Abstract

SINGH, Shri Krishna et al. The 6 September 1997 (Mw 4.5) Coatzacoalcos-Minatitlan, Veracruz, Mexico earthquake: implications for tectonics and seismic hazard of the region. Geofís. Intl [online]. 2015, vol.54, n.2, pp.191-199. ISSN 2954-436X.  https://doi.org/10.1016/j.gi.2015.04.014.

We analyze the earthquake of 6 September 1997, which occurred about 25 km SE of Coatzacoalcos, Veracruz. The earthquake was recorded by the local broadband station of TUIG (S-P time = 5 s). P-wave polarities at regional and teleseismic distances, and modeling of the displacement waveforms at TUIG yields a thrust-faulting focal mechanism (ϕ = 150°; δ = 70°; λ = 90°). In the same region a destructive earthquake occurred on 26 August 1959 (Mw 6.4) at a similar depth and with a similar mechanism. The analysis of the 1997 event reinforces a previous conclusion that the lower crust of the coastal plains of Gulf of Mexico, in the Coatzacoalcos-Minatitlan basin, is in compressive stress regime; this is in contrast to the shallow part of the crust which is characterized by normal-faulting. It agrees with observations that suggest that the state of stress in sedimentary basins may differ from the one at greater depth. Focal mechanisms are available for seven earthquakes in and near Gulf of Mexico. All of these events demonstrate a mid- and lower-crust in the region under compressive regime. Trend of P axes of these earthquakes may be explained by one or more of the following causes: strong coupling along the subduction plate interface offshore Tehuantepec, absolute motion of the North American plate, and downwarping of the lithosphere due to sediment accumulation.

Using the recordings of the 1997 event as empirical Green's function, we simulate ground motions in the epicentral region of a postulated Mw 6.4 earthquake in the Coatzacoalacos- Minatitlan basin. Under reasonable assumptions, the expected PGA , PGV, and PGD are 120-260 gal, 12-28 cm/s, and 6-11 cm, respectively. The extensive liquefaction reported in Coatzacoalcos during the 1959, Mw 6.4, earthquake suggests that the sediments of the basin behave nonlinearly under such excitation.

Keywords : Jaltipan earthquake; strong motion; tectonic of the Gulf of México; seismic hazards of the Gulf of México.

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