The 1976 Guatemala earthquake case study: A contribution to the seismic hazard evaluation

Guatemala has been repeatedly rocked by violent earthquakes in the last century. February 4th, 2016 recur forty years by the disastrous earthquake of the 1976, the most destructive seismic event associated with the Motagua fault (M=7.5), that caused 23,000 deaths, 77,200 injuries and produced severe damage over a wide area of the country. The coseismic geological effects were a 230 km long strike-slip fault with remarkable displacement and also 50.000 induced landslides, numerous liquefaction phenomena, ground cracks, ground deformation over an area of about 18.000 km2. In spite of this, the estimated intensity was relatively low (Imax= IX MM, in few localities). The aim of this study is to analyse the primary and secondary earthquake ground effects, in order to assess the local intensities according to the ESI scale 2007 (Michetti et al., 2007) thus drawing the new macroseismic field. The analysis of environmental earthquake effects has permitted the evaluation of the ESI intensity in some localities, and to establish a new epicentral Intensity, characterized by Io=XI ESI. The new macroseismic field shows a direct correlation with the fault zone at the surface, infact the general pattern of ESI isoseismal lines is compatible with the rupture mechanism of the Motagua fault and its propagation from ENE to WSW. The recalculated seismic intensity is hence two degrees higher than previous estimates and definitely more consistent with the scenario of the damage caused by the 1976 Guatemala earthquake allowing in the future, a better and more realistic definition and evaluation of seismic hazard of the Guatemala. Michetti A.M., et al.,(2007). Intensity Scale ESI 2007. Mem. Descr. Carta Geol. d'Italia. 74 Porfido S., et al. (2015) Impact of ground effects for an appropriate mitigation strategy in seismic area: the example of Guatemala 1976 earthquake. Springer -DOI: 10.1007/978-3-319-09057- 3_117