Regional path effects on seismic wave propagation in central Mexico

We present results of the analysis of a set of seismograms recorded in a temporary array of seven broadband seismic stations located in central Mexico. Our purpose was to investigate path effects on ground motion in central Mexico for earthquakes occurring in the subduction zone, along the Pacific coast. All our stations, were located on rock. Nineteen earthquakes (3.6 < M(c) < 4.9), recorded by all seven stations, were analyzed. The results show that path effects result in significant amplification and an increase of the duration of the seismic motion at two stations located in the central part of the Transmexican Volcanic Belt (TVB). Analysis of the records for periods larger than 5 sec shows that the 3D crustal structure acts as an efficient wave guide for surface waves coming from distances greater than 200 km, only for paths oriented north-south or east-west. In the band of 2-6 sec, we observe large amplification and an increase in the duration of ground motion, associated to the regional propagation of higher modes of surface waves. After correlating our results with recent geological and gravimetric studies, we suggest that this amplification could be due to the impedance contrast of low-velocity volcanic rocks of the TVB overlying higher velocity limestones. If we are right, the increase of ground-motion duration would result from mode conversion at the southern boundary of the TVB. These path effects in the 2- to 6-sec period band are very important for ground motion in Mexico City because the very soft surficial deposits in that basin amplify greatly ground motion in this same period band.