DEEP-STRUCTURE OF ARC VOLCANOS AS INFERRED FROM SEISMIC OBSERVATIONS

Possible evidence for deep-seated magmatic activity beneath the northeastern Japan are has been obtained from micro-earthquake observations. Tomographic inversions for P-wave velocity clearly delineate low-velocity zones, which are inclined to the west and continuously distributed from the uppermost mantle to the upper crust beneath active volcanoes. Exceptionally deep (22-40 km) micro-earthquakes are found at 10 locations around the low-velocity zones beneath active volcanoes. All these events have extremely low predominant frequencies, suggesting a close relation to the magmatic activity in this depth range. At shallower depths (8-15 km) distinct reflectors of 8-waves from shallow events are found at five locations in or around the low-velocity zones. Their locations and reflection coefficients suggest that the reflectors are very thin magma bodies existing in the mid-crust. These observations shed some light on the state of magma at depths beneath volcanic arcs.