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Asamori, Koichi; Makuuchi, Ayumu; Umeda, Koji
no journal, ,
Two shallow moderate earthquakes occurred in the northwestern part of Kagoshima province, on March 26 (Mw 6.1) and on May 13 (Mw 6.0) in 1997. In this study, we conduct magnetotelluric (MT) soundings in and around the source region of the 1997 Kagoshima earthquake sequence and perform a 3-D inversion of wideband MT data above a depth of 30 km. The obtained 3-D resistivity model acquired through the inversion shows as follows. (1) Two anomalous conductive bodies are clearly visible in the upper and lower crust beneath the seismic source region, and seem to be combined and extend down to the uppermost mantle. (2) These conductive bodies are located just beside the two mainshocks in the upper crust. (3) These results suggest that the generation of two large earthquakes is not a pure mechanical process, but is closely related to heterogeneities in the material property and stress field due to the relatively effective transfer of mantle fluids from the upper mantle.
Umeda, Koji; Asamori, Koichi; Kobori, Kazuo
no journal, ,
Following the Mw 9.0 Tohoku-Oki earthquake, an unusual, shallow normal-faulting swarm sequence occurred near the Pacific coast in the southeast Tohoku district. In order to provide geochemical constraints on the source of the fluids triggering the swarm activity, new helium isotope data were acquired from gas and water samples around the seismic source region. The observed 
He/
He ratios in these samples are significantly lower than the atmospheric value of 1.4
10
, indicating that mantle helium contributed less than 10% of the total helium. Plausible sources of the fluids can be attributed to waters released from sediment porosity collapse and from smectite-illite and opal-quartz reactions in the subducting sediments, rather than dehydration reactions in subducting, altered basalts and/or hydrated mantle. The swarm earthquake sequence would have been triggered by stress changes associated with the Tohoku-Oki earthquake, enhanced by fluid flow along inherited weakened zones in the crust.