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Redalyc.A Simplified Method to Invert Slow Slip Events: Examples for The Geofísica Internacional ISSN: 0016-7169 [email protected] Universidad Nacional Autónoma de México México Castro Artola, Oscar A.; Iglesias, Arturo; Kostoglodov, Vladimir; Franco-Sánchez, Sara I.; Hjörleifsdóttir, Vala; Cabral-Cano, Enrique; Santoyo, Miguel A. A Simplified Method to Invert Slow Slip Events: Examples for the 2002, 2006 and 2014 events in Guerrero, Mexico Geofísica Internacional, vol. 56, núm. 4, octubre-noviembre, 2017, pp. 377-389 Universidad Nacional Autónoma de México Distrito Federal, México Available in: http://www.redalyc.org/articulo.oa?id=56853159005 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative GEOFÍSICA INTERNACIONAL (2017) 56-4: 377-389 ORIGINAL PAPER A Simplified Method to Invert Slow Slip Events: Examples for the 2002, 2006 and 2014 events in Guerrero, Mexico Oscar A. Castro Artola*, Arturo Iglesias, Vladimir Kostoglodov, Sara I. Franco-Sánchez, Vala Hjörleifsdóttir, Enrique Cabral-Cano and Miguel A. Santoyo Received: February 02, 2017; accepted: August 11, 2017; published on line: October 01, 2017 Resumen Abstract Se desarrolló un método eficiente para An efficient method to obtain a simple model obtener un modelo simple de la distribución of the slip distribution on a fault plane was de deslizamientos en un plano de falla a partir developed from estimates of static offsets de deformaciones estáticas observadas en from continuous GPS measurements. Transfer mediciones continuas de GPS. Se calcularon functions were computed for the displacement funciones de transferencia; éstas representan due to a unit force on a fault plane (with a el desplazamiento debido a una fuerza unitaria predetermined geometry, subdivided in a dense en un plano de falla (con una geometría grid of subfaults) for all combinations of stations predeterminada, subdividida en una malla densa and subfaults, considering a layered media. The de subfallas) para todas las combinaciones de forward problem is built as a linear combination estaciones y subfallas, considerando un medio of elliptical slip-patches over the fault plane and estratificado. El problema directo se construye the transfer functions between each patch and como una combinación lineal de parches each station. To test the method, the 2002, 2006 elípticos de deslizamiento sobre el plano de and 2014 slow slip events (SSE) were modeled falla y las funciones de transferencia. Para in Guerrero, Mexico, assuming that slip occurred probar el método, se modelaron los eventos along the subduction interface between the de deslizamiento lento (SSE, por sus siglas en Cocos and North America plates. This subduction inglés) de 2002, 2006 y 2014 en Guerrero, fault was modeled as two adjacent planes, both México, suponiendo que el movimiento ocurrió striking 289° for 600 km approximately parallel a lo largo de la interfase de subducción entre to the Middle America trench and with a width las placas de Cocos y Norteamérica. Se modeló of 120 km (240 km total width). The shallower la interfase de subducción como dos planos plane starts at the trench and dips 14° and the adyacentes, ambos con rumbo de 289° y por deeper plane is sub-horizontal (2° dip). The 600 km aproximadamente, y paralelos a la merged fault plane is segmented in 120x48, 5x5 trinchera de América Central y con un ancho de km2 subfaults. A pre-calculated set of transfer 120 km (240 km de ancho total). El plano más functions is kept for future slow slip events, so somero comienza en la trinchera con un echado that these only need to be computed for new de 14°, mientras que el plano más profundo GPS stations. A strong agreement between es sub-horizontal (2 ° dip). El plano de falla observations and synthetic data was found for all combinado está dividido en 120x48 subfallas de analyzed events, and the results are consistent 5x5 km2. Un conjunto pre-calculado de funciones with those reported by previous studies. de transferencia se guarda para futuros eventos, de modo que éstas sólo necesitan ser calculadas Palabras clave: Eventos de deslizamiento lento, para nuevas estaciones GPS. Se encontró un inversión de deslizamiento, aplicación de GPS, buen ajuste entre las observaciones y los datos trinchera de Centro América, subducción, brecha cos para todos los eventos analizados, así como sísmica de Guerrero. consistencia con resultados reportados en estudios previos. A. Iglesias Key words: Slow slip events, slip inversion, GPS Vladimir Kostoglodov S. I. Franco-Sánchez application, Middle America trench, subduction, V. Hjörleifsdóttir Guerrero seismic gap. E. Cabral-Cano Instituto de Geofísica Universidad Nacional Autónoma de México O. A. Castro Artola* Ciudad Universitaria Posgrado en Ciencias de la Tierra Delegación Coyoacán, 04510 Instituto de Geofísica México CDMX, México Universidad Nacional Autónoma de México Ciudad Universitaria M. A. Santoyo Delegación Coyoacán, 04510 Instituto de Geofísica Unidad Michoacán México