Coda wave attenuation parallel and perpendicular to the Mexican Pacific coast

We calculated the quality factor, Qc, at frequencies from 6 to 24 Hz using coda waves of 97 aftershocks of the Petatlan, Mexico, earthquake (March 14, 1979; MS=7.6). The data were recorded parallel (between Acapulco and Playa Azul) and perpendicular (between Petatlan and Mexico City) to the coast. The results are the following: at 12 and 24 Hz there is no significant difference in the attenuation (Qc−1) along the two paths; at 6 Hz, Qc−1 has a large scatter in both directions. This observation indicates strong site effects at this frequency; average Qc−1 is slightly higher between Petatlan–Acapulco (toward SE) than between Petatlan–Playa Azul (toward NW); and at high frequencies, Qc−1 remains essentially constant perpendicular to the coast. These results show that the large seismic wave amplifications in Mexico City are caused by shallow site effects.