ABSTRACT To estimate predicted ground motion from a teleseismic slip model, we use a low- and high-frequency hybrid method to simulate the regional, strong ground motions observed following the 18 April 2014 moment magnitude (Mw) 7.3 Papanoa, Mexico, earthquake. To generate the regional ground motion at low frequencies (<1 Hz), a teleseismically derived, finite-fault, kinematic model is used to define the earthquake source, taking into account slip-model variations identified with a parameter sampling approach that considers possible errors in the fault geometry, the hypocenter depth, and the rupture velocity. A 3D crustal model is used to calculate the low-frequency ground motions using a finite-element calculation that includes topography and considers variations in the source model to estimate the uncertainty in the calculations. High frequencies (>1 Hz) are added using a 1D full-wave propagation code that estimates uncertainties by considering multiple random distributions of slip with different spatial correlation lengths. The synthetic, broadband (0.05–10.0 Hz) ground motions are obtained by combining the low- and high-frequency portions match filtered at 1 Hz. These synthetic ground motions are compared with the regional observations using velocity records, peak ground acceleration, and medians of the orientation-independent response spectra of the horizontal components (RotD50) calculated at periods of 0.2, 0.3, 0.5, 1.0, 2.0, 3.0, 5.0, 7.5, and 10.0 s. The results indicate that ground motions estimated at these periods using our hybrid approach based primarily on a teleseismically derived source model are comparable to the values observed for the 2014 Papanoa earthquake at regional distances. The approach could be used to estimate strong-motion spectral levels expected for regions with limited local and regional recordings and could also fill in magnitude or distance gaps in ground-motion prediction relations utilized in the assessment of seismic hazard.

ABSTRACT Thirty-seven portable accelerometers were deployed in the eastern San Francisco Bay communities of Walnut Creek and Concord to study site response in a fault-bounded, urban, sedimentary basin. Local earthquakes were recorded for a period of two years from 2017 to 2019 resulting in 101 well-recorded events. Site response is estimated by two methods: the reference site spectral ratio method and a source-site spectral inversion method. The reference site spectral ratio method allows investigation of the variability of site amplification with source azimuth and frequency. The source-site spectral inversion method yields the best least-squares fit to site response for a database of ground-motion records. Both methods show substantial amplification in the Walnut Creek–Concord basin below 2 Hz indicating strong surface-wave development. Greater amplification is seen for sources aligned along the long axis of the basin. Inversion using close-in sources at short distances yields lower amplification at longer periods than the entire data set due to reduced surface-wave generation for steeper angles of incidence. Inversion of site response spectra for shallow shear-wave velocity using a global search algorithm yields VS30 values consistent with generalized mapping results based on geology and topography but with greater variability due to local site variations. 3D finite-element modeling shows greater amplification in the Walnut Creek–Concord basin with a basin-edge effect likely contributing to higher ground motions. Topography is also seen to lead to increased scattering and shadowing effects.

SUMMARY This paper proposes the use of geostatistical techniques to estimate dispersion curves between other known ones. To do it, we introduce two novel methodologies: the stacking method and the group-velocity mapping method. We obtain our set of group-velocity fundamental mode dispersion curves from seismic noise correlation. Consequently, we first assign their attribution point at the mid-distance between the stations used for the dispersion curves calculation. The stacking method uses the range of the omnidirectional semivariogram of a regionalized variable that quantifies the similarity between dispersion curves to stack them according to their spatial correlation. We test this technique with dispersion curves obtained in Mexico City and get a range of ∼400 m for the omnidirectional semivariogram. We also calculate directional semivariograms and observe a maximum range (∼500 m) in the NW-SE direction, agreeing with the city's spatial distribution of natural periods. On the other hand, the group-velocity mapping method uses the ordinary kriging estimator in the group velocities for all the ranges of periods to generate maps and then dispersion curves. Estimated dispersion curves retrieved from both, the stacking and the group-velocity mapping method, were compared with those obtained with the fast marching tomographic method. We also establish analogies between getting group-velocity maps with the tomographic method and with the group-velocity mapping method. Finally, we observe that the range of the omnidirectional semivariogram used in the stacking method may be related to the tomographic method resolution.

The Mexican subduction zone is prone to large earthquakes that impact populated coastal and inland cities due to strong shaking and can generate potentially disastrous tsunamis. Even though the population and infrastructure on the Mexican Pacific coast may experience tsunamis, only sparse instrumentation and scarce tsunami observations exist for both, historical and instrumental, records. This study presents a probabilistic tsunami hazard analysis that can provide information for managing and assessing risks in the region. We use ensembles of numerical tsunami simulations to estimate the probability of exceedance of the maximum tsunami amplitudes, considering the contribution of near sources and slip heterogeneities, for return periods of 100, 500, and 1,000 years. According to 7,946 simulated scenarios, the amplitudes can reach up to 9.5 m along the shoreline. Moreover, tsunami hazard maps, curves, and disaggregation analyses reveal critical hazard points along the coasts of the states of Jalisco, Colima, Michoacán, Guerrero, and Oaxaca.

