Continental crust anisotropy measurements from tectonic tremor in Cascadia

Abstract

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.

Margin-wide continental crustal anisotropy in the Mexican subduction zone

“…On the other hand, in Oaxaca state, fast polarization directions are generally oriented NW-SE and are oblique to the trench. The splitting times range between 0.07 and 0.34 s and they are consistent with other observations in the continental crust (Cassidy & Bostock 1996;Huang et al 2011;Yang et al 2011;Balfour et al 2012;Huesca-Pérez & Ghosh 2015;Huesca-Pérez et al 2016;Huesca-Pérez et al 2017). the anisotropy.…”

“…The existence of tremor activity all along the subduction margin opens the possibility of analysing the polarization of these recurrent signals. Anisotropy studies using TT signals have been performed successfully in Vancouver Island, Canada (Bostock & Christensen 2012), in northern Washington State (Huesca-Pérez & Ghosh 2015), southern Cascadia (Huesca-Pérez et al 2017) and in Guerrero state at the MSZ (Huesca-Pérez et al 2016). However, crustal anisotropy has also been studied using other methods such as receiver functions (Castellanos et al 2017), P-wave azimuthal variations (Crampin et al 1980;Ji et al 1993), and surface wave analysis (Stubailo et al 2012).…”

Geneses of Two Contrasting Antigorite Crystal Preferred Orientations and Their Implications for Seismic Anisotropy in the Forearc Mantle

“…Because the vertical thickness of the mantle wedge is small above the shallowly subducted slab, a weak trench‐parallel seismic anisotropy is still expected (Figure 8d). This may be the case for some warm and low‐angle subduction zones such as Cascadia and Nankai ( T 30 km ~ 260–400°C and θ ~ 20–30°; see Syracuse et al, 2010), where despite extensive serpentinization (Bostock et al, 2002; Brocher et al, 2003; Kamiya & Kobayashi, 2000), very short delay times (<0.1–0.3 s) are contributed by forearc mantle wedge (Huesca‐Pérez et al, 2017; Long, 2016; Salah et al, 2009).…”