Abstract
A non double-couple earthquake was found in an oceanic crust of the Philippine Sea plate subducting into the mantle in the Kanto district, central Japan. Its magnitude and focal depth are 4.6 and 57 km, respectively. This subducting oceanic crust consists of low-density gabbro or basalt at depths shallower than about 60 km, while it consists of high-density eclogite to which gabbro transformed at depths deeper than about 60 km. The non double-couple earthquake occurred at this density boundary in the oceanic crust. Since the first motion of seismic P waves is predominantly dilatational and shows volume contraction, we consider that this event was probably caused by a sudden phase transformation from gabbro to eclogite. Its predominant frequency implies that this phase transformation occurred instantaneously in the metastability field. Since this event emanated not only compressional waves but also shear waves, we conclude that the sudden volume contraction triggered a double-couple earthquake immediately. This interpretation is supported by the aftershock distribution and the P-wave first-motion distribution of an eyeball type, which has only one cone on the focal sphere.
