Attenuation of Coda Waves in the Central Region of the Gulf of California, Mexico

Coda waves were analyzed from events recorded by NARS seismic network deployed along both margins of the Gulf of California, Mexico, to estimate coda attenuation Qc. Sato's (1977) single scattering model was used for a coda window of 20 to 25 s beginning at twice the S-wave travel time. Events recorded from 2003 to 2007 located in the central region of the Gulf of California were analyzed. Source-to-receiver distances are between 40 and 500 km. Assuming a power law of the form Q(c)(f) = Q(o)f(a), Q(c) values were averaged and a value of Q(0) = 83 +/- 3 and a frequency dependence a value of 1.06 +/- 0.03 in the frequency range from 1 to 7 Hz was obtained. Q(o) value and the high frequency dependency agree with the values of other regions characterized by a high tectonic activity. Based on source-station distribution two subregions (north and south) were defined. Q(c) values were calculated and correlated with tectonics and morphology of each area. The observed higher attenuation in the south region can be attributed to the fact that south region is more fractured since the greater earthquakes occur in central to south Gulf of California and the oceanic crust is reported to be thinner in the southern region.