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

A deep earthquake (Mw 7.9) beneath the Ogasawara Islands, occurring at an unusual depth of 680 km, produced strong ground motion across Japan with distinct low-frequency characteristics unlike typical subducting slab earthquakes. Unlike conventional deep earthquakes that generate high-frequency waves guided by the Pacific slab, this event primarily radiated low-frequency S wave pulses (<1 Hz) and long-period signals (<0.1 Hz), resulting in significant shaking at regional distances (1,000–2,000 km). Numerical simulations revealed that these low-frequency waves originated from direct S waves and SV-to-P conversions at the free surface, forming a shear-coupled PL wave reinforced by continuous S wave input. The weak high-frequency slab-guided signal was attributed to the earthquake's exceptional depth, outside the main subducting slab, highlighting the unique seismic behavior of very deep earthquakes.

Furumura, T., & Kennett, B. L. N. (2017). Unusual Strong Ground Motion Across Japan From the 680 km Deep 30 May 2015 Ogasawara Islands Earthquake. Journal of Geophysical Research: Solid Earth, 122(10), 8143–8162. Portico. https://doi.org/10.1002/2017jb014519