A major tsunamigenic earthquake occurred off the Pacific coast of Nicaragua at 18.16 local time, (00h 16m GMT) on September 2, 1992 . The initial surface wave magnitude was estimated at 7.2 and its epicenter was at 11.761° N, 87.419 West, about 120 Kms West/Southwest of the city of Managua. The magnitude was later revised to a moment magnitude (Mw 7.6.) The main earthquake was followed by several strong aftershocks. Tsunami waves of up to 10 meters reached the Nicaraguan coast.

Magnitude and Energy Release

Analysis of the seismological record showed the fault motion of this earthquake to be unusually long in duration and occurring in the top 10 km of oceanic crust - a depth much shallower than that of typical subduction-zone earthquakes. It was determined that the magnitude of this quake had been underestimated because most of the seismic stations measured only seismic waves of short period. Short period waves had not been readily produced by this quake because ot its long duration of faulting. Subsequent analysis of seismic records made with long period seismometers showed that the moment magnitude of this earthquake was larger (Mw -7.6), and the energy release to be five times greater.

Tectonic Setting

The earthquake occurred along a convergence zone of subduction that has created the Middle America Trench (MAT). This is the region in the Pacific where the Cocos tectonic plate is subducted under the western end of the Caribbean plate. It is an active zone of stress and deformation, characterized by major, shallow-focus, and potentially tsunamigenic, erthquakes. Seismic wave analysis of the 1992 event indicated that this event was much shallower than other typical subduction-zone earthquakes.

Bathymetric Map of Central America showing the Middle America Trench where the Cocos plate underthrust the western end of the Caribbean plate

The Tsunami of 2 September 1992 in Nicaragua

The earthquake of 2 September 1992 in Nicaragua generated a greater tsunami than expected from its surface wave magnitude, thus known as a 'tsunami earthquake". The unusual wave heights and greater destructiveness are attributed to the earthquake's shallow focal depth and to the slow fault rupture within subducted sediments.

The earthquake generated a very destructive local tsunami which struck along a 220 km coastal section of the Pacific coast of Nicaragua with waves of up to 10 meters in certain areas. The first of the tsunami waves reached the coast of Nicaragua about 20 minutes after the earthquake. The first wave was relatively small but the second and third waves were higher and more destructive.

Tsunami Generating Area

Based on the plate tectonics of the convergence zone that has formed the Middle America Trench, on aftershock distribution, and on the fact that the tsunami struck along a 220 km of the Pacific coast of Nicaragua, the tsunami generating area is believed to be a somewhat irregular , broken up ellipsoid. Its major axis is estimated to be approximately 220-250 km and its minor axis to be about 35-40km. Its total area is estimated to be anywhere from 8,000 to 10,000 Km2. Slip displacements are estimated to be up to 3m.