We present a source inversion of the 2008 Wenchuan, China earthquake, using strong-motion waveforms and geodetic offsets together with 3-D synthetic ground motions. We applied the linear multiple time window technique considering geodetic and dynamic Green's functions computed with the finite-element method and the reciprocity and Strain Green's Tensor formalism. All ground motion estimates, valid up to 1 Hz, accounted for 3-D effects, including the topography and the geometry of the Beichuan and Pengguan faults. Our joint inversion has a higher moment (MO) than a purely geodetic inversion and the slip distribution presents differences when compared to 1-D model source inversions. The moment is estimated to be MO = 1.2 x 10(21) N.m, slightly larger than other works. Our results show that considering a complex 3-D structure reduces the size of large areas of 10 m slip or greater by distributing it in wider zones, with reduced slips, in the central portion of the Beichuan and the Pengguan faults. Finally, we compare our source with a relocated aftershock catalogue and conclude that the 4-5 m slip contours approximately bound the absence or presence of aftershocks.

Mexico is a seismically active country Earthquakes with magnitudes larger than 7 0 happen, on average, every other year This fact requires a rapid and consistent response from the national monitoring agency, the Servicio Sismológico Nacional, SSN (Mexican National Seismological Service) For this purpose, in 2014, the SSN created a set of procedures for the daily operations and rapid response called “Protocolo de Respuesta Inmediata ante Sismos Amenazantes” (PRISA, protocol of immediate response to threatening earthquakes) This protocol has been triggered for 292 events with a magnitude larger than or equal to 5 0 that occurred between April 2014 and July 2020 Here we present the response of the SSN, based on this protocol, to three significant earthquakes: the 8 and 19 September 2017 events (Mw 8 2 and 7 1, respectively) and the 23 June 2020 (Mw 7 4) The first two quakes caused severe damage in southern and central Mexico, whereas the third occurred during the coronavirus disease 2019 pandemic and confinement in Mexico Having PRISA in place contributed to the efficient SSN response in the three events, even though some activities for the 2020 earthquake were performed remotely © Seismological Society of America

ABSTRACTWe present an analysis of the information collected by the ¿Sintió un sismo? (SUS) web‐based system. One of the most devastating events in central Mexico in the past 35 yr struck near the Mexican states of Puebla and Morelos on 19 September 2017. At the moment of the event, several programs and projects were in place to monitor and perform quick assessments of the magnitude of the earthquake and the severity of its effects on the population and infrastructure. The SUS platform gathers questionnaires designed in Spanish to estimate macroseismic intensities. The availability of such a system in the dominant language of the country permits a broad reach, only limited by the disparity of the services and internet access. By analyzing residuals of the median attenuation intensity of the event, we confirm previous observations on the site and regional effects in Central Mexico such as the strong influence of the Trans‐Mexican volcanic belt on the ground‐motion amplification. In addition, we obtained correlations between peak parameters and macroseismic intensities that reveal the character of the affected structures’ responses. We emphasize the potential usability of systems similar to the SUS at the regional level and their impact on the decision‐making process and support for further research using all available datasets.

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.

AbstractWe present results of a slip model from joint inversion of strong motion and static Global Positioning System data for the Mw7.1 Puebla‐Morelos earthquake. We find that the earthquake nucleates at the bottom of the oceanic crust or within the oceanic mantle with most of the moment release occurring within the oceanic mantle. Given its location at the edge of the flat slab, the earthquake is likely the result of bending stresses occurring at the transition from flat slab subduction to steeply dipping subduction. The event strikes obliquely to the slab, we find a good agreement between the seafloor fabric offshore the source region and the strike of the earthquake. We argue that the event likely reactivated a fault first created during seafloor formation. We hypothesize that large bending‐related events at the edge of the flat slab are more likely in areas of low misalignment between the seafloor fabric and the slab strike where reactivation of preexisting structures is favored. This hypothesis predicts decreased likelihood of bending‐related events northwest of the 2017 source region but also suggests that they should be more likely southeast of the 2017 source region.

