VERY LOW FREQUENCY WAVE PROPAGATION NUMERICAL MODELLING: APPLICATION TO T-WAVES PROPAGATION

摘要

The submarine seismic and volcanic activity in the ocean generate a large amount of low-frequency (< 40 Hz) acoustic waves that propagate in the water column over very large distances (> 1000 km). The purpose of this paper is to test the ability of the SPECFEM2D numerical model [1] to generate and simulate the long-distance propagation of such low-frequency acoustic waves and to compare the results with actual data recorded in the Atlantic Ocean. Seismic waves convert into acoustic waves at the sea-bottom, which in turn propagate in the water column and, at low frequencies (0-40Hz), can be carried over very long distances with little attenuation. This is due to the efficient propagation properties of the Sound Fixing and Ranging (SOFAR) channel, a low-sound velocity layer present in most of the oceans. Earthquake generated acoustic waves in the ocean are referred to as T-waves (or tertiary waves), since in certain conditions, when they reach the shore, they may convert back to seismic waves and arrive third after the P- and S-seismic waves, on near-shore seismological stations. Acoustic waves travel at 1500 m/s in the ocean whereas seismic P- and S-waves travel at velocities from 3700 to 5000 m/s in the crust. The first documented record of T-waves dates from 1927 [2] but the origin of these late seismic arrivals was understood a decade later [3]. Hydrophones, originally deployed to detect ships and submarines or to monitor biological sounds, are now currently used to record T-waves for monitoring the seismic and volcanic activity of the sea-floor (e.g. [4-6]).