Near-source, along-path, and near-site contributions to the spectral parameter kappa from earthquakes located in the central Gulf of California, Mexico

Near-source \({\kappa }_{s}\), along-path \(\widetilde{\kappa }\), and near-site \({\kappa }_{0}\) contributions to the spectral parameter kappa (\(\kappa\)) were studied from earthquakes located along the Canal de Ballenas-Guaymas fault system and recorded by stations sited around the central-north Gulf of California, Mexico. The dataset consists of 26 earthquakes (M 3.4-6.0) recorded by six stations with hypocentral distances ranging from 31 to 270 km. We used the Anderson and Hough (1984) approach to estimate \(\kappa\), followed by a one-step least-squares inversion to separate \(\kappa\) contributions. We found that \(\kappa\) has a range of 0.0260 to 0.1012 s, with a mean of 0.0600 \(\pm\) 0.0170 s, while \({\kappa }_{s}\) and \({\kappa }_{0}\) exhibit means of 0.0088 \(\pm\) 0.0059 s and 0.0200 \(\pm\) 0.0205 s, respectively. We also observed significant inter-event \({\kappa }_{s}\) and inter-station \({\kappa }_{0}\) variabilities. The \(d\widetilde{\kappa }/dr\) regional average is 0.00023 \(\pm\) 0.00001 s, equivalent to a regional quality factor of 1242 \(\pm\) 54 for an S-wave velocity of 3.5 km/s. Furthermore, our results suggest that \(d\widetilde{\kappa }/dr\) tends to decrease with distance. We demonstrate that a well-designed least-squares inversion scheme can effectively address the limitations associated with estimating \({\kappa }_{0}\) using the Anderson and Hough approach in situations where recordings per station have a narrow distance range and where no recordings at small source-site distances are available for most stations. We found no correlation of \({\kappa }_{s}\) with earthquake magnitude. Instead, relatively higher \({\kappa }_{s}\) values tend to cluster along the ridge flanks of the Canal de Ballenas Basin, where hydrothermal fluid circulation is expected.