Geochemical constraints on the earthquake swarm following the 2011 Tohoku earthquake

Swarm earthquakes, often interpreted to result from fluids invading the brittle seismogenic zone, have seismicity patterns that are significantly different from an aftershock sequence. 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 were acquired. The observed 3He/4He ratios in these samples are significantly lower than the atmospheric value of 1.4×10-6, 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 aqueous fluids driven off the subducting slab at shallow depths of ~40 km 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 changes associated with the Tohoku-Oki earthquake, enhanced by fluid flow along inherited weakened zones in the crust.