A collaborative research project between Universidad Nacional Autónoma de México (UNAM) and Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE) about the major 1887 earthquake in the Basin and Range Province of northeastern Sonora and the persisting background seismicity in its epicentral region yielded numerous results that are summarized in this paper. Three first-order range-bounding north-south normal faults (from south to north: Otates, Teras, and Pitáycachi) ruptured in this earthquake as evidenced by a multi-segment surface rupture with the maximum vertical displacement of 5.2 m and a length of 102 km, which is the longest extensional dip-slip surface rupture in the worldwide historical record. Empirical scaling relations between surface rupture length and moment magnitude for normal faults estimate a magnitude Mw of 7.5 ± 0.3 for this earthquake. The rupture was arrested in the north as well as in the south at major cross faults, and the rupture displacement profile tapers rapidly toward these faults. The 1887 rupture is characterized at the surface and at focal depth in the mid-to-lower crust by extensional dip slip on 55° to 72°W dipping faults. The rupture kinematics and epicenter of the 1887 earthquake can be inferred from the bilateral branching pattern observed along the trace of the Pitáycachi segment. The focal mechanisms and the inversion of slickenlines exposed on the 1887 surface rupture indicate a normal-fault-type tectonic strain field with an ENE-WSW oriented regional extension. The long-term geological slip rates of the faults that ruptured in 1887 is estimated at 0.02 to 0.08 mm/yr, and the average repeat time of 1887 size ruptures at 15 to 42 kyr. A regional seismic network, Red Sísmica del Noreste de Sonora (RESNES), was installed within this project in the epicentral region of the 1887 earthquake. The network consisted of nine digital autonomous seismological stations and operated from 2002 to 2011. Its recordings indicate ongoing microseismicity not only along the faults that ruptured in 1887, but also along two major Basin and Range Province normal faults located farther south, in the Granados-Huásabas region. In addition to the parametric data of the seismicity distribution, the high quality of the RESNES digital recordings permitted the characterization of the regional seismic attenuation and a spatial analysis of the seismic energy radiated by the recorded earthquakes. A model of the change in static Coulomb stress throughout this region resulting from the slip on the three individual segments of the 1887 rupture can explain the long-term seismicity pattern in the area. The model suggests that stress increases caused by singularities at the tips of the three rupture segments triggered seismicity in the step-over between the northern and central segments of the surface rupture, as well as in the Granados-Huásabas region, to the south of the documented surface rupture. Furthermore, the model explains the moderate earthquakes that occurred after the 1887 event in the region of the neighboring Fronteras fault. According to the model, these were the result of bending of the hanging-wall blocks of the Pitáycachi and Teras segments.

ABSTRACT We investigate the radiated seismic energy of P and S waves in the north-central region (27.5–31.5° N) of the Gulf of California (GoC), Mexico, from 83 earthquakes recorded at stations of the Broadband Seismological Network (RESBAN) of the GoC, Mexico, and the Mexican National Seismological Service for the period 2015–2021. The earthquakes occurred within the limits of transform and normal faults at depths between 10 and 20 km and had moderate magnitudes (3.4

We used accelerograms from foreshocks recorded by the closest strong-motion stations to the epicentral location of the 2010 El Mayor-Cucapah earthquake (Mw7.2) to estimate S-wave near-source attenuation and to investigate the spatial–temporal evolution of the spectral decay parameter kappa (κ). We found that κ estimated from the foreshocks has significantly higher values compared with those estimated using the mainshock recordings. Since κ is inversely proportional to the quality factor Q and this may vary depending on the state of stress and the presence of fluids, this observation indicates that Q was higher in the epicentral region during the mainshock rupture process, probably due to a higher concentration of stress. We calculated the average regional S-wave attenuation κ(r) before and during the mainshock using a nonparametric approach, and we also found higher attenuation before the occurrence of the main event, suggesting a possible role of fluid flow in the rupture process of the main rupture. Before and during the mainshock κ(r) increased with increasing hypocenter distance, but at short distances (r < 17 km) κ(r) increased faster before the main shock. However, during the main rupture κ(r) increased faster than during the foreshock sequence for r > 17 km, suggesting that the tectonic stress probably decreased beyond that distance. 17 days before the mainshock the near-source attenuation ( κ_s ) was very low in the ruptured area, κ_s=0.0374 s, increasing during the next 30 h to κ_s= 0.0490 s, then κ_s started to decrease to the value of κ_s=0.0024 s during the occurrence of the main event. We interpret this decrease in κ_s as due to increasing Q resulting from an important increase of tectonic stress before the occurrence of the Mw7.2 earthquake. We conclude that κ, in combination with other geophysical parameters, it is useful to understand the preparatory phases of the earthquake rupture process.

We used seismic records from the Central Italy seismic network to estimate the S-wave quality factor Q in the rupture area of the 6 April 2009 L'Aquila (Mw 6.1) mainshock and seven foreshocks with local magnitudes ML 3.2-4.3 that occurred the week preceding the main event. We separated the attenuation along the source-station path by calculating individual nonparametric spectral attenuation functions for 21 frequencies between 0.20 and 25 Hz for each foreshock and the mainshock. Then, we estimated Q in the rupture zone using the omega 2 source model as a reference. The rate of amplitude decay of the average source-station path attenuation functions is approximately the same for all the events except for two foreshocks located southwest of the main shock. We found that nearby the rupture-zone Q increases with frequency from 0.27 to 30.63 for the mainshock, and from 0.01 to 3.0 for the foreshocks. These low values of near-source Q obtained suggest the likely presence of fluids in the rupture areas of the foreshocks and the mainshock zone of the 2009 L'Aquila seismic sequence.

We investigate temporal and spatial variations of the spectral decay parameter kappa (kappa) before and after the 6 April 2009, L'Aquila earthquake (Mw 6.1), in Central Italy. We analysed foreshocks 10 days before and aftershocks occurring 10 days and 6 months after this main event. We select earthquakes with magnitudes M w >= 3.2 registered by the seismic network of Central Italy within a radius of 20 km from the epicentre of the L'Aquila main shock and having hypocentre distances of less than 170 km. We separate near-source, along-path and near-site contributions of kappa for each group of events and we detected temporal variations of this S-wave attenuation parameter. We find that 10 days before the main shock kappa along the path has the lowest values, probably due to high tectonic stress accumulated, in agreement with previous investigations performed with other techniques, then kappa increases during the main event and remains constant during the first 10 days of aftershocks. The aftershocks that occurred 6 months after show an increase in the regional attenuation probably due to the tectonic stress released during the main shock and the earlier aftershocks. From the spatial point of view, 10 days before the principal event the foreshocks located to the south show an increase in the near-source attenuation towards the northeast, in the direction of the main shock. These spatial variations of kappa may be related to the presence of crustal fluids near the rupture area, as evidenced by other previous studies. The first 10 days of aftershocks that concentrate around the main earthquake have high near-source kappa , and those located north of the main rupture have lower values. These observations are consistent with previous investigations that show variations of elastic and anisotropic crustal properties during the L'Aquila earthquake sequence due to dilatancy and fluid diffusion processes within the nucleation zone. We conclude that temporal variations of the spectral decay parameter kappa provide important clues for the earthquake cycle in Central Italy.

Near-source κ_s , along-path κ , and near-site κ_0 contributions to the spectral parameter kappa ( κ ) 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 κ , followed by a one-step least-squares inversion to separate κ contributions. We found that κ has a range of 0.0260 to 0.1012 s, with a mean of 0.0600 ± 0.0170 s, while κ_s and κ_0 exhibit means of 0.0088 ± 0.0059 s and 0.0200 ± 0.0205 s, respectively. We also observed significant inter-event κ_s and inter-station κ_0 variabilities. The dκ/dr regional average is 0.00023 ± 0.00001 s, equivalent to a regional quality factor of 1242 ± 54 for an S-wave velocity of 3.5 km/s. Furthermore, our results suggest that dκ/dr tends to decrease with distance. We demonstrate that a well-designed least-squares inversion scheme can effectively address the limitations associated with estimating κ_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 κ_s with earthquake magnitude. Instead, relatively higher κ_s values tend to cluster along the ridge flanks of the Canal de Ballenas Basin, where hydrothermal fluid circulation is expected.

Aftershock forecasts can help reduce seismic risk by communicating how many aftershocks can be expected following a large earthquake, and how the expected number of aftershocks evolves over time and space. Prior work finds that graphical forecast products may communicate this information better than only text or numbers. To identify which visual products can serve numerous user groups, we held workshops with members of target professions, including emergency managers, engineers, critical infrastructure operators, science communicators, and more. We conducted these workshops in the United States, Mexico and El Salvador to understand which forecast products may be effective across countries. Many users reported needing maps of shaking hazard to support their work. We find a greater variation in user needs across profession than country, and that user needs also vary with time. We discuss practical implications for effective visual communication of aftershock forecasts and natural hazards.

We determine average attenuation functions and estimates of the quality factor Q for both P - and S -waves in the northern Gulf of California, Mexico. We use seismograms from the Broadband Seismological Network of the Gulf of California (RESBAN) operated by the Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE). The database consisted of 64 earthquakes with Mw between 4.5 and 6.6 and hypocentral distances between 30 and 350 km. Attenuation functions were determined from a nonparametric model obtained by inverting observed spectral amplitudes of 25 frequencies between 0.1 and 25.12 Hz. To estimate Q_p and Q_s , three geometric dispersion functions were defined: one frequency dependent and two frequency independent. We find that the estimates of Q depend strongly on the geometric dispersion function adopted. Estimates of Q obtained for the hypocentral distance from 30 to 350 km indicate that P -wave attenuation is larger than S -wave attenuation regardless of the geometrical spreading function used. When using the frequency-dependent geometric dispersion, we estimate that Q_P=224.6f^1.10 and Q_S=244.7f^1.17 . In general, the high values of Q_p and Q_s suggest that the northern Gulf of California consists of a continental crust, possibly containing rocks with low fluid content and that the likely high pressure present in that region could generate a decrease in attenuation due to the closure of pores in the rock. In addition, the values of the Q_s/Q_p ratio suggest that the rocks in that region must have a low fluid content.

We present a ground-motion prediction equation ( GMPE ) of peak ground acceleration (PGA) for the Gulf of California, Mexico, derived from 1326 PGA values of 720 earthquakes recorded between 2015 and 2018 by the Red Sismológica de Banda Ancha del Golfo de California (RESBAN), operated by the Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE). We evaluated and adjusted two different parametric forms of GMPE developed for transcurrent tectonic regimes that resemble the tectonic setting in the Gulf of California, along with an additional parametric form developed for central Mexico that incorporates the focal depth as an independent variable. All three adjustments exhibit appropriate behavior in terms of residual analysis and variables stability, with the adjusted parametrical form of Joyner and Boore (Bull Seismol Soc Am 71:2011–2038, 1981. https://doi.org/10.1785/BSSA0710062011 ) providing the best overall performance. The proposed GMPE is: logPGA=-2.27+0.4M+0.065log(r)-0.0034 (r)+σ_A+σ_E , with r=√(d^2+11.86^2) . PGA is peak horizontal acceleration in cm/s 2 , M is magnitude and d is epicentral distance. σ A = 0.18 and σ E = 0.4 correspond to the intra-event and inter-event standard deviations, respectively. Although the inclusion of focal depth as an independent variable enhanced adjustments in various tectonic regions, it did not provide a specific advantage in this context. To account for depth variations, we incorporated a fixed value representing the average focal depth into the distance definition (11.86 km) effectively capturing the seismic characteristics of the region and emphasizing the importance of regional-specific factors in GMPE development and refinement.

ABSTRACTWe investigated the temporal variation of the spectral decay parameter κ before and after two main earthquakes that occurred in the central Italy region, namely the Amatrice (Mw 6.0) of 24 August 2016 and the Norcia (Mw 6.5) of 30 October 2016 earthquakes. For this analysis, we used seismograms from the central Italy dense seismic array stations, and earthquakes located at hypocenter distances r < 80 km, having magnitudes Mw 3.4–6.5. The dataset consists of 393 events recorded at 92 stations. We estimated, for both earthquake sequences, average functions κ˜(r) that describe the distance dependence of κ along the S-wave source-station paths using acceleration spectra from foreshocks, mainshock, and aftershocks. We observed that there was a regional attenuation drop within approximately two months after the Amatrice earthquake. Then, κ˜(r) tends to return toward the attenuation values observed before the occurrence of the main event, namely to the values of κ˜(r) obtained from the foreshocks, when the earthquake cycle is probably completed. We also estimated the near-source kappa (κs) using aftershocks from 24 August 2016 to 3 September 2016. The results show that the values of κs are lower than those from aftershocks located to the north near the epicenter of the Amatrice earthquake, suggesting that the tectonic stress was probably high near the rupture zone, and that there may be a likely fluid flow of crustal fluids. κ˜(r) obtained from the foreshocks of the Norcia earthquake is like that calculated with the records of the Amatrice aftershocks. Then, κ˜(r) drops to lower attenuation values during the Norcia main event and tends to increase again during the aftershocks. From the analysis of these two earthquake sequences that occurred in a short-time interval in central Italy, we conclude that the temporal variation of κ˜(r) could be a valuable indicator to monitor the earthquake cycle.

We study the spectral decay parameter kappa using S-wave recordings from the central Italy dense regional array. The data set used consists of 266 earthquakes, 353 stations, and 13,952 observations of. with a mean value of 0.0412 +/- 0.0177 within the distance range of 7.1-168.8 km. We model the variation of. with hypocenter distance r as kappa(r)=kappa(0)+kappa(s)+kappa(r), in which kappa(0) and kappa(s) represent the near-site and the near-source decay parameters, respectively, and kappa(r) the average kappa along the S-wave source-station paths. We first determine kappa(r) with a nonparametric inversion approach and then we solved for.0 and.s with a second inversion. We found that kappa(r) increases with distance within the whole distance range analyzed (9.2-80.6 km). The near-source decay parameter takes values in the range 0:0 < kappa(s) <= 0:026 with a mean value of 0.003 +/- 0.006, which represents 7.52% of the mean value of the observed kappa. The values of the near-site decay parameter vary in the range 0:0035 <= kappa(0) <= 0:0823 with a mean value of 0.0298 +/- 0.0133, that is, 72.28% of the mean value of the kappa observed. We conclude that most of the highfrequency attenuation takes place near the site, because kappa(r) contributes with only 20.2% of the spectral decay. We also investigate the spatial variability of. by determining kappa(r) within four quadrants that divide the studied region taking as a reference axis the Apennines chain orientation. We found higher values of kappa(r) in the southern quadrants, where seismicity and faulting are more active, and less attenuation in the more stable northeast quadrant.

SUMMARYIn the last 15 yr, the use of surface waves retrieved from the cross-correlation of ambient seismic noise has significantly increased its applications to determine or monitor changes in the elastic properties of the Earth's interior. We designed a methodology for laterally constrained surface wave inversion based on a two-stage technique to estimate the 3-D distribution of the S-wave velocity (Vs). The first stage inverts traveltimes to estimate group or phase velocity dispersion maps and their inverse covariance matrix for different periods. The inverse covariance matrix is constructed explicitly using the ray tracing information. The inverse covariance matrix adds the lateral sensitivity of the maps to the structure, whereas the period-dependency provides sensitivity to the structure at depth. The second stage applies a nonlinear conjugate gradient scheme to estimate the 3-D distribution of S-wave velocity using the ensemble of velocity dispersion maps and their estimated inverse covariance matrix. We validate the methodology using a synthetic model. The results show an improved estimation of subsurface S-wave velocity structures compared to conventional point-wise inversion.

We present attenuation relations for peak ground velocity (PGV) and acceleration (PGA) for northeastern Mexico, where seismic activity has been underestimated over the years. We tested four models derived from a parametrical form previously proposed for intraplate earthquakes in Central Mexico but using regional data recorded by the Mexican National Seismological Service (Servicio Sismologico Nacional de Mexico, SSN). For comparison and as a selection criteria, we used the decaying trend of the models, the ratio between the observed and the predicted data (logarithmic) as a function of the distance and its standard deviation, and we determined the following attenuation relations: log PGV = -4.36 + 0.8Mw - 0.00015R - 0:97 log(R) - 0.0041H + sigma(A) + sigma(E) log PGA = 0.31 + 0.23Mw + 0.0011R 1:48log(R) 0:0022 + sigma(A) Where M-W is the moment magnitude, R the hypocentral distance, and H the focal depth (both in km). sigma(A) and sigma(E) correspond to the intra-event and inter-event standard deviations. For PGV sigma(A) and sigma(E) are 0.18 and 0.004, while for PGA sigma(A) and sigma(E) are 0.25 and 0.30, respectively. We found that by using the original distance definition R, (dependent on the hypocentral distance and on the magnitude altogether), the results are basically the same as when using the hypocentral distance directly. We also tried to incorporate an independent variable to characterize the local soil conditions of the recording stations. Despite the fact of finding a good residual behavior with that incorporation, the empirical functions of acceleration (PGA) stopped decaying around 250 km, implying a physical impasse and making the models inadequate for greater distances, situation that did not occur with velocities (PGV). The obtained constants found for the local site conditions on both PGA and PGV were similar between stations, showing no strong impact on the site effects on the predictive equations. The proposed models fulfill the purposes of having small residual fitting and simple attenuation relations that depend only on magnitude, hypocentral distance and focal depth, making them practical and easy to apply for engineering or seismic risk assessment purposes with the available data from local or regional networks.

SUMMARYWe apply a new technique for a laterally constrained surface wave inversion (LCSWI) to estimate the 3-D sedimentary structure of the Cerro Prieto Basin, Baja California, Mexico. The basin contains the Cerro Prieto geothermal field, which is considered one of the most productive in the world. The data used consist of group velocity measurements of Rayleigh waves extracted from cross-correlations of ambient noise registered at 12 stations distributed in an 18 × 12 km area. We estimated an S-wave velocity model that clearly shows three relatively homogeneous geoseismic units that correlate with the stratigraphic column reported in previous studies. The deepest geoseismic unit is the most heterogeneous and shows low-velocity zones likely associated with fluids. The resulting velocity model shows similarity with the conceptual geological model of the geothermal field reported in the literature and recent geophysical studies that suggest the potential existence of another deeper reservoir west of the current exploitation area.

ABSTRACT Generalized inversion techniques (GITs) have become popular for determining seismological parameters (e.g., source, attenuation, and site response), particularly in low-to-moderate seismicity regions. Indeed, GITs can potentially provide reliable site-response estimates when a minimum number of recordings is available, as well as valuable information about source parameters and regional attenuation characteristics. Significant advances have been made on GITs in which different approaches and hypotheses were investigated, such as the application of “nonparametric” and “parametric” inversion schemes. In this context, several scientific questions have arisen that depend on the final scope of the GITs: What is the optimal inversion strategy for a given dataset configuration? What is the impact of the different choices, assumptions, and implementations on the reliability of the results? Is it possible to quantify the associated epistemic uncertainties? Here, we have considered and compared the different approaches of GITs to improve the understanding of each for use in different applications. A methodological benchmark that includes different GIT methods and dataset configurations is set up to fulfill the objective, using a simple synthetic dataset, a French regional sparse dataset, and an Italian national dense dataset. The benchmark is developed in two phases: (1) phase I: a free phase with no common constraints; and (2) phase II: a constrained phase with unified reference conditions. Despite unifying the reference conditions in the different inversions, the variability was not reduced. Discrepancies are observed between different terms of GITs. Site responses appear to be the most robust estimates, compared to source and attenuation terms. The way that stress drops of earthquakes and quality factors for crustal attenuation are parameterized appears to lead to significant variability between different approaches. Finally, uncertainties are addressed by quantification of the inter-method variability for the different terms and parameters.

We compile a new catalog of earthquakes that occurred in the region of the Gulf of California, Mexico, between 1901 and 2018. We include events reported by the catalog of the seismic network RESNOM (Red Sismica del Noroeste de Mexico) operated by the CICESE (Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Baja California) and complemented with events listed in the ISC (International Seismological Centre) and the SSN (Servicio Sismologico Nacional, UNAM) catalogs. The minimum magnitude of completeness of this catalog is Mc = 3.6, and we find that many hypocenters concentrate between 5 km and 10 km depth, the likely depth of the seismogenic zone of the earthquakes in the Gulf of California. We identify active and inactive faults in this region, and we detected faults located approximately 300 km SE of April 2010 El Mayor-Cucapah (Mw 7.2) earthquake that were probably reactivated by this earthquake sequence. We additionally compiled a catalog of focal mechanisms and found that most strike-slip fault earthquakes are located on transform faults and normal fault earthquakes near the spreading centers. We also detected seismicity lineaments and strike-slip events off the major transform faults that tend to occur parallel to the ridge direction either at the continental crust or at the oceanic-continental transition zone as previously reported by Ortega and Quintanar (2010) in the southern gulf.

We used 1029 earthquakes, with magnitudes ranging from M 3.0 to M 6.5, located in central Apennines, Italy, and recorded by 414 local stations to study the variation of the quality factor QS of shear waves with depth. We first determined average nonparametric attenuation functions in the frequency band from 0.5 to 20 Hz and hypocenter distances less than 155 km to correct the observed acceleration spectra for attenuation effects. Then, we separated source and site effects from the corrected spectral records to determine the changes of QS with depth. We used a 1D local shear-wave velocity model to calculate the travel times of the source-station paths, and we inverted the observed spectra to determine QS in three different depth intervals (0–4 km, 4–10 km and 10–15 km) and five frequencies (0.5, 1, 4, 10 and 20 Hz). We found that QS increases with frequency at all depths considered and tends to have lower values at shallow depths. The average value of QS is consistent with previous studies made in central Italy and can be approximated by QS = 43f0.94. To describe the frequency dependence of QS with depth (H), we determine the following relations: QS = 5.5f1.39, 0.5 ≤ f ≤ 10 Hz and QS = 151.5, f > 10 Hz for 0–4 km, QS = 52f0.87 for 4 < H < 10 km and QS = 51f0.92 for 10 ≤ H ≤ 15 km. We conclude that the Q-depth-dependent model can be useful to improve estimates of source parameters and ground motion prediction in the central Apennines region of Italy.

We image seismic properties of the plate boundary through northern Baja California and southernmost California using a double-difference tomography and earthquake relocation algorithm that incorporates both event-pair and station-pair data. The data are derived from seismic networks in southern California (SCSN) and northern Baja California (CICESE). It comprises >700,000 travel times (P and S) associated with similar to 200 seismic stations and >21,000 local earthquakes from 2003-2018, along with >2,000,000 event-pair and >1,500,000 station-pair double-differences derived from these travel times. In the upper crust, low V-P values in the resultant models imply deep sediments and sedimentary rocks northwest of the Salton Sea and around the northern end of the Imperial fault. Low V-S values indicate regions of rock damage at the northern ends of the Sierra Juarez fault zone and Laguna Salada fault, and the southern end of the San Jacinto fault zone (SJFZ). Shallow seismicity (d(95) < 6km) highlights the Salton Buttes and Cerro Prieto volcanoes as high heat-flow thin crust regions. At mid to lower crustal depths, the sharpest contrast in crustal V-P and most prominent low V-S anomaly (V-S similar to 3.4km/s, V-P/V-S similar to 1.9) occurs along a corridor extending northwestward from the Cerro Prieto fault (CPF), east of and subparallel to the Sierra El Mayor/Cucapah and into the southern SJFZ, indicating a continuous weak fluid-rich zone. At similar to 25 km depth, we observe uppermost mantle velocities (V-P similar to 7.8km/s) in the eastern Salton Trough, crustal velocities (V-P < 7km/s) around the Sierra El Mayor/Cucapah and Peninsular Ranges, and near mantle velocities (7.4 < V-P < 7.8 km/s) beneath Laguna Salada. These results illuminate a thin crust in the Salton Trough, except for the Sierra El Mayor/Cucapah, and a thicker crust in the Peninsular Ranges. In all, high seismic activity, rapid changes in crustal structure and the prevalence of damage/deformation and fluids in a corridor extending from the CPF to the southern SJFZ emphasize this zone as a key component of the active plate boundary. (C) 2021 Elsevier B.V. All rights reserved.

We apply an iterative inversion scheme, initially developed by Hashida and Shimazaki (1984) and later modified by Joshi et al., (2010), to estimate three - dimensional shear - wave quality factor, Q(s)(f), of south-central Gulf of California, Mexico. An area of 230 km x 288 km in this region is divided into 108 rectangular blocks of different Q(s)(f). We use 25 well-located earthquakes recorded at three broadband stations of the regional network RESBAN operated by CICESE (Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Baja California) and three Ocean Bottom Seismographs (OBS) of the Sea of Cortez Ocean Bottom Array (SCOOBA) experiment. This dataset permits us to obtain Q(s)(f) estimates of different blocks using the modified inversion algorithm. Q(s)(f) is obtained at various frequencies in 0.16 similar to 8.0 Hz range. We found that the estimated Q(s) structure correlates with geological and tectonic models of the region proposed in previous studies. A regional frequency-dependent relation using all 1944 values of shear-wave quality factor is obtained at 18 different frequencies in all blocks can be approximated by a function of the form Q(s)(f) = 20 f(1.2). This relation is typical in a tectonically active region with high S-wave attenuation and is similar to attenuation relations reported by other authors for the Imperial Valley, California region.

We estimated the S-wave quality factor Q(f) of the upper crust in northeastern Sonora, Mexico, using a two-layer model. The first layer of the model has a thickness of 1 km, and the frequency dependence of Q can be represented by the relation Q(f) = 127f 0.7 in the frequency range of 0.5 ≤ f ≤ 20 Hz. The second layer varies in depth between 1 and 21 km, and Q(f) = 181f 0.7 in the same frequency range. The average Q of the model, weighted with the thickness of the layers, gives the relation Q(f) = 179.1f 0.7. We used nonparametric attenuation functions, determined in a previous study, to correct the observed spectral amplitudes, and then we retrieved source and site effects using a spectral inversion scheme. We used the resulting functions to separate source and site effects from the original spectral records and modeled Q for the 1-D model’s depths. We analyzed 38 local earthquakes recorded by 12 stations of the regional network RESNES (Red Sísmica del Noreste de Sonora). We found that the S-wave Q of the superficial layer is a factor of only 1.4 smaller than the more profound layer. This result validates the use of an average Q to correct spectral amplitudes by the effect of anelastic attenuation from waves traveling within the upper crust.