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

The M_w 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.