9:30 AM - 9:45 AM
[J05-3-04] 3d distribution of fluids and their origins in a seismogenic zone, Northern Miyagi, NE Japan
Northern Miyagi is a seismically active region located in one of the strain concentration zones in NE Japan. This area experienced three large earthquakes, in 1900 (M7.0), 1962 (M6.5) and 2003 (M6.2). The 2003 earthquake was well studied and its focal mechanism and aftershock distribution support that the earthquake was a high angle reversed fault, which is a reactivation of an originally normal fault, created in the Miocene during the Japan opening. Geologically, the area is mostly simply covered with thick sediment and is surrounded by granitic rocks of Kitakami Mountains to the east and to the north. The objectives of this study are to image the geofluid in three dimensions using magnetotellurics (MT) and to relate them to earthquake activities in the region.
We used MT data at 67 sites in total, covering 50km x 50km area. 39 sites are new and 32 sites are from the compilations of the previous three MT profiles in the area (Nagao, 1997; Mitsuhata et al, 2001; Sato et al., 2005). The previous profile data were inverted assuming the structure is quasi-two-dimensional, but we have found strong three-dimensionality in the dataset. We inverted the data using 3d inversion code, WS3dMTINV (Siripunvaraporn and Egbert, 2009). The model showed three deep conductors around the focal depths of the three large earthquakes. This suggests that high pore pressure fluid is episodically expelled from the reservoir to induce earthquakes in a less permeable zone..
Current high seismicity is seen at the outer rim of the deep conductors. The northernmost conductor at 1962 earthquake has a deeper extension to northwest toward Kurikoma volcano. However the conductors at 1990 and 2003 earthquakes have no deeper extensions. The former conductor at 1962 earthquake implies fluid supplied from the volcanoes. In contrast, the latter conductors at 1990 and 2003 earthquakes imply fluids trapped in sediment at the rim of the Miocene rift.
We used MT data at 67 sites in total, covering 50km x 50km area. 39 sites are new and 32 sites are from the compilations of the previous three MT profiles in the area (Nagao, 1997; Mitsuhata et al, 2001; Sato et al., 2005). The previous profile data were inverted assuming the structure is quasi-two-dimensional, but we have found strong three-dimensionality in the dataset. We inverted the data using 3d inversion code, WS3dMTINV (Siripunvaraporn and Egbert, 2009). The model showed three deep conductors around the focal depths of the three large earthquakes. This suggests that high pore pressure fluid is episodically expelled from the reservoir to induce earthquakes in a less permeable zone..
Current high seismicity is seen at the outer rim of the deep conductors. The northernmost conductor at 1962 earthquake has a deeper extension to northwest toward Kurikoma volcano. However the conductors at 1990 and 2003 earthquakes have no deeper extensions. The former conductor at 1962 earthquake implies fluid supplied from the volcanoes. In contrast, the latter conductors at 1990 and 2003 earthquakes imply fluids trapped in sediment at the rim of the Miocene rift.