CONTINUOUS HIGH-VELOCITY ASEISMIC ZONE BENEATH THE IZUBONIN ARC

Since 1904, eleven unusually deep earthquakes have been reported near the southern end of the Izu-Bonin arc (about 24-degrees-N, 142-degrees -E). These isolated events are separated from ordinary deep earthquake s in the Wadati-Benioff (W-B) zone of the Izu-Bonin (I-B) arc by a dis tance of about 300 km parallel to the trench axis. They also are locat ed 200 km closer to the trench than the natural extension of the shall ower earthquakes above them. Using 3-D ray tracing, a detailed analysi s is performed of the difference in travel time residuals between the anomalous events and the deep events within the W-B zone to the north. This analysis yields higher reliability and resolution than can be ob tained from ordinary tomographic imaging. We find that these unusual e vents occurred in a high velocity zone which is connected to the north ern W-B zone of the I-B arc. The aseismic high velocity zone has a P-v elocity at least 3% faster than the surrounding mantle. Both our resul t and previous evidence for a horizontally lying slab west of the W-B zone at 27-degrees-N (e.g., Okino et al., 1989) are consistent with th e existence of a prominent contortion of the subducted slab in the I-B region.