Seismic structure around the Nicaragua megathrust fault and the rupture characteristics of the 1992 tsunami earthquake

Abstract

The mega-thrust fault zone is a complex system where seismically stable, unstable and conditionally stable areas coexist. The factors controlling seismic rupture are poorly understood, especially in the frontal part of the margin wedge, key to understand tsunami generation. In this work we investigate the relationships between upper plate structure and properties and earthquake rupture characteristics. This hypothesis is tested for the particular case of Nicaragua margin, where a Mw~7.2, slow tsunami earthquake took place in 1992. We present 2D seismic velocity (Vp) and inter-plate geometry models obtained by joint refraction and reflection travel-time inversion of OBS data, together with coincident multichannel seismic reflection images of the overriding plate and the inter-plate boundary. We show that the Vp at the base of the igneous upper plate increases by over 40% from near the trench to the downdip limit of the seismogenic zone, and this increase is best explained by a rock fracturing decrease of 15% to 0%. The fracturing degree of the upper plate near the trench is below the critical limit for fractured basalts, meaning that the highly-fractured upper plate can still store elastic energy to be released during large earthquakes. We show that the rigidity variations inferred from the Vp at the base of the upper plate agree with those required to explain the large co-seismic slip near the trench and slow rupture propagation velocity of the 1992 earthquake and tsunami. The trench-ward reduction in upper plate rigidity, combined with the increased normal stress caused by local asperities such as subducted seamounts, can explain the anomalous tsunamigenic character of this earthquake. In summary, this work shows that modern active seismic data can provide essential information which, combined with seismological and tsunami data, can help to understand the rupture characteristics of megathrust earthquakes