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Shear Wave Splitting and Mantle Flow in Mexico: What Have We Learned?

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GEOFÍSICA INTERNACIONAL (2017) 56-2: 187-217 ORIGINAL PAPER Shear Wave Splitting and Mantle Flow in Mexico: What Have we Learned? Raúl W. Valenzuela* and Gerardo León Soto Received: November 15, 2016; accepted: December 08, 2016; published on line: April 01, 2017 Resumen con un énfasis en las zonas de subducción. Una justificación importante para el estudio de la El presente artículo es un resumen y análisis anisotropía sísmica es que permite conocer las de los estudios de partición de ondas características del flujo en el manto superior transversales (shear wave splitting) para así como su relación con procesos tectónicos. el manto superior que se han realizado en México tiene muchos y diversos ambientes México durante la última década. Cuando una tectónicos. Algunos de ellos se encuentran onda sísmica entra en un medio anisótropo actualmente activos y otros lo fueron en el se parte (o se separa), esto quiere decir que pasado, pero en cualquier caso han dejado se producen una onda rápida y otra lenta. Se su marca en la forma de anisotropía sísmica. necesitan dos parámetros para cuantificar la Esto ha dado lugar a una gran variedad anisotropía. Dichos parámetros son la dirección de mecanismos para producir el flujo del de polarización rápida y el tiempo de retardo manto. De manera general la presentación entre la onda rápida y la lenta. Se presenta se ha organizado en las siguientes regiones: un ejemplo de la aplicación de la técnica península de Baja California, la región Mexicana empleando la fase SKS ya que la mayoría de Occidental de Cuencas y Sierras, el norte y las observaciones usan datos telesísmicos. Sin noreste de México, la Fosa Mesoamericana, embargo, también se incluyen los resultados la península de Yucatán y la anisotropía en la de dos estudios que usaron ondas S locales base del manto. La relación entre la anisotropía de sismos intraplaca. Se explican aspectos y el flujo del manto se analiza con base en las importantes para interpretar las mediciones de características particulares de cada región. partición. Entre ellos se incluyen la ubicación de la anisotropía en función de la profundidad, Palabras clave: partición de ondas S, la relación entre la estructura cristalina de la anisotropía del manto superior, flujo del manto, olivina y el flujo del manto, el papel que juega movimientos de placas, Fosa Mesoamericana, el movimiento absoluto de placas y el papel placas de Cocos, Rivera, Pacífico y América del que juegan los movimientos relativos de placas Norte. R. W. Valenzuela* Departamento de Sismología Instituto de Geofísica Universidad Nacional Autónoma de México Ciudad Universitaria Delegación Coyoacán, 04510 Mexico CDMX, México *Corresponding author: [email protected] G. León Soto Instituto de Investigaciones en Ciencias de la Tierra Universidad Michoacana de San Nicolás de Hidalgo Morelia, Mich., Mexico 187 R. W. Valenzuela and G. León Soto Abstract motivation for studying seismic anisotropy is that it makes it possible to constrain the A review is presented of the shear wave characteristics of upper mantle flow and its splitting studies of the upper mantle carried relationship to tectonic processes. Mexico has out in Mexico during the last decade. When a many diverse tectonic environments, some of seismic wave enters an anisotropic medium which are currently active, or were formerly it splits, which means that a fast and a slow active, and have left their imprint on seismic wave are produced. Two parameters are anisotropy. This has resulted in a wide variety used to quantify anisotropy. These are the of mechanisms for driving mantle flow. Broadly fast polarization direction and the delay time speaking, the discussion is organized into the between the fast and the slow wave. An following regions: Baja California peninsula, example of the measurement technique is Western Mexican Basin and Range, northern presented using an SKS phase because most and northeastern Mexico, the Middle America observations are based on teleseismic data. Trench, the Yucatán peninsula, and lowermost Results of two studies using local S waves mantle anisotropy. Depending on the unique from intraslab earthquakes are also discussed. characteristics encountered within each region, Key aspects of the interpretation of splitting the relationship between anisotropy and mantle measurements are explained. These include the flow is explored.. depth localization of anisotropy, the relation- ship between olivine fabrics and mantle flow, Key words: shear wave splitting, upper mantle the role of absolute plate motion, and the anisotropy, mantle flow, plate motions, Middle role of relative plate motions with a special America Trench, Cocos, Rivera, Pacific, and focus on subduction zones. An important North American plates. Introduction component of the upper mantle and it is an anisotropic mineral (Stein and Wysession, Seismic anisotropy is a process whereby elastic 2003). Mantle anisotropy is the result of the waves travel faster in a preferred direction strain induced lattice preferred orientation and slower