Estimation of ground motion in Xalapa, Veracruz, Mexico during the 1920 (M~6.4) crustal earthquake, and some significant intraslab earthquakes of the last century

摘要

Ground motions in Xalapa, Veracruz, Mexico, during the earthquake of January 4, 1920 (M~6.4), and three significant intraslab earthquakes (Mw7.0) of the last century were estimated. These events are reasonable scenario earthquakes for Xalapa. Towards this goal, portable broadband seismographs at nine sites in the city and an additional one at a reference hard site outside the city were deployed. Peak ground acceleration (Amax) and peak ground velocity (Amax) in Xalapa were estimated based on Brune ω-2 source model and the site effect, obtained from earthquake recordings by using the standard spectral ratio (SSR) technique, and the application of a stochastic method. During the 1920 Xalapa earthquake the estimated Amax values corresponding to a stress drop, Δσ, of 50 bar are between 100 and 250 cm/s2, except at two sites where the site effect is very large and Amax values reach 300 and 600 cm/s2. Estimated Vmax values are between 10 and 20 cm/s, except at the site with the largest site effect where it is ~ 40 cm/s. Δσ of 30 and 100 bar produce about half and twice of these peak values, respectively. The main uncertainty in the present estimations is due the Δσ value, because although a range of 30 to 100 bar for crustal earthquakes in the Trans-Mexican Volcanic Belt (in which Xalapa is located) seems reasonable, it is not constrained by the data. The mean stress drop for intraslab events, ~ 300 bar, is better constrained from previous studies. A median Amax of ~ 30 cm/s2 and a median Vmax of 4 cm/s in Xalapa during the 1973 (Orizaba) and 1999 (Tehuacán) earthquakes was estimated; the corresponding values during the 1980 (Huajuapan) earthquake are ~ 10 cm/s2 and 2 cm/s. The uncertainty in the estimation is probably within a factor of 2 to 3.The ground motion prediction equations developed from data in the forearc region with less attenuation (than the backarc region) and recorded at hard sites appear to work reasonably well for Xalapa sites, which lie in the back arc. This observation suggests that the seismic waves from intraslab earthquakes, traveling through the mantle wedge before arriving Xalapa, suffer relatively large attenuation. However, these waves get amplified due to local site effects. It seems that in Xalapa these two effects, roughly, balance each other.