使用远震、强震动波形和大地测量偏移的一个大规模数据集来研究2008年中国汶川地震的破裂历史。使用线性多时间窗方法来对破裂进行参数化。由于汶川断层作用的复杂性,使用三个单独的平面来表示断裂面。该地震清楚地表明了大地测量、远震和强震动数据集的优点和局限性。大地测量数据(静态偏移)对于确定浅部滑动的分布很有用,但对深部断层作用不敏感,且不能揭示滑动时间的任何信息。震中距在30°~90°范围内的远震数据,由于射线路径简单,通常建模不困难,并能区分浅部和深部滑动。然而,当涉及多个断层面时,由于陡峭的离源角导致定时模糊,远震数据不能区分不同的滑移情况。另一方面,地方强震动数据对于从方向性确定破裂方向是理想的,但也容易因格林函数的不准确而过度建模,导致对滑移分布的错误解释。我们发现,要对汶川破裂进行准确的描述需要所有这三种数据集。地震矩估计约为1.0×1021N·m,断层滑动特征是在多个大的断层阻延区上有高达10m的滑动。破裂开始于彭灌断层的南端并朝东北边单侧破裂。当到达交叉的小鱼洞断层时,相邻的北川断层从这个结合部开始并双侧破裂到东北和西南。

Subduction zones, where two tectonic plates converge, are generally dominated by large thrust earthquakes. Nonetheless, normal faulting from extensional stresses can occur as well. Rare large events of this kind in the instrumental record have typically nucleated in and ruptured the top half of old and cold lithosphere that is in a state of extension driven by flexure from plate bending. Such earthquakes are limited to regions of the subducting slab cooler than 650 °C and can be highly tsunamigenic, producing tsunamis similar in amplitude to those observed during large megathrust events. Here, we show from analyses of regional geophysical observations that normal faulting during the moment magnitude M w 8.2 Tehuantepec earthquake ruptured the entire Cocos slab beneath the megathrust region. We find that the faulting reactivated a bend-fault fabric and ruptured to a depth well below the predicted brittle–ductile transition for the Cocos slab, including regions where temperature is expected to exceed 1,000 °C. Our findings suggest that young oceanic lithosphere is brittle to greater depths than previously assumed and that rupture is facilitated by wholesale deviatoric tension in the subducted slab, possibly due to fluid infiltration. We conclude that lithosphere can sustain brittle behaviour and fail in an earthquake at greater temperatures and ages than previously considered.

Eight seismic stations were placed in a linear array with a topographic relief of 222 m over Mission Peak in the east San Francisco Bay region for a period of one year to study topographic effects. Seventy-two well-recorded local earthquakes are used to calculate spectral amplitude ratios relative to a reference site. A well-defined fundamental resonance peak is observed with individual station amplitudes following the theoretically predicted progression of larger amplitudes in the upslope direction. Favored directions of vibration are also seen that are related to the trapping of shear waves within the primary ridge dimensions. Spectral peaks above the fundamental one are also related to topographic effects but follow a more complex pattern. Theoretical predictions using a 3D velocity model and accurate topography reproduce many of the general frequency and time-domain features of the data. Shifts in spectral frequencies and amplitude differences, however, are related to deficiencies of the model and point out the importance of contributing factors, including the shear-wave velocity under the topographic feature, near-surface velocity gradients, and source parameters.

We examine the variability of long-period (T >= 1 s) earthquake ground motions from 3D simulations of M-w 7 earthquakes on the Salt Lake City segment of the Wasatch fault zone, Utah, from a set of 96 rupture models with varying slip distributions, rupture speeds, slip velocities, and hypocenter locations. Earthquake ruptures were prescribed on a 3D fault representation that satisfies geologic constraints and maintained distinct strands for the Warm Springs and for the East Bench and Cottonwood faults. Response spectral accelerations (SA; 1.5-10 s; 5% damping) were measured, and average distance scaling was well fit by a simple functional form that depends on the near-source intensity level SA(0) (T) and a corner distance R-c: SA (R,T) = SA(0) (T) (1 + (R/R-c))(-1). Period-dependent hanging-wall effects manifested and increased the ground motions by factors of about 2-3, though the effects appeared partially attributable to differences in shallow site response for sites on the hanging wall and footwall of the fault. Comparisons with modern ground-motion prediction equations (GMPEs) found that the simulated ground motions were generally consistent, except within deep sedimentary basins, where simulated ground motions were greatly underpredicted. Ground-motion variability exhibited strong lateral variations and, at some sites, exceeded the ground-motion variability indicated by GMPEs. The effects on the ground motions of changing the values of the five kinematic rupture parameters can largely be explained by three predominant factors: distance to high-slip subevents, dynamic stress drop, and changes in the contributions from directivity. These results emphasize the need for further characterization of the underlying distributions and covariances of the kinematic rupture parameters used in 3D ground-motion simulations employed in probabilistic seismic-hazard analyses.