EDGE WAVE AND NON-TRAPPED MODES OF THE 25 APRIL 1992 CAPE-MENDOCINO TSUNAMI

The 25 April 1992 Cape Mendocino earthquake generated a tsunami charac terized by both coastal trapped edge wave and non-trapped tsunami mode s that propagated north and south along the U.S. West Coast. Both obse rved and synthetic time series at Crescent City and North Spit are con sistent with the zero-order edge wave mode solution for a semi-infinit e sloping beach depth profile. Wave amplitudes at Crescent City were a bout twice that observed at North Spit, in spite of the fact that the source region was three times farther from Crescent City than North Sp it. The largest observed amplitude was due to an edge wave which arriv ed almost three hours after the initial onset of the tsunami; since su ch waves are highly localized nearshore, this suggests that the enhanc ed responsiveness at Crescent City is at least partly due to local dyn amic processes. Furthermore, the substantially delayed arrival of this wave, which was generated at the southern end of the Cascadia Subduct ion Zone, has significant implications for hazard mitigation efforts a long the entire U.S. West Coast. Specifically, this study demonstrates that slow-moving but very energetic edge wave modes could be generate d by future large tsunamigenic earthquakes in the CSZ, and that these might arrive unexpectedly at coastal communities several hours after t he initial tsunami waves have subsided.