[1] Epicentral locations of non-volcanic tremors (NVT) in the Mexican subduction zone are determined from the peak of the energy spatial distribution and examined over time. NVT is found to occur persistently at a distance of $215 km from the trench, which we term the "Sweet Spot" because this region probably has the proper conditions (i.e., temperature, pressure, and fluid content) for the NVT to occur with minimum shear slip. High-energy NVT episodes are also observed every few months, extending $190 km to $220 km from the trench with durations of a few weeks. During the 2006 slow slip event (SSE) the duration and the recurrence rate of the NVT episodes increased. Low-energy episodes were also observed, independent from the high-energy episodes, $150 km to $190 km from the trench during the 2006 SSE. Both the high and low energy episodes were made up of many individual NVT's that had a range of energy-release-rates. However, the highest energy-release-rates of the high-energy episodes were consistently double those of the low-energy episodes and the persistent activity at the Sweet Spot. We suggest that all of the high-energy episodes are evidence of small, short repeat interval SSE. Given this model, the increased recurrence rate of the high-energy NVT episodes during the 2006 long-term SSE implies that short-term SSE's also increase during the SSE and are therefore triggered by the SSE.Components: 6100 words, 7 figures.

We find very low frequency earthquakes (VLFEs) in Cascadia under northern Washington during 2011 episodic tremor and slip event. VLFEs are rich in low‐frequency energy (20–50 s) and depleted in higher frequencies (higher than 1 Hz) compared to local earthquakes. Based on a grid search centroid moment tensor inversion, we find that VLFEs are located near the plate interface in the zone where tremor and slow slip are observed. In addition, they migrate along strike with tremor activity. Their moment tensor solutions show double‐couple sources with shallow thrust mechanisms, consistent with shear slip at the plate interface. Their magnitude ranges between Mw 3.3 and 3.7. Seismic moment released by a single VLFE is comparable to the total cumulative moment released by tremor activity during an entire episodic tremor and slip event. The VLFEs contribute more seismic moment to this episodic tremor and slip event than cumulative tremor activity and indicate a higher seismic efficiency of slow earthquakes in Cascadia than previously thought. Spatiotemporal correlation of VLFE and tremor activity suggests that they are the results of the same physical processes governing slow earthquakes.

We present new observations of crustal shear wave anisotropy extracted from nonvolcanic tremor in Cascadia under Washington State. Measurements of crustal anisotropy are extremely sparse and limited in this area mainly due to low level of seismicity. Abundance of tremor activity during slow earthquakes offers a unique opportunity to measure anisotropy parameters of the continental crust using tremor signal. To accomplish this, polarization and splitting analyses of nonvolcanic tremor are performed using three-component broadband seismic stations. Splitting times measurements range between 0.08 and 0.17 s and similar to the splitting magnitude typically observed in the continental crust. Fast direction of shear wave anisotropy generally trends ESE-WNW. Fast polarization directions are, in general, perpendicular to the prevailing maximum compressive stress field but tend to be parallel to several mapped EW and ESE-WNW trending faults in this area. The observed spatial pattern of anisotropy is likely controlled by faulting that accommodates NS compression resulting from the tectonic movement of the Oregon block toward north. Existence of several EW trending crustal faults and source parameters of crustal earthquakes at depth, consistent with the regional stress regime, indicate that these faults may be the dominant factor causing the observed pattern of shear wave anisotropy.

We present new shear wave anisotropy measurements in the continental crust in northern Guerrero obtained from tectonic tremor. Measurements of crustal anisotropy had not been performed in this area due to the lack of seismicity. However, tectonic tremor activity is abundant and offers an opportunity to determine anisotropy parameters. Polarization and splitting analyses were performed using broadband three‐component seismograms. Results show that splitting times range between 0.07 and 0.36 s. These values are similar to the splitting magnitudes typically observed in the continental crust. The state of stress in the continental crust was investigated by inverting focal mechanisms determined in this study, and also from previous structural geology studies. Unfortunately, no stress measurements were possible in the area where tectonic tremor occurs. It was determined that, to the south of the study area, near the Pacific coast, and to the north, in the volcanic arc, the maximum compressive stress shows a general E‐W trend. The fast polarization directions are oriented NE‐SW and are oblique to the observed maximum compressive stress surrounding the study area. Thus, the relationship between the maximum compressive stress and the observed anisotropic pattern cannot be conclusively established. Several factors such as nonlinear strain in the continental crust as a result of Slow Slip Events, variations of pore fluid pressure, deep crustal mineralogy, and/or upper crust foliations and schistosity could be inducing the observed anisotropy pattern. In general, the fast axes tend to parallel the regional Laramide and Tertiary folds‐and‐thrusts which strike NNE‐SSW. This system of folds‐and‐thrusts is highly foliated in low‐grade schist and seems likely to control the anisotropic structure observed within the tectonic tremor region in Guerrero.

