Based on analysis of data from a trans‐Mexico temporary broadband seismic network centered on Mexico City, we report that the subducting Cocos Plate beneath central Mexico is horizontal, and tectonically underplates the base of the crust for a distance of 250 km from the trench. It is decoupled from the crust by a very thin low viscosity zone. The plate plunges into the mantle near Mexico City but is truncated at a depth of 500 km, probably due to an E‐W propagating tear in the Cocos slab. Unlike the shallow slab subduction in Peru and Chile, there is active volcanism along the Trans Mexican Volcanic Belt (TMVB) that lies much further inland than regions to either side where subduction dip is not horizontal. Geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction geometry.

Nonvolcanic tremor (NVT) activity is revealed as episodes of higher spectral amplitude at 1–8 Hz in daily spectrograms from the continuous seismological records in Guerrero, Mexico. The analyzed data cover a period of 2001–2007 when in 2001–2002 a large slow slip event (SSE) had occurred in the Guerrero‐Oaxaca region, and then a new large SSE occurred in 2006. The tremor burst is dominated by S‐waves. More than 100 strong NVT bursts were recorded in the narrow band of ∼40 × 150 km2 to the south of Iguala City and parallel to the coastline. Depths of NVT hypocenters are mostly scattered in the continental crust between 5 and 40 km depth. Tremor activity is higher during the 2001–2002 and 2006 SSE compared with that for the “quiet” period of 2003–2005. While resistivity pattern in Guerrero does not correlate directly with the NVT distribution, gravity and magnetic anomaly modeling favors a hypothesis that the NVT is apparently related to the dehydration and serpentinization processes.

[1] Shear wave velocity of the crust below central Mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense, 500 km long linear seismic network. Vertical components of regional records from 90 well-located earthquakes were used to compute Rayleigh-wave group-velocity dispersion curves. A tomographic inversion, with high resolution in a zone close to the array, obtained for periods between 5 and 50 s reveals significant differences relative to a reference model, especially at larger periods (>30 s). A 2-D S wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions. The results show large differences, especially in the lower crust, among back-arc, volcanic arc, and fore-arc regions; they also show a well-resolved low-velocity zone just below the active part of the Trans Mexican Volcanic Belt (TMVB) suggesting the presence of a mantle wedge. Low densities in the back arc, inferred from the low shear wave velocities, can provide isostatic support for the TMVB.

No abstract

Artículo de publicación ISIWe determine here for the first time the geometry and location of the hydrothermal and magmatic reservoirs in the Lazufre volcanic area. This furthers the understanding of the origin of one of the largest worldwide volcanic uplift regions, both in space and amplitude. The exact locations and shapes of the sources generating a double-wide uplift region in the Lazufre found by past deformation data (InSAR and GPS) and generating hydrothermal and magmatic fluids found by geochemical gas analysis have not been well-delimited. In this study, we use seismological data to perform a 3-D high-resolution S-wave velocity model, which allows defining better the locations and shapes of the sources of the deformations and the hydrothermal and magmatic reservoirs. We find three anomalies. Two of them (with S-wave velocity of about 1.2–1.8km/s) are located below the Lastarria volcano. The shallow one (<1km below the volcano base) has a funnel-like shape. The deeper one is located between a depth of 3 and 6km below the volcano base. Both are strongly elliptical in an EW direction and separated by a 2–3km thick zone with Vs of ∼1.5–2km/s. As far as these anomalies are located under the hydrothermal activity of Lastarria volcano, they are interpreted as a double hydrothermal (the shallow part) and magmatic source (the deeper part). The lattercan feed the former. This double hydrothermal and magmatic source is in agreement with previous geochemical, deformation (GPS and InSAR) and magneto-telluric studies. In particular, it explains the double origin of the gases (hydrothermal and magmatic). The third low-velocity zone (with S-wave velocity of about 2.3km/s) located at 5km depth and deeper is centered beneath an area of surface uplift as determined by InSAR data. We compare the seismic tomography and InSAR results to propose that this low-velocity zone is at the top of a large reservoir, hosting hydrothermal fluids and possibly also magma.PLUTONS project funded by the National Science Foundation (grant #EAR-0909254). The research was supported by the Chilean CONICyT-Fondecyt project #1061253, and the Mexican CONACyT projects #129820 and #221165

In the wake of death and destruction left by the 2017 earthquake in Mexico City, it is natural to ask whether the event was unexpected and anomalous. Although such an intraslab earthquake (M w 7.1; depth 57 km; epicentral distance = 114 km from the city) was considered likely, the recordings in the city during the last 54 yrs reveal that the ground motion during the 2017 earthquake was anomalously large in the critical frequency range to the city (0.4-1 Hz). The intraslab earthquakes occur closer to Mexico City, at greater depth, and involve higher stress drop than their interplate counterparts. Consequently, the ground motion is relatively enriched at high frequencies as compared with that during interplate earthquakes, which is dominated by lower frequency waves (f < 0:5 Hz). This explains the observed difference in the damage pattern during the 2017 and the disastrous interplate earthquake of 1985 (M w 8.0). Electronic Supplement: Figures showing spectral ratios, peak ground acceleration (PGA) and peak ground velocity (PGV) as function of distance R, comparison of observed response spectra SA, and predicted median and 1 s SA from a site-specific ground-motion prediction equation (GMPE) model at CU, plot of accelerograms, Fourier acceleration spectra, and SA at SCT of interplate 1985 M w 8.0 and intraslab 2017 M w 7.1 earthquakes, accelerographic stations in Mexico City, which recorded the 2017 M w 7.1 earthquake, SA of the 2017 earthquake at sites in and near Condesa and Roma colonies, and basis for the estimation of exceedance rate of PGA at CU in Mexico City from intraslab earthquakes, and tables providing a list of 20 (interplate and intraslab) earthquakes with largest recorded PGA at CU in the 1964-2017 period, significant pre-1975 intraslab earthquakes, and a comparison of observed PGA and PGV at CU during the 2017 M w 7.1 earthquake with predictions from GMPE.

Abstract.Near-source strong motions are inverted to estimate the rupture history of intraslab, normalfaulting September 30, 1999, Oaxaca, Mexico earthquake. Two focal mechanisms (Harvard and NEIC CMT solutions) are tested for the source geometry.

No abstract