Since 1904, eleven unusually deep earthquakes have been reported near the southern end of the Izu‐Bonin arc (about 24°N, 142°E). These isolated events are separated from ordinary deep earthquakes in the Wadati‐Benioff (W‐B) zone of the Izu‐Bonin (I‐B) arc by a distance of about 300 km parallel to the trench axis. They also are located 200 km closer to the trench than the natural extension of the shallower earthquakes above them. Using 3‐D ray tracing, a detailed analysis 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 obtained from ordinary tomographic imaging. We find that these unusual events occurred in a high velocity zone which is connected to the northern W‐B zone of the I‐B arc. The aseismic high velocity zone has a P‐velocity at least 3% faster than the surrounding mantle. Both our result and previous evidence for a horizontally lying slab west of the W‐B zone at 27°N (e.g., Okino et al., 1989) are consistent with the existence of a prominent contortion of the subducted slab in the I‐B region.
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)