Estimation of the spectral parameter kappa in the region of the Gulf of California, Mexico

We analyzed records from the Broadband Seismological Network of the Gulf of California (RESBAN) and from stations of the NARS-Baja array, operated by CICESE, Ensenada, Baja California, Mexico, to make estimates of the spectral decay parameter kappa (κ). This attenuation parameter is important for evaluating the seismic risk and hazard of this region. Thirteen shallow earthquakes with focal depths less than 20 km and magnitudes between 5.1 and 6.6 were selected to calculate κ and the near-site attenuation κ0. We used three different approaches to estimate κ0: (a) with individual measurements of κ from vector modulus of three-component spectral amplitudes at different epicentral distances and extrapolating to zero distance to estimate κ0, (b) with individual measurements using vertical component spectra, and (c) measuring from the high-frequency part of the site transfer function determined calculating the horizontal-to-vertical spectral ratio (HVSR) method. For most stations, the three methods give similar results. At short distances (50–60 km), κ takes values close to 0.04 s at NE76, the station located in the middle of the array. κ increases with distance taking an average value of up to 0.18 s for distances close to 500 km. κ0 at most sites is close to 0.03 s, except for GUYB (Guaymas) that has a κ0 = 0.05 s and NE83 (Navolato) with κ0 = 0.065 s, both stations located in the continent, on the eastern side of the gulf, where the soils are less consolidated. Finally, we analyze if κ0 correlates with magnitude and back azimuth, and we found that for most stations, κ0 does not correlate with either one. However, station TOPB, located on basalt, shows a moderate correlation with magnitude, with κ0 increasing with increasing MW in a short back-azimuth range. We also found that for station NE74, located on soft soil, κ0 correlates with back azimuth, having lower values for azimuths near 120°.