It is believed that the ellipsoid type of block movement occurred along an oblique but very shallow subduction angle. According to geologic studies of the offshore Nicaragua region - and in contrast to the northern belt of convergence, crustal motions near Nicaragua are accomodated by "bookshelf" faulting that may have an overall northwest-trending direction that parallels the trench, but may also include northeast-striking left-lateral faults. Additionally, most of the block motions occurred wihin subducted sediments with a great deal of shear which must have been the factor s responsible for the slower rupture velocity. Such a mechanism would explain the long duration of the rupture which lasted 100 seconds, and the sediment contribution to tsunami excitation, that characterizes this event as a "tsunami earthquake".
Historical Tsunamis in the Region
The historic tsunami record shows that, prior to 1992, two other major earthquakes along this convergence zone generated two destructive tsunamis along the western coast of Central America. On 5 August 1854 an earthquake with an estimated magnitude of 7.3 generated a tsunami that struck Costa Rica. On 26 February 1902 an earthquake of magnitude 7.0 generated a tsunami that was extremely destructive in El Salvador and was responsible for more than 185 deaths.
Effects of the 2 September 1992 Tsunami in the Immediate Area - Deaths and Damages
There was major damage to coastal towns and villages from San Juan Del Sur to Masachapa. At least 70,000 people living in the coastal areas of 27 communities s were affected by the tsunami.
El Transito - Tsunami runup ranged from 6 - 9.9m. The maximum runup of 9.9m, occurred in the central part of the inundation area near El Transito. This extreme may be partially due to reflection of the waves. The waves were particularly damaging at El Transito, a small town of about a thousand people located about 55 km W of Managua. The town faces the Pacific openly without any breakwater or bank. It was the closest town to the earthquake's epicenter. According to local reports, waves of up to 9.9 meters destroyed most of the houses of this town (about 200), killed 16 people (14 children and injured 151. Fortunately, the first wave was not very large and people had time and managed to escape the second and third waves that were larger.
Masachapa - Hard hit was the town of Masachapa where the tsunami waves penetrated 300 to 400 meters inland. Nine people lost their lives there
El Popoyo - The maximumum wave height at El Popoyo was 5.6 meters. The waves caused extensive damage and killed 15 people.
The most affected coastal communities by the 2 September 1992 tsunami in Nicaragua were also La Boquita, Casares and Huehuete . Other towns that sustained loss of life and damage to property were San Martin, Pochomil and Poneloya. Sixty percent of all houses were destroyed in San Juan Del Sur, a small town with 13,000 people. At Corinto, the tsunami caused extensive damage to food and fuel storage facilities. The town was partially protected by a large seawall . Fresh water wells were flooded by sea water. There were no deaths reported from other Central American countries but waves of 2 to 4 meters damaged harbors and boats in Costa Rica.
According to the report a total of 116 people lost their lives, 63 were missing, 489 were injured and 13,000 were left homeless. The total death toll was later revised to 170 people. The tsunami was responsible for the destruction of most of Nicaragua's Pacific fishing fleet. Total property losses were estimated at around $ 25 million.
The 2 September 1992 event indicates that the potential tsunami hazard for the Central America region has been underestimated. Major tsunamigenic earthquakes occur primarily near the Middle America Trench along the intra-oceanic convergent margins of the Cocos and Caribbean plates - a zone of active, oblique, shallow subduction. The quantity and thickness of subducted sediments along certain sections of these margins can alter seismic focal mechanisms and the geometry and velocity of ruptures. Certain large earthquakes, along such zones, can be expected to be "slow" - that is to have lower rupture velocities and to generate seismic waves of longer periods. If measured with short period seismic instruments, the energy release of such major earthquakes will always be underestimated. The greater tsunamigenic potential of such earthquakes -along such zones of oblique shallow subduction - is not only the result of greater energy release but of the contribution of subducted sediments to the geometry of ocean floor deformation and volumes of displacements. "Bookshelf" faulting of subducted sediments, as that which characterizes the offshore area of Nicaragua (and which probably occurred during the 1992 earthquake) can generate the so-called "tsunami earthquakes" that can be potentially more catastrophic.
