ACOUSTIC-GRAVITY WAVES FROM SUBMARINE EARTHQUAKES - TOWARDS AN EARLY TSUNAMI DETECTION SYSTEM

Abstract:

An uplift of the ocean bottom caused by a submarine earthquake can generate Acoustic-Gravity Waves (AGW), progressive compression-type waves that travel at near the speed of sound in water. Recent studies indicate that as AGW travel they leave measurable bottom pressure signatures, which can act as tsunami precursors. In this regard, it is anticipated that such utilization of AGW would enhance current early tsunami detection systems. To this end, there is an increasing need to characterize the spatio-temporal evolution of the pressure field induced by AGW in more realistic scenarios.

We analyze and simulate the fundamental AGW modes generated by the 2004 Indian Ocean earthquake. We consider the first five AGW modes and show that they may all induce comparable temporal variations in pressure at different water depths in regions far from the epicenter. An example for the dynamic pressure induced by AGW is given in Figure 1.

We show that the pressure field depends on the presence of the leading AGW modes. Each AGW mode becomes evanescent at a critical time, at which energy is transferred to the next higher modes. Consequently, the frequency associated with the most energetic mode changes as the leading mode varnishes. Correspondingly, the main pattern of the pressure field changes as the leading mode change. As an example, for a reference point located at 1000 Km from the epicenter, and 4km deep, the first five AGW become evanescent after 1.6, 4.6, 7.7, 10.8 and 13.8 hours, respectively.

Our analysis and simulations shed light on the spatio-temporal evolution of the pressure field induced by AGW that radiate during submarine earthquakes. Practically, this can assist in the implementation of an AGW early tsunami detection system, starting from applying the appropriate earthquake models, to identifying the relevant measurement equipment and their optimal locations.