in other directions. It occurs for (LPO) of upper mantle minerals, predominantly both P and S waves, e. g. Savage (1999) olivine (Silver and Chan, 1991; Savage, 1999). and Park and Levin (2002). Different seismic As explained below, seismic anisotropy can phases, analyzed with different methods, can oftentimes be used to tell the direction of be used to measure anisotropy. These include mantle flow and can also be related to different studies relying on the refracted Pn phase, tectonic processes. tomography of both body and surface waves, shear wave splitting, surface wave scattering, Worldwide, shear wave splitting studies of and the receiver function technique; see the upper mantle, using teleseismic phases, Park and Levin (2002) and Long (2013) for a were published starting in the 1980s once review. It is the purpose of this paper to focus broadband seismometers became widely avai- on shear wave splitting and its relationship to lable (e. g. Ando and Ishikawa, 1982; Ando upper mantle flow in Mexico. et al., 1983; Ando, 1984; Kind et al., 1985; Bowman and Ando, 1987; Silver and Chan, When a shear wave propagates through an 1988, 1991; Vinnik et al., 1989a, 1989b, anisotropic medium, its component polarized 1992; Vinnik and Kind, 1993). The methods, parallel to the fast direction gets ahead of its as well as the results, have been extensively orthogonal component, which is thus known discussed in several excellent reviews (Silver, as the slow wave. In this case the fast wave 1996; Savage, 1999; Park and Levin, 2002; “splits” from the slow one. This phenomenon Long and Silver, 2009a; Long and Becker, is the equivalent of the birefringence observed 2010). Given that subduction zones play a for light (electromagnetic) waves traveling dominant role as drivers of plate tectonics, at different speeds within a calcite crystal, their anisotropy structure has been extensively as described in optics textbooks, e. g. Hecht studied (e. g. Long and Silver, 2008, 2009b; (1987). Two parameters are needed to quantify Long, 2013; Long and Wirth, 2013; Lynner and shear wave splitting. These are the polarization Long, 2014a, 2014b; Paczkowski et al., 2014a, direction of the fast wave, f, an angle usually 2014b). Shear wave splitting results have been measured clockwise from north, and the delay compiled in excellent databases by Liu (2009) time, dt, between the fast and the slow wave, for North America, and by Wüstefeld et al. e. g. Silver and Chan (1991). Olivine is a major (2009) worldwide. 188 VOLUME 56 NUMBER 2 GEOFÍSICA INTERNACIONAL Many studies in Mexico have used and temporary networks together provides teleseismic, core-transmitted phases such a thorough picture, both geographically and as SKS. In the earliest work, Barruol and in time. Permanent networks are made up of Hoffmann (1999) made a few measurements fewer stations located farther apart, but cover of shear wave splitting parameters at UNM, the a larger area over a long period of time. On the only Geoscope station in Mexico. Later on, van other hand, temporary networks densely cover Benthem (2005) made an attempt to present a smaller area for a few years. a unified view of upper mantle anisotropy and flow for the entire country, but only a handful Measurement of Shear Wave Splitting of stations were available then. He used data Parameters from the permanent network operated by Mexico’s Servicio Sismológico Nacional, SSN Many shear wave splitting studies of the (Singh et al., 1997), and from the temporary upper mantle in Mexico (van Benthem, 2005; NARS-Baja California deployment (Trampert et Obrebski et al., 2006; Obrebski, 2007; Stubailo al., 2003; Clayton et al., 2004). Later studies and Davis, 2007, 2012a, 2012b, 2015; van were focused, for the most part, on particular Benthem et al., 2008, 2013; Bernal-Díaz et al., regions of the country, using data mostly from 2008; León Soto et al., 2009; Long, 2009a, temporary arrays. Research in northwestern 2010; Rojo-Garibaldi, 2011; Ponce-Cortés, Mexico (Obrebski et al., 2006; Obrebski, 2007; 2012; Stubailo, 2015; Bernal-López, 2015; van Benthem et al., 2008; Long, 2010) used Bernal-López et al., 2016) have worked with data from the permanent networks Red Sísmica core-transmitted phases such as SKS, sSKS, del Noroeste de México, RESNOM (Grupo SKKS, and PKS because they offer several RESNOM, 2002), and Red Sísmica de Banda advantages that will be discussed shortly. Ancha del Golfo de California, RESBAN (Castro Most importantly, in the isotropic case *KS et al., 2011), in addition to the temporary waves are radially (i. e. SV-) polarized and NARS-Baja California deployment (Trampert thus should not be observed on the transverse et al., 2003; Clayton et al., 2004). A number component, making it easier to verify that the of dense, temporary arrays have been used to measured parameters (f, dt) are reliable. The study subduction of the oceanic Cocos (MASE, technique used to quantify splitting, however, 2007; Pérez-Campos et al., 2008; VEOX, is more general and can be applied to shear 2010; Melgar and Pérez-Campos, 2011; Kim waves containing both SV and SH energy from et al., 2011) and Rivera (Yang et al., 2009) local events (e.
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