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We assemble a dataset which enhances existing Central America and Mexico data for the NGASubduction (NGA-Sub) project, including now additional earthquake ground motions and site parameters from Mexico. This data has been used to provide regional customization of NGA-Sub global Ground Motion Models (GMMs) for application in Mexico, paying particular attention to the site response of Mexico City (CDMX). The expanded database for Mexico incorporates smaller magnitude earthquakes (M < 6) and three significant events (M 7.2-8.3) that occurred in 2017 and 2018. These latter events are particularly important, because they are well recorded over a broad distance range and apply for hazard-critical conditions. Our focus here is on presenting the observed site response in CDMX, which we model based on the time-averaged shear wavevelocity in the upper 30 meters (VS30) and the Peak Ground Acceleration at a reference rock site (PGAr). Theempirical model we propose is different from previous work in several ways. First, it is properly centered withrespect to GMMs. Second, it is referenced to VS30 = 760 m/s, which is significantly firmer than the reference sitepreviously used in practice, taken from a location on the UNAM campus with VS30 ~ 300 m/s. Third, we identifysite amplification as nonlinear, whereas linear response has been assumed in prior models. The extent of thisnonlinearity is characterized using an established seismic zonation for CDMX, being more pronounced in softersites (Zone III) compared to stiffer ones (Zone I). This nonlinear effect is prominent at short periods and disappears at long periods. The advantages of the provided model include a unified framework for both withinand outside CDMX (contrasting current practices) and the integration of applicable features from global modelswith necessary local customization.

Seismic anisotropy in the flat slab region of Mexico is compared with tectonic tremor (TT) activity. The anisotropy is observed in three separate horizontal layers using a novel technique with receiver functions. Those layers are identified as the continental crust and the subducted flat oceanic slab and a thin (∼10 km thick) remnant mantle wedge between those two layers. The TT is located in two zones: (1) the Sweet Spot where most of the tremor is observed (∼160–180 km from the coast) and (2) the Transient Zone (∼80–110 km from the coast). Anisotropy within each layer is observed to be different within each of the tremor zones than just outside them. The changes are explained as due to hydration within those zones. Water releasing phase changes have previously been modelled to occur within those two zones in the subducted slab (Manea & Manea). Water rising through each of the layers should generate the observed differences in anisotropy in those zones as the fast polarization direction and split times can differ between dry and hydrated material. This observation also correlates with the many observations of high pore fluid pressure associated with TT. [ABSTRACT FROM AUTHOR]

Abstract: We 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. [ABSTRACT FROM AUTHOR]

Subduction zones are among the most dynamic tectonic environments on Earth. Deformation mechanisms of various scales produce networks of oriented structures and faulting systems that result in a highly anisotropic medium for seismic wave propagation. In this study, we combine shear wave splitting inferred from receiver functions and the results from a previous SKS-wave study to quantify and constrain the vertically averaged shear wave splitting at different depths along the 100-station MesoAmerican Subduction Experiment array. This produces a transect that runs perpendicular to the trench across the flat slab portion of the subduction zone below central and southern Mexico. Strong anisotropy in the continental crust is found below the Trans-Mexican Volcanic Belt (TMVB) and above the source region of slow-slip events. We interpret this as the result of fluid/melt ascent. The upper oceanic crust and the overlying lowvelocity zone exhibit highly complex anisotropy, while the oceanic lower crust is relatively homogeneous. Regions of strong oceanic crust anisotropy correlate with previously found low Vp/Vs regions, indicating that the relatively high Vs is an anisotropic effect. Upper-mantle anisotropy in the southern part of the array is in trench-perpendicular direction, consistent with the alignment of type-A olivine and with entrained subslab flow. The fast polarization direction of mantle anisotropy changes to N-S in the north, likely reflecting mantle wedge corner flow perpendicular to the TMVB. [ABSTRACT FROM AUTHOR]

Receiver functions (RFs) from 224 permanent and temporary stations in central and southern Mexico were used to characterize the upper-mantle transition zone in that region. Discontinuities at 410 and 660 km depth are both deeper compared to iasp91, which reflects a slow velocity anomaly in the upper mantle. They show topography on the interfaces that is consistent with the interaction of the subducted slab or its broken off extension. A low-velocity layer on top of the 410 is identified mainly on the continental side of where the slab pierces it (i.e. in the lee of the slab roll-back). In general the RFs show a complex behaviour where the mantle has been disturbed by the lateral motion of the subducted slab, and are simple where it has not. Complexity on the 660 coincides with the place where the broken off portion of the Farallon Plate would have penetrated this interface or is possibly lying on top of it. [ABSTRACT FROM PUBLISHER]

Receiver functions from teleseismic events, recorded by stations around the Gulf of California, are used to map the upper-mantle seismic discontinuities. We observe a mean transition zone thickness comparable to the global average for most of the region. A low-velocity layer is detected above the 410 discontinuity that varies in thickness along the Gulf of California. The 660 discontinuity shows complex waveforms south of latitude 30°N as a result of the phase change of garnet to perovskite. Within the transition zone, a complex behaviour of the receiver functions is observed mainly at the southern end of the Gulf. The north–south variations of this zone are likely associated with a slab window at the northern Gulf, resulting from the cessation of subduction of the Farallon plate 12 Ma, and the subduction of the Guadalupe and Magdalena microplates at the southern end, resulting in a hydrated upper mantle. Our results suggest that change in rifting styles occurring along the Gulf of California mirrors deeper processes in the upper mantle. [ABSTRACT FROM AUTHOR]