The 6 September 1997 (Mw4.5) Coatzacoalcos-Minatitlán, Veracruz, Mexico earthquake: implications for tectonics and seismic hazard of the region

Abstract

We analyze the 6 September 1997 M _w 4.5 earthquake, which occurred about 25 km SE of Coatzacoalcos, Veracruz. The earthquake was recorded by the local broadband station 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 M _w 6.4 earthquake occurred on 26 August 1959 at a similar depth and with a similar mechanism. The analysis of the 1997 event reinforces a previous conclusion that the lower crust beneath the Comalcalco basin is in a thrust-faulting stress regime, in contrast to the shallow part of the crust, which is characterized by normal-faulting; this implies a permutation with depth of the maximum and minimum principal stresses. It agrees with observations elsewhere that the state of stress in sedimentary basins can be different from the one at greater depth.
Focal mechanisms are available for seven earthquakes in and near the Gulf of Mexico. All of these events indicate a thrust-faulting type stress regime at mid- and lower-crustal levels. The observed trend of the P axes of these earthquakes can 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 loading.
By using the recordings of the 1997 event as empirical Green's function, we simulate the ground motions in the epicentral region of a postulated M _w 6.4 earthquake in the Comalcalco basin. Under reasonable assumptions, the expected peak acceletion, velocity and displacement are 120-260 gal, 12-28 cm/s, and 6-11 cm, respectively. The extensive soil liquefaction in Coatzacoalcos during the 1959, M _w 6.4, earthquake suggests that the sediments of the basin behave nonlinearly under such excitation.