The influence of megathrust depth-varying elastic properties on the rupture of the 1992 Nicaragua tsunami earthquake

Over the last decades researchers have emphasized on the important role that elastic properties - particularly rigidity - and structural features play in the "anomalous" shallow rupture of tsunami earthquakes. Yet, we have little understanding on the depth-distribution of such properties and how they influence the rupture of these particular events.

Here, we focus on the rupture zone of the 1992 Nicaragua tsunami earthquake (Mw ~7). This event occurred ~ 20 km down-dip of the megathrust interface but it released large seismic moment near the trench, triggering a devastating tsunami. We analyze the elastic and structural properties of the rupture area from a set of 2D coincident wide-angle (WAS) and multichannel seismic (MCS) lines acquired across and along the rupture area of the 1992 event. We have jointly inverted refracted and reflected travel-times (TT) from both WAS and MCS data to derive P-wave velocity ( Vp ) information of the overriding plate and the geometry of the megathrust. Combining MCS TTs of reflected arrivals at the megathrust with the ensemble of WAS TTs allowed to enhance the tomographic resolution of our results. This way, we are able to retrieve small-scale topographic variations of the plate interface that would have been omitted with the inversion of WAS TT alone. From Vp we derive other elastic parameters namely, density, S-wave velocity and rigidity using wel l -established empirical relationships.

The results obtained from two parallel sections across the megathrust show that, despite minor local variations, the down-dip distribution of megathrust elastic properties does not present significant variations, sharing a similar trend with depth. The depth-dependence of these properties is consistent with the along-dip distribution of megathrust elastic properties inferred from many other subduction zones of the Pacific, Indonesia, and the Caribbean. We finally use the inferred down-dip distribution of rigidity to compute the slip and the rupture time of the 1992 event. The results agree remarkably well with the estimates of rupture times derived from the inversion of surface waves spectra of the event. These results demonstrate that the along-dip variation of rigidity in the plate interface is key in the up-dip rupture of megathrust earthquakes, particularly, that of tsunami earthquakes.