Triggering of earthquake swarms following the 2011 Tohoku megathrust earthquake

Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Kobori, Kazuo; Hama, Yuki*

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. The observed He/He ratios are significantly lower than the atmospheric value, indicating mantle helium contributed less than 10%. The plausible source of fluids can be attributed to waters released owing to sediment porosity collapse, and smectite-illite and opal-quartz reactions in the subducting sediments, rather than dehydration reactions of the subducting altered basalts and/or hydrated mantle. The aqueous fluids driven off the subducting slab migrate into the fore-arc crust, because of the pressure gradient between lithostatic pore pressure along the plate interface and hydrostatic pore pressure in the overriding crust. The swarm earthquake sequence would have been triggered by stress change associated with the Tohoku-Oki earthquake, enhanced by fluid flow along inherited weakened zones in the crust.