CDMX, México Universidad Nacional Autónoma de México *Corresponding author: [email protected] Morelia, Michoacán, México 377 O. A. Castro Artola, et al. Introduction The suggested approach requires a calculation of the predicted displacement at While modern computation capabilities allow each GPS station, due to a unitary force on each the use of powerful methods (e.g. finite element sub-fault; reference to these displacements will and finite difference methods) based on data- be as transfer functions (TF). One of the main intensive computing, the trade-off between objectives of this work is to set up a library of TF parameterization and the non-uniqueness in order to use them in future slow slip events problem makes it difficult to find reliable in the region. As the fault plane geometry will solutions especially for underdetermined not change with time, the functions for every inverse problems. It is not always possible to station could be readily used for impending add a priori information to constrain the inverse SSE. problem but sometimes a re-parameterization helps to reduce the non-uniqueness and allows The Guerrero Pacific coast is an over 400 obtaining simpler patterns. For example, km long, active seismic region, within the measurements of deformation obtained by Mexican Subduction Zone (MSZ). This region analyzing GPS time series have disclosed the has been intensively studied because its NW existence of aseismic slip over active fault part has been defined as a mature ~100 km planes. However, frequently the number of GPS length seismic gap (Singh et al., 1981) which stations and the quality of data are not enough has not produced any large earthquake since to adequately constrain the inverse problem, 1911. Several important cities in central making it difficult to distinguish the details of Mexico were strongly affected by earthquakes slip distribution over the fault plane. occurred in the MSZ (e.g. 1911: M~7.5, 1957: 7.8, 1985: 8.0, 1995: 7.3, etc.). The Okada’s (1985 and 1992) approach is first permanent GPS station deployed in the usually applied to obtain displacement at the region began to operate early 1997 and at the surface due to slip on a fault plane embedded beginning of 1998 a signal corresponding to a in a half-space. For example, in a global transient displacement was recorded (Lowry et inversion scheme (Iglesias et al., 2004) or al., 2001), later attributed to a slow slip event. a linear inversion method (Outerbridge et Since then, the GPS networks have grown al., 2010) it is used to get the distribution gradually and have recorded slow slip events of magnitude and direction of slip. There are with different complexity, about every 4 years. different approaches to retrieve slip distribution Slow slip events could have an important role over a fault plane (e.g. Radiguet et al., 2012, in the seismic cycle, not only because slip may and 2016). Different methods using distinct have reached the seismogenic (coupled zone) approximations can provide equally good fits part of the subduction interface (Graham et al., between data and synthetics with very different 2015) but also because they may trigger large slip distributions (Mai and Beroza, 2000), magnitude subduction thrust earthquakes (e.g. which begs the question; which model is most Graham et al.; 2014; Radiguet et al., 2016). realistic? To answer the question, Occam’s razor principle was invoked assuming that Methodology the simplest solution is the most appropriate. Therefore, a modified version of a frequency Forward model domain scheme was used to find kinematic parameters from displacement records (Cotton A frequency domain representation was and Campillo, 1995) to retrieve slip within a modified to model the faults kinematics fault plane at depth. Re-parameterization was assuming a discretized fault plane on which performed by introducing slip only on simple the kinematic parameters (slip, rise time geometrical patches within the fault plane. and timing of rupture initiation) change from This is a restriction in which slip is distributed subfault to subfault (Cotton and Campillo, symmetrically within each ellipse, reducing 1995). This approach has been successfully the model parameters and therefore the applied in several previous studies (e.g., number of model solutions. Furthermore, the Hernandez et al., 2001, Castro-Artola, 2013). A computational time and non-uniqueness of the static problem is considered, so there is no rise problem are reduced by this simplification. A time and initiation of rupture, only cumulative simulated annealing inversion procedure was slip. The displacement (ui) on the surface due used to search for the best model parameters, to a slip distribution within elliptical patches on as this procedure has been proven effective to a fault plane at depth is described by: explore a set of models and find a good solution n in a complex misfit space (e.g. Kirkpatrick et ,, ,,,, al., 1983; Iglesias et al., 2001).
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