Impact of Ground Effects for an Appropriate Mitigation Strategy in Seismic Area: The Example of Guatemala 1976 Earthquake

Guatemala is one of the most seismically active countries of Central America. It has suffered several dramatic earthquakes during the last century, the most destructive of which was the event associated with the Motagua fault, occurred on 4, February 1976, that caused 23,000 deaths, and 77,200 injuries. Noteworthy were also the coseismic geological effects, either directly linked to the earthquake source, such as a remarkable displacement along a 230 km long strike-slip fault, or caused by the ground shaking, such as 50,000 landslides, numerous liquefaction phenomena, ground cracks and ground deformation over an area of about 18,000 km2. In spite of this, the estimated intensity was relatively low (Imax = IX MM, only in few localities). For this reason, in order to achieve a better definition of seismic hazard, we have conducted a re-evaluation of the 1976 earthquake taking into account the environmental effects that have contributed substantially to the dramatic level of recorded impact, both in terms of loss of human life and damage affecting housing and infrastructures. The analysis of primary and secondary effects has permitted a better evaluation of seismic intensity in 24 localities that were hit by the earthquake and allowed for establishing a new epicentral Intensity, characterized by XI ESI. The recalculated seismic intensity is consequently two degrees higher than previous estimates and definitely more consistent with the scenario of the actual damage caused by the earthquake.