S wave attenuation in the coastal region of Jalisco-Colima, Mexico

Aftershock data from the October 9, 1995, M = 7.6 earthquake, that occurred in the coastal subduction region of Colima-Jalisco, Mexico, are used to ob tain estimates of the frequency independent quality factor Q(s) and spectra l decay parameter kappa, in the approximate frequency range 3 Hz less than or equal to f less than or equal to 40 Hz, as functions of hypocentral dist ance r. A regression analysis results in the relations for S waves: Q(s) = 261.397 + 3.198 r +/- 15.536 kappa. = 0.009651 + 0.000462 r +/- 0.0012. The observed distance dependence of Q(s) and kappa is similar to that repor ted for the Mexican regions of Oaxaca and Guerrero. Our Q(s) values agree w ith those reported for Oaxaca and northern Baja CA for frequencies around 8 Hz, and are somewhat lower (a factor of similar to 0.8 to similar to 0.5) for higher frequencies around 20 Hz; however, they are significantly lower than all but the lowest values reported for the Guerrero region, attaining factors similar to 0.25 and similar to 0.13 for frequency ranges around 8 a nd 20 Hz, respectively. It thus appears that the Guerrero region has an ano malously low attenuation compared with the flanking coastal regions of Oaxa ca-Chiapas to the SE and Jalisco-Colima to the NW. (C) 1999 Elsevier Scienc e B.V. All rights reserved.