Anomalous propagation of long-period sPL waves from an intermediate-depth earthquake in the Mariana region

Seismic waveforms are sensitive to the Earth’s structure of seismic velocity and attenuation. Unusually long-period sP waves from an intermediate-depth earthquake (Mw 7.5, Depth = 260 km) in the northern Mariana region were observed by the broadband seismic network in Japan. The dominant period ranges from 20 to 50 s, which is significantly longer than the source time function of the earthquake. On the basis of the epicentral distance range of their appearance and the prograde particle motions in the vertical‒radial plane, we identified them as sPL waves, i.e., PL waves converted from the upgoing S waves at the surface or the ocean bottom. Wavelet analysis revealed two series, the island arc type and the oceanic type, in which the dominant period of sPL waves increases linearly with epicentral distance. The propagation paths of the former type are mainly through the Izu‒Mariana arc and the Japan islands, and those of the latter are mainly through the Philippine Sea Basin. Synthetic seismograms of spherically symmetric Earth structures indicate that the difference in the linear relationships between them is explained primarily by the difference in crustal thickness. Two important results concerning the upper mantle structure are obtained: 1) a small velocity reduction (0 ~ 1%) in P waves is observed in the upper mantle beneath the Philippine Sea plate, and 2) high attenuation of S waves, \(\:{Q}_{\mu\:}=20\sim30,\) is observed in the mantle wedge above the hypocenter. The sPL waves provide useful information about the crust–upper mantle structure, independent of direct P waves and S waves.