During the past years, significant work has been done for studying the crustal anisotropy and state of stress of the Mexican subduction zone. At the same time, there is new evidence of the geometry of the subducted slab proposing subduction tearing. Here, we present a study of the Earth crust using three different methods: azimuthal anisotropy based on ambient noise, shear-wave splitting of tectonic tremors, and moment tensor inversions of the earthquakes of 7 September 2017 Mw 8.2 Tehuantepec, Mexico. This earthquake initiated a seismic sequence that triggered shallow seismicity and aftershocks. The shallow earthquakes fall into a region where there were few published focal mechanism higher than Mw 4.5. Two slab tearings: in the Michoacán–Guerrero border and in central Oaxaca, best represent the slab geometry of the Mexican subduction zone. At the Michoacán–Guerrero, the subducted slab is subhorizontal, whereas in central Oaxaca the plate is characterized by northeast vergence. We interpret that the mantle’s flow in this part of the subducted slab produces multiple alignments in the crust and differentiates the tectonostratigraphic terranes of the southern region of Mexico.

Se desarrollaron dos tomografías de tiempo de viaje para las ondas P y S, así como un mapa del cociente Vp/Vs en el sur de México. Se utilizaron datos provenientes de la red temporal de banda ancha Meso-American Subduction Experiment (MASE). Los perfiles de las tomografías tienen su origen en la costa del Pacífico y corren tierra adentro 205 km perpendiculares a la trinchera; muestrean hasta una profundidad de 55 km. Los resultados muestran, para ambas ondas, velocidades altas desde la costa hasta 40 km en la sección descendiente de la placa subducida de Cocos, una anomalía lenta para la onda P entre los 50 km y 90 km, por arriba del doblez donde la placa se vuelve subhorizontal y velocidades bajas para las ondas P y S por encima de la placa entre los 90 km y 205 km desde la costa. El mapa del cociente de Vp/Vs exhibe dos zonas de valores altos: (1) la región donde la placa desciende desde la costa hasta 60 km; y (2) entre los 90 km y 160 km, donde se han detectado los Tremores No-Volcánicos (NVT). Por otro lado, se encuentran valores bajos de Vp/Vs donde la placa dobla (60 km - 90 km), lo que probablemente indica que la corteza está seca y sometida a esfuerzos intensos. Se estimaron valores normales de Vp/ Vs en la corteza al norte de 160 km de la costa, a pesar de que existe mucha evidencia de alta presión de fluidos en aquella región. Este hecho, muy probablemente, describe una combinación de reducciones proporcionales entre las velocidades de P y S debido a altas temperatura y bajas presiones efectivas.

Summary Shear wave splitting measurements in the Isthmus of Tehuantepec (IT), southern Mexico, inferred from teleseismic core phases are presented. Measurements were made along a south-to-north profile across the IT. The results show a predominantly trench-normal pattern of fast polarization orientations with averaged delay times up to 2.2 s. Fast orientations near the trench suggest a corner flow in the mantle wedge and an entrained flow in the subslab region. Away the trench, fast orientations are parallel to the Absolute plate Motion, suggesting that the anisotropy in that region is driven by a simple asthenospheric flow. A comparison with splitting measurements made in the Mexican subduction zone shows a 17º clockwise rotation of the fast orientations of between east and west Mexico. This is consistent with the observed change in orientation of 19º clockwise in the Middle America Trench (MAT). This suggests that the rotation of the fast orientations is controlled by the change of orientation in the MAT.

We present new observations of crustal anisotropy in the southern Cascadia fore arc from tectonic tremor. The abundance of tremor activity in Oregon and northern California during slow‐slip events offers an enormous amount of information with which to measure and analyze anisotropy in the upper brittle continental crust. To accomplish this, we performed analyses of wave polarization and shear wave splitting of tectonic tremor signals by using three component broadband seismic stations. The splitting times range between 0.11 and 0.32 s and are consistent with typical values observed in the continental crust. Fast polarization azimuths are, in general, margin parallel and trend N‐S, which parallels the azimuths of the maximum compressive stresses observed in this region. This pattern is likely to be controlled by the stress field. Comparatively, the anisotropic structure of fast directions observed in the northern section of the Cascadia margin is oblique with respect to the southern section of Cascadia, which, in general, trends E‐W and is mainly controlled by active faulting and geological structures. Source distribution analysis using a bivariate normal distribution that expresses the distribution of tremors in a preferred direction shows that in northern California and Oregon, the population of tremors tends to distribute parallel to fast polarization azimuths and maximum compressive stresses, suggesting that both tremor propagation and anisotropy are influenced by the stress field. Results show that the anisotropy reflects an active tectonic process that involves the northward movement of the Oregon Block, which is rotating as a rigid body. In northern Cascadia, previous results of anisotropy show that the crust is undergoing a shortening process due to velocity differences between the Oregon Block and the North America plate, which is moving more slowly with respect to the Oregon Block, making it clash against Vancouver Island.