Tsunami-Responses of San Pedro Bay and Shelf, Calif.

The lowest modes of free oscillation of the continental shelf off San Pedro Bay, CA, are determined analytically from geometric models which approximate the shelf as the sector of a cone overlying a vertical-convex circular-cylindrical continental slope. The shelf is also modeled as a sloping basin coupled with the deep San Pedro Channel between the Santa Catalina Island and the mainland. The lowest modes of oscillation of the adjacent oceanic basins most likely to affect San Pedro Bay are also determined by modeling them geometrically as open-mouth, semi-ellipsoidal (Gulf of Santa Catalina) and closed, ellipsoidal (Sant Cruz Basin to Gulf of Santa Catalina). Marigrams from several tide-stations in Los Angeles-Long Beach Harbors, which recorded the tsunamis of Nov. 11, 1922 (Chile), April 1, 1946 (Aleutian Trench), May 23, 1960 (Chile) and March 28, 1964 (Alaska), have been analyzed for their power spectra and reveal numerous peaks of high energy recurrent at the widely separated times. These periods of resonance agree quite closely with many of the calculated periods of free oscillations. Knowledge of these responses is important to harbor planning because of dangerous surge currents generated in the harbors.