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

Abstract

The 24 May 2013 earthquake beneath the Sea of Okhotsk (610 km,Mw8.3) producedsignificant ground motion across the whole span of the Japanese islands, from 1,300‐to 4,200‐kmepicentral distance. The largest shaking was concentrated along the back‐arc side of the subductionzone, which is the opposite of the normal pattern for deep earthquakes in the Pacific slab. Observationsfrom the dense Hi‐net and F‐net arrays across Japan show that the largest shaking in northernJapan (near 2,000‐km epicentral distance) was caused by near‐causticSwaves, with triplication ofupgoing and downgoing waves from the deep source and reflected waves from the 660‐km discontinuity.Three‐dimensionalfinite difference method simulations confirm that the antiwaveguide effect of thehigh‐wave speed slab is to push the zone of larger intensity 300 km farther to south than might beexpected. TheSwavefront distorted by the slab has near‐critical incidence at the free surface producinglarge sP and generating shear‐coupled PL (s‐PL) waves with period >3 s. With increasing epicentraldistance theSincident angle exceeds critical, then total sS reflection creates large ground motion atlarge distance (>3,000 km) and even farther (>6,000 km) with sSS. The propagation of sS, sSS linking tosS‐PL, and sSS‐PL wave trains is very efficient in continental structures with thicker crust. The feltreports at large (4,000–8000 km) distances from the 2013 Sea of Okhotsk earthquake can be explained bylengthy, long‐period ground motion in the continental environment with amplification in sedimentarybasins and in tall buildings. Show more