The Significance of Long-Period Ground Motion at Regional to Teleseismic Distances From the 610-km Deep M_w 8.3 Sea of Okhotsk Earthquake of 24 May 2013

The 24 May 2013 earthquake beneath the Sea of Okhotsk (610 km, M_w 8.3) produced significant ground motion across the whole span of the Japanese islands, from 1,300- to 4,200-km epicentral distance. The largest shaking was concentrated along the back-arc side of the subduction zone, which is the opposite of the normal pattern for deep earthquakes in the Pacific slab. Observations from the dense Hi-net and F-net arrays across Japan show that the largest shaking in northern Japan (near 2,000-km epicentral distance) was caused by near-caustic S waves, with triplication of upgoing and downgoing waves from the deep source and reflected waves from the 660-km discontinuity. Three-dimensional finite difference method simulations confirm that the antiwaveguide effect of the high-wave speed slab is to push the zone of larger intensity 300 km farther to south than might be expected. The S wavefront distorted by the slab has near-critical incidence at the free surface producing large sP and generating shear-coupled PL (s-PL) waves with period >3 s. With increasing epicentral distance the S incident angle exceeds critical, then total sS reflection creates large ground motion at large distance (>3,000 km) and even farther (>6,000 km) with sSS. The propagation of sS, sSS linking to sS-PL, and sSS-PL wave trains is very efficient in continental structures with thicker crust. The felt reports at large (4,000–8000 km) distances from the 2013 Sea of Okhotsk earthquake can be explained by lengthy, long-period ground motion in the continental environment with amplification in sedimentary basins and in tall buildings.