W phase

Abstract

The recent Nicaragua tsunami earthquake (September 2, 1992) produced a distinct ramp‐like long‐period (up to 1000 sec) phase which begins between P and S waves on displacement seismograms. In terms of ray theory, this phase consists of long‐period P, PP, S, SS, SP, PS, etc and its propagation mechanism is similar to that of a whispering gallery. In terms of normal‐mode theory, it represents a group of higher‐mode Rayleigh waves with a group velocity close to, but slower than, that of P wave. This phase has not been recognized as a distinct phase in the seismological practice because of clipping of seismograms for very large earthquakes. With the advent of modern wide‐dynamic range seismographs, this phase can be easily identified for all large earthquakes. In view of its use for identifying slow earthquakes, determining whether slow deformation is precursory or coseismic to the regular short‐period energy release, and determining velocity structures between the source and the station, we propose that this phase be called the “W phase”.