Regional Distance PL Phase in the Crustal Waveguide—An Analog to the Teleseismic W Phase in the Upper-Mantle Waveguide

Abstract

Dense strong ground motion observations of the shallow Mw 6.6 2004 Mid-Niigata earthquake, Japan, show a strong, moderately long-period disturbance (5–20 s) immediately following P. The associated ground motion is as large or larger as that in the S waves and surface waves. The nature of this PL phase is revealed with the aid of 3-D finite difference simulations for heterogeneous crust and mantle structures. A strong near-field contribution at the nearest stations grades into an interference packet of multiple PmP waves beyond 100 km that develops normal dispersion with prograde motion. This partially trapped P wave in the crustal waveguide loses energy by conversion to SV at the surface, and these S waves are then lost to the mantle. However, the amplification of PL is strong in the presence of near-surface low-velocity sediments since conversion to S waves is weakened. The behavior of the PL phase in the crustal waveguide at regional distances is a direct analog of the well-recognized W phase that travels in the upper-mantle waveguide. The W phase is a very long period disturbance (100–1,000 s) between P and S phases that can travel to teleseismic distances (1,000–10,000 km) as a superposition of multiple reflections from the mantle that has widely been applied for rapid source inversion for large earthquakes. The PL-W analogy gives the possibility to extend W-phase type source inversion to much smaller events, exploiting the early arrival of PL to give rapid warning of damaging long-period ground motion at regional distances carried by S and surface waves.