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.