Abstract
Seismic velocity (Vp, Vs) and Poisson's ratio (σ) images show that the hypocenter of the 2008 Iwate‐Miyagi earthquake (M7.2) is located in a distinctive zone with low‐Vp, slightly low‐Vs and low‐σ within the upper 10 km depths, where the mechanical strength might be weaker than the normal seismogenic layer with high velocity and low Poisson's ratio immediate below. A prominent low‐Vp, low‐Vs and high‐σ zone is revealed in the lower crust and uppermost mantle under the Iwate mainshock hypocenter, which may reflect fluids resulting from the dehydration of the subducting Pacific slab under northeast Japan. The aqueous component of fluids intruded into the source region may have reduced the mechanical strength of the rock matrix and so triggered the Iwate earthquake. Our results indicate that crustal fluids beneath the seismogenic layer can play an important role in the initiation of large crustal earthquakes.
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