The elastic properties of the upper plate could determine the tsunamigenic character of the 1992 Nicaragua tsunami earthquake

Abstract

We present 2D seismic velocity models and coincident multichannel seismic reflection images of the overriding plate and the inter-plate boundary in the rupture area of the Mw~7.2, 1992 Nicaragua tsunami earthquake. The trench-perpendicular profiles run over a subducting seamount located at ~20 km from the trench, just where seismological observations and tsunami run-up models predict a patch of large co-seismic slip. The bulk of the upper plate is made of igneous rock, and its increasing P-wave velocity with depth reflects a progressive decrease in the degree of rock fracturing. The velocity variation indicates that fracturing must increase from ~0% at the upper end of the shallow mantle wedge, located ~10 km beneath the fore-arc Sandino basin, to ~10% above the seamount. The inter-plate seismogenic zone, as defined by the aftershocks of the 1992 earthquake, extends from near the mantle wedge to near the trench. This indicates that the highly-fractured upper plate may still store elastic energy that is released during large earthquakes. We show that the slow rupture propagation velocity and low rigidity required to explain the characteristics of the 1992 earthquake and tsunami, agree with those inferred from the P-wave velocity in a rock-volume around the plate boundary zone. We hypothesize that the trench-ward reduction in upper plate rigidity, combined with the increased normal stress by local asperities such as subducted seamounts, may determine the anomalous tsunamigenic character of this, and maybe other, earthquakes