Crustal anisotropy from tectonic tremor in Guerrero, Mexico

Abstract

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.

Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico

“…Kaneshima 1990;Silver & Chan 1991;Silver 1996;Crampin & Gao 2006). Furthermore, the crustal, upper limit set for the time delay in this study is consistent with earlier anisotropy measurements in this same region of flat slab subduction (Song & Kim 2012a,b;Audet 2013;Huesca-Pérez et al 2016). Final ϕ and δt values for a particular depth at each station were obtained by discarding low-quality estimates and averaging the remaining Table 1.…”

Continental crust anisotropy measurements from tectonic tremor in Cascadia

“…However, some exceptions can be noted, such as station FAC1 or FACD, that show a high angle with respect to the north. The splitting times observed in the region studied range between 0.11 and 0.32 s. These splitting time magnitudes are consistent with what is usually observed in the crystalline continental crust worldwide [e.g., Cassidy and Bostock , ; Yang et al , ; Huang et al , ; Balfour et al , ; Huesca‐Pérez and Ghosh , ; Huesca‐Pérez et al , ]. The fast polarization directions tend to be margin parallel (Figures and a) and aligned with respect to the maximum horizontal compressive stress that is observed in Central and Southern Cascadia (blue bars in Figure b), as reported by the World Stress Map Project [ Heidbach et al , ] (see http://dc-app3-14.gfz-potsdam.de).…”

“…Seismic anisotropy refers to the dependence of the velocity of a seismic wave on its direction of propagation. Seismic anisotropy in layered sedimentary rocks can have many causes, including mineral alignment, as in schist [e.g., Valcke et al, 2006;Bostock and Christensen, 2012;Huesca-Pérez et al, 2016], alignment of grain-scale fabrics [e.g., Hall et al, 2008], and nonhydrostatic stresses that align microfracture sets [Zatsepin and Crampin, 1997;Verdon et al, 2008]. Shear wave splitting is probably the least ambiguous indicator of seismic anisotropy [Verdon et al, 2008].…”

“…Cassidy and Bostock [1996] and Yang et al [2011] have observed that crustal anisotropy splitting times are stronger in the upper 20 km of the crust than at the bottom because the upper part is more brittle and fractured [Cassidy and Bostock, 1996]. Anisotropy whose origin is the lower part of the crust could be explained in terms of several factors, such as mineral alignments from metamorphosed rocks at the base of the crust, strain induced from slow slip, and high pore fluid pressures [Huesca-Pérez et al, 2016]. The most common anisotropic models are horizontal layers and/or horizontally aligned mineral and grain-scale fabrics.…”

Margin-wide continental crustal anisotropy in the Mexican subduction zone

“…It is important to note that this is a companion article that extends our results to the subduction zone of Mexico. A previous article of Huesca et al (2016) studied the region of Guerrero and here we analyse the entire subduction zone of Mexico adding two new regions, Jalisco and Oaxaca, that flank the Guerrero region, at the west and east, respectively. (2) We describe the data processing and (3) we discuss the causes of anisotropy in the MSZ, with focus on the Jalisco-Colima-Michoacán and Oaxaca provinces.…”