On Tsunami Resonance of the Gulf of California

Abstract

The influence of the local bathymetry and the shape of the contour in the resonant response of the Gulf of California (Mexico) to incident tsunami waves of various approach directions and frequencies is analyzed. The role of the bottom topography is studied by comparing the analytical long-wave solutions for a rectangular basin of constant depth with those for a rectangular basin of variable depth adjusted to the real bathymetry. Results show that the role of the non-flat bottom topography in the response to the excitation is limited mainly to the tidal regime, where the energy is redistributed; but no substantial influence is observed in the tsunami wavelength range. The role of the contour shape is studied by analyzing the Weber's integral solutions of a long-wave inviscid linear bidimensional model for coupled basins of arbitrary shape and constant depth, modified for oblique incidence. Results show that the role of the contour shape is important in the excitation by oblique incident waves of transversal modes of oscillation through multiple boundary reflections of trapped wave energy as in a Bragg scattering resonator. The model results for the most oblique wave incidence agree reasonably with the high energy concentration at the low frequencies appearing in the spectra of two past tsunamis recorded at coastal sites on the Gulf. INTRODUCfION The Gulf of California is an elongated basin enclosed between the Peninsula of Baja California and the northwest mainland Mexico except at the southern end, where it opens to the Pacific Ocean. It is approximately 1100 Ion long, with an average width of 150 Ion. Though it has an average depth of about 729 m (Figure 1), the bottom is a structurally complex depression whose depth increases toward the southern opening. Bray and Robles (1990) have reviewed both the historical and most recent studies of the physical oceanography of the Gulf; many features of its dynamics remain unknown.