Shear‐Wave Attenuation Study in the South Region of the Gulf of California, Mexico

We study the variability of the quality factor Q(S) with depth (1D model) using S-wave recordings from regional stations of the Broadband Seismological Network of the Gulf of California (GoC) (RESBAN) operated by Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE). We analyzed earthquakes located by Sumy et al. (2013) in the southern gulf with hypocenters determined using ocean-bottom seismographs of the Sea of Cortez Ocean Bottom Array (SCOOBA) experiment and onshore stations of the Network of Autonomously Recording Seismographs (NARS)-Baja array. We also used events relocated by Castro et al. (2017b) in the same region. This catalog of seismicity within the North America-Pacific plate boundary permits us to study the attenuation characteristics of the S waves in greater detail than previous studies, because the new data set samples more densely the gulf region. We determined nonparametric attenuation functions in the 10-250 km distance range to estimate Q(S) at different frequencies and depths. To estimate Q(S), the geometrical spreading function G(r) is taken as 1/r for r < 100 km and 1/root r for r >= 100 km. We found that at 4 Hz Q(S) similar to 770 for the first 5 km, increasing to similar to 1200 up to 10 km depth, where Q(S) starts to gradually decrease, reaching a value of Q(S) similar to 970 between 25 and 40 km depth. This increase in attenuation at 10-40 km depth is consistent with estimates of Q(S) near the East Pacific Rise (Yang et al., 2007). In general, our average estimates of Q(S), which include spreading centers and basins, are higher than those reported in other ridge zones. This indicates that zones of high attenuation in the GoC may be restricted to regions near the spreading centers. We also used the function G(r) = 1/r, which corresponds to the dominance of body waves at all distances, and we found that at low frequencies (f < 4 Hz) this function gives similar values of Q(S) as those estimated with the spreading function that introduces a weaker decay with distance at r > 100 km.