Geofísica Internacional ISSN: 0016-7169 [email protected] Universidad Nacional Autónoma de México México Rodríguez-Lozoya, Héctor E.; Domínguez R., Tonatiuh; Quintanar Robles, Luis; Aguilar Meléndez, Armando; Rodríguez-Leyva, Héctor E.; Plata Rocha, Wenseslao; García Páez, Fernando Attenuation of Coda Waves in the Central Region of the Gulf of California, México Geofísica Internacional, vol. 56, núm. 2, enero-marzo, 2017, pp. 137-145 Universidad Nacional Autónoma de México Distrito Federal, México Available in: http://www.redalyc.org/articulo.oa?id=56850598001 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) 56-2: 137-145 ORIGINAL PAPER Attenuation of Coda Waves in the Central Region of the Gulf of California, México Héctor E. Rodríguez-Lozoya, Tonatiuh Domínguez R., Luis Quintanar Robles, Armando Aguilar Meléndez, Héctor E. Rodríguez-Leyva, Wenseslao Plata Rocha and Fernando García Páez Received: January 06, 2016; accepted: October 25, 2016; published on line: April 01, 2017 Resumen frecuencias de 1 a 7 Hz. El valor y la alta QO dependencia de la frecuencia están de acuerdo Se analizan las ondas de coda de eventos con los valores reportados para otras regiones registrados por la red sísmica de NARS que caracterizadas por una alta actividad tectónica. cuenta con instrumentos instalados a lo largo Con base en la distribución de estaciones de ambas márgenes del Golfo de California, respecto a las fuentes, se definieron dos México, para estimar atenuación . Se utilizó Qc subregiones (norte y sur). Se calcularon modelo de dispersión simple de Sato (1977) los valores de y se correlacionaron con QC para ventanas tiempo de 20 a 25 segundos la tectónica y morfología de cada zona. comenzando en dos veces el tiempo de viaje Se observa una mayor atenuación en la de la onda S. Se analizaron eventos registrados región sur que puede ser atribuida a que entre 2003 y 2007 ocurridos en la región esa zona esté más fracturada dado que los central del Golfo de California. Las distancias eventos sísmicos mayores ocurren de la fuente-receptor analizadas son entre 40 y 500 zona centro del Golfo de California hacia el km. Suponiendo una relación de dependencia sur. Por otro lado, la corteza de la zona sur de de la frecuencia de la forma Qc QC (f) es de menor espesor que la zona norte. = a, los valores promedio encontrados QO f fueron de = 83±3 y una dependencia de QO Palabras clave: Atenuación de coda, atenuación- la frecuencia a de 1.06±0.03 en el rango de frecuencia, Golfo de California, Mexico. H. E. Rodríguez-Lozoya* A. Aguilar Meléndez H. E. Rodríguez-Leyva Universidad Veracruzana F. García Páez Campus Poza Rica, México Facultad de Ingeniería Universidad Autónoma de Sinaloa, México W. Plata Rocha *Corresponding author: [email protected] Facultad de Ciencias de la Tierra y el Espacio Universidad Autónoma de Sinaloa, México T. Domínguez R. Centro Universitario de Estudios e Investigación en Vulcanología Universidad de Colima, México L. Quintanar Robles Instituto de Geofísica Universidad Nacional Autónoma de México Ciudad Universitaria Delegación Coyoacán 04510 Mexico CDMX, México 137 H. E. Rodríguez-Lozoya, et al. Abstract value and the high frequency dependency QO agree with the values of other regions Coda waves were analyzed from events characterized by a high tectonic activity. recorded by NARS seismic network deployed Based on source-station distribution two along both margins of the Gulf of California, subregions (north and south) were defined. Mexico, to estimate coda attenuation Qc. Sato’s values were calculated and correlated QC (1977) single scattering model was used for a with tectonics and morphology of each area. coda window of 20 to 25 s beginning at twice The observed higher attenuation in the south the S-wave travel time. Events recorded from region can be attributed to the fact that south 2003 to 2007 located in the central region of region is more fractured since the greater the Gulf of California were analyzed. earthquakes occur in central to south Gