ALBERT

Description: One important element of understanding basin response to strong shaking is the analysis of spectral ratios, which may provide information about the dominant frequency of ground motion at specific locations. Spectral ratios computed from accelerations recorded by strong‐motion stations in Mexico City during the mainshock of the 19 September 2017 M 7.1 Puebla‐Morelos earthquake reveal predominate periods consistent with those mapped in the 2004 Mexican seismic design code. Furthermore, the predominant periods thus computed validate those studies using mainshock and aftershock recordings of the handful strong‐motion stations that recorded the 19 September 1985 M 8.1 Michoacán earthquake. Even though the number of stations in each of the zones (zones I, II, IIIa, b, c, and d) is not the same, they still allow confirmation of site frequencies (periods) attributable to the specific zones (particularly those in zones IIIa, b, c, and d). Spectral ratios are computed with two different methods: (1) horizontal to horizontal (H/H) ratio of smoothed amplitude spectrum of a horizontal channel in direction X of a station with respect to the smoothed amplitude spectrum of the horizontal channel in the same X direction of a reference stiff soil (or rock) station and (2) horizontal to vertical (H/V) ratio (or also known as the Nakamura method) of both horizontal (H) and vertical (V) channels of the same station. We show a comparison of the identified frequencies (periods) derived by both methods and find they are very similar and in good agreement with those indicated in the zoning maps of Mexico City in the 2004 seismic design code.

Description: The 2017 M 8.2 Tehuantepec and M 7.1 Puebla‐Morelos earthquakes were deep inslab normal‐faulting events that caused significant damage to several central‐to‐southern regions of Mexico. Inslab earthquakes are an important component of seismicity and seismic hazard in Mexico. Ground‐motion prediction equations (GMPEs) are an integral part of seismic hazard assessment as well as risk and rapid‐response products. This work examines the observed ground motions from these two events in comparison to the predicted median ground motions from four GMPEs. The residuals between the observed and modeled ground motions allow us to study regional differences in shaking, the effects of each earthquake, and basin effects in Mexico City, Puebla, and Oaxaca. We find that the ground motions from these two earthquakes are generally well modeled by the GMPEs. However, the Tehuantepec event shows larger than expected ground motions at greater distances and longer periods, which suggests a waveguide effect from the subduction zone geometry. Finally, Mexico City and the cities of Puebla and Oaxaca exhibit very large ground motions, indicative of well‐known site and basin effects that are much stronger than the basin terms included in some of the GMPEs. Simple and rapid ground‐motion parameter estimates that include site effects are key for hazard and real‐time risk assessments in regions such as Mexico, where the vast majority of the population lives in areas where the aforementioned effects are relevant. However, GMPEs based on site correction terms dependent on topographic slope proxies underestimate, at least in the three cities tackled in this work, the observed amplification. Therefore, there is a need to improve models of seismic amplification in basins that could be included in GMPEs.

Description: An earthquake of magnitude 6.6 occurred on 4 January 2006, at 28.081° N and 112.381° W along a transform fault that joins the San Pedro Martir and the Guaymas basins in the Gulf of California extensional province. We located 17 foreshocks and 38 aftershocks. The foreshocks occurred on a fault perpendicular to the transform fault, where the main event occurred. The aftershocks were located along a fault length of approximately 18 km with a northwest–southeast trend. The average Brune static stress drop of the San Pedro Martir event was 8 MPa. From a time‐domain moment tensor inversion, we obtained the fault geometry given by strike of 129°±1°, dip of 86°±4°, and rake of 168°±12°, which was constrained to have a nonisotropic component and a source depth of 6±2  km. We used the inversion code from Yagi et al. (1999) to invert near field and teleseismic P waves to obtain the spatial slip distribution over the fault. The event had a single source and a moment rate function (MRF) displaying a triangular shape with a duration of 12 s. The rupture propagated toward the northwest from the hypocenter over a rupture area of 28×12  km2 with a maximum slip displacement of 2.3 m and a seismic moment of 8.79×1018  N·m. The directivity confirmed that the rupture propagated from the southeast to the northwest. Few aftershocks were located in the rupture area obtained from the inversion. Most aftershocks occurred toward the southeast of the epicenter. All these source analyses were performed to have a well‐calibrated excitation term for future regional modeling of ground‐motion parameters. The magnitudes of the foreshocks preceding this peculiar earthquake were higher than those of the aftershocks. Our results show that earthquakes with magnitudes of five or higher present a simple and self‐scaling law using a constant stress parameter, but for earthquakes with magnitudes lower than five, the high frequencies are depleted, and the earthquake can be replicated by a low‐stress parameter of 0.28 MPa. We also observed that the aftershocks and foreshocks differ in their frequency content. Although the foreshocks follow Brune’s omega‐squared source term, the aftershocks have larger contents of high frequencies.