Geochemical constraints on the earthquake swarm following the 2011 Tohoku earthquake

Umeda, Koji; Asamori, Koichi; Kobori, Kazuo

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 1.410, 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.