Ringing P waves and submarine faulting

Abstract

Long‐period teleseismic records from shallow focus oceanic events sometimes exhibit ‘ringing’ P waves. The ringing portion of these prolonged wave groups characteristically has a narrow band frequency content, an amplitude which monotonically decreases from values comparable to the primary P pulse, and a high station to station coherence. These traits are suggestive of leaking compressional modes of the oceanic water layer. This layer is an efficient wave guide for elastic body waves once their energy is effectively introduced into the system. Through the modeling of point and extended sources interior to oceanic crust it is found that to account for the large excitation of these modes a particular type of faulting is required. Specifically, the fault must originate in a setting such that a substantial fraction of the dislocation surface is situated above the Moho and be oriented so that a significant amount of compressional energy is propagated upward. These provisions are met by steeply dipping or thrust faults in continental margins and subduction zones where thickened oceanic crustal and sedimentary layers are commonly found. Events of this type are likely to generate perturbations in submarine topography. Hence real‐time observation of ringing P waves from large earthquakes in these geographical provinces could be diagnostic of shallow faulting with high tsunamigenic potential.