Abstract

The reverse propagation of a surface wave traveling through the Japanese Islands from southwest to northeast was detected for major shallow earthquakes that occurred in northeastern Japan. The wave is predominant in the transverse component, having a period of 20 sec and lasting longer than 200 sec. The propagation velocity is around 3.8 km/sec, which is characteristic of a typical surface wave. Semblance analysis using a dense seismograph network shows that the wave was generated near the Kyushu-Palau ridge southeast of Kyushu Island. Numerical simulation with a 3D finite-element method can explain the surface-wave reflection predominant in the transverse component as reflection from seamounts with a crustal root 10 km deeper than the Moho discontinuity at the surrounding ocean floor. The observed long duration of the reverse-propagating surface wave might represent the superposition of a reflected wave from other seamounts and/or the thick sedimentary layer along the Nankai trough.

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