Fluid-driven seismicity activation in northern Nagano region after the 2011 M9.0 Tohoku-oki earthquake

The dynamic triggering of earthquakes is well documented; however, the underlying physical mechanisms remain obscure. Here we analyze the seismicity in northern Nagano, central Japan, following the Tohoku‐oki quake, until the occurrence 13 h later of an Mw6.2 local earthquake. We use waveform detection techniques to identify 17 times more earthquakes than those in the Japan Meteorological Agency catalog. The activation of seismicity in the epicentral region of the Mw6.2 event is weak and delayed, culminating with the occurrence of the moderate shock preceded by two small foreshocks. The seismicity activation to the south is shallower, abundant, and starts during the passage of Tohoku‐oki surface waves of high dynamic stresses. The early activation occurs in areas of relatively high near‐surface fluid temperature, indicating that the dynamic triggering is likely caused by excitation of geothermal crustal fluids. The Mw6.2 Northern Nagano earthquake might have been delay‐triggered by fluid migration from a deep source. Key Points Immediate and delayed inland earthquake activation after Tohoku‐oki megathrustSeismicity induced by dynamic stress changes through excitation of geofluidsEarthquake activation pattern correlates with crustal fluid properties