Excitation of Tsunami By Horizontal Motion in Subduction Zones and Its Impact on Earthquake Source Inversions.

Abstract:

The strong development of tsunami instrumentation in the last decade now provides observations of tsunami wave propagation in most ocean basins. This evolution has lead to the wide use of tsunami data to image earthquake sources. In particular, the 2011 Mw9.0 Tohoku earthquake is the first large event with an almost complete azimuthal coverage of tsunami records. Source studies have taken advantage of these observations to demonstrate the ability of tsunami data to constrain the distribution of coseismic slip, especially in the shallow part of subductions where other standard datasets often lack resolution (e.g. onland geodetic data). This gain in resolution comes with the need to improve tsunami Green’s functions. One source of error in these Green’s functions calculations is the contribution of horizontal motion of the sea-bottom to the tsunami excitation (here referred to as “bathymetry effect”). For the specific case of the Tohoku earthquake, because of the steep bathymetry and relatively flat slab geometry near the surface, we find that the bathymetry effect can double the amplitude of the tsunami. The inversion of tsunami data to image the Tohoku earthquake slip distribution without taking into account the bathymetry effect results in slip over-prediction of 30 to 50 m over a wide area. Because the bathymetry gradient is not as strong and the slab as flat in all subduction zones, the bathymetry effect has often been overlooked in forward simulations and source inversions. In view of the importance of this bathymetry effect for the Tohoku earthquake, we extend our calculations to all the other major subduction zones to evaluate where it is indeed critical. We show that in several subduction zones, the bathymetry effect can locally double the water elevation due to a unit of slip. We discuss the global repartition of this bathymetry effect and what we know about the ability of the concerned subduction zones to generate large earthquakes and tsunami.