Tsunami generation by horizontal displacement of ocean bottom

TLDR

Tsunami generation is usually modeled from vertical ocean‑bottom displacement, but when the source lies on a steep slope and horizontal displacement dominates, the horizontal motion can significantly affect tsunami size. The 1994 June 2 Java earthquake, a shallow‑dipping thrust near a steep trench, and the 1994 Nov 14 Mindoro strike‑slip fault extending from ocean to land perpendicular to the coast, both involved substantial horizontal motion of the slope. These two events produced tsunamis far larger than predicted from seismic waves alone, demonstrating that horizontal slope motion is a key contributor to tsunami generation.

Abstract

Tsunami generation by an earthquake is generally modeled by water surface displacement identical to the vertical deformation of ocean bottom due to faulting. The effect of horizontal deformation is usually neglected. However, when the tsunami source is on a steep slope and the horizontal displacement is large relative to the vertical displacement, the effect becomes significant. We show this for two recent earthquakes which generated much larger tsunamis than expected from seismic waves. In the case of the 1994 June 2 Java, Indonesia, earthquake, the focal mechanism was a very shallow dipping thrust and the source was near a very steep trench slope. In the case of the 1994 Nov. 14 Mindoro, Philippines, earthquake, strike‐slip faulting extended from ocean to land perpendicular to the coast line. In both cases, we found that the horizontal motion of slope had an important contribution to the tsunami generation.