in the central Tohoku region covering the 100 km × 100 km area. The period band extends from 0.003s to 2,000s. The dataset also includes the past data from Asamori et al. (2010), Ishizu et al. (2022), Mitsuhata et al. (2001), and Sato et al. (2006). We obtained the 3-D resistivity structure using the 3d inversion code WSINV3DMT (Siripunvaraporn & Egbert, 2009). In this analysis, the impedance and tipper error floors were set to 10% and 20%, respectively, and a final model with an RMS of 2.69 was obtained.
The final model showed a continuous low resistivity zone along the volcanic arc from the uppermost mantle (40 km depth) to the lower crust (20 km depth) in the NNE-SSW direction. The conductors rise further into the upper crust by splitting toward volcanos.
The low resistivity under each volcano can be discriminated between melt and crustal fluids using seismic velocity. The low velocity and high Vp/Vs regions are interpreted as melt regions, whereas the low velocity and low Vp/Vs regions are crustal fluid regions (Okada et al., 2014). Thus, the melt region is shared up to 20 km depth beneath Mt. Kurikoma and Takamatsu-dake, but at shallower depths, it rises to a depth of 10 km only in the direction of Takamatsu-dake. Similarly, beneath the Onikobe caldera, the melt rises to a depth of 15 km.
The InSAR study (Takada & Fukushima,2013) detected multiple volcanic subsidences around Mt. Kurikoma after the 2011 Tohoku earthquake. We have found that the long wave-length subsidence areas correspond to the melt area at 20 km depth and that the peaks subsidence areas at Mt. Takamatsu and Onikobe caldera correspond to the shallower melt area.
We also found isolated blocks of low resistivity on the forearc side at 10 to 20 km depths. Low resistivities are located around the hypocenters of the northern Miyagi earthquakes (1900 and 1962), supporting the fluid valve action of seismogenesis (Sibson, 2009).