Unusual Strong Ground Motion Across Japan From the 680 km Deep 30 May 2015 Ogasawara Islands Earthquake

Abstract

AbstractThe Mw 7.9 earthquake beneath the Ogasawara Islands on 2015 May 30 at 680 km depth (about 100 km deeper than other seismicity in the vicinity) produced significant shaking over a broad area of Japan in the epicentral distance range 1,000–2,000 km. Usually, deep earthquakes in the subducting Pacific slab develop a band of large ground motion along the east coast of northern Japan, due to efficient guiding of high‐frequency (>1 Hz) waves in the subducting slab. However, for this very deep earthquake, the large ground acceleration arises from relatively low‐frequency (<1 Hz) S wave pulses with a long tail of long‐period (<0.1 Hz) signals. The arrival of the slab‐guided high‐frequency signal was very late, and weak, compared with normal Pacific slab events. Numerical simulation of seismic wave propagation reveals that the observed low‐frequency pulses are the direct S wave from the very deep source followed by a large SV‐to‐P conversion at the free surface with near‐critical S wave incidence. These phases produce a long‐period shear‐coupled PL (S‐PL) wave by constructive interference between wide‐angle P and S reflections in the crust and incoming S wave to the crust. The S‐PL wave travels several hundred kilometers at regional distances with reinforcement from continuing S wave input from the very deep source. Such an unusual observation of strong low‐frequency ground motion was due to its very great source depth, outside the main slab, so the high‐frequency Pacific‐slab guided wave was weak.