THE 1992 NICARAGUA EARTHQUAKE - A SLOW TSUNAMI EARTHQUAKE ASSOCIATED WITH SUBDUCTED SEDIMENTS

THE 1992 Nicaragua earthquake was a 'tsunami earthquake'; that is, it generated tsunamis1 disproportionately large for its surface-wave magnitude, M(s). The moment magnitude, M(w), determined from long-period (approximately 250-s) surface waves2, was 7.6, significantly larger than the 20-s M(s) of 7; this M(s)-M(w) disparity is also characteristic of tsunami earthquakes3,4. The Nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks, allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms. We conclude that the Nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the Cocos and North American plates, and because of the absence of sediments on the trench floor offshore of Nicaragua, the slip propagated up-dip all the way to the ocean bottom, exciting large tsunamis. The occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction-zone thrust earthquakes. Our results reinforce the idea that tsunami warning systems using long-period (greater-than-or-equal-to 100-s) waves5-7 are necessary to reduce the hazard from this type of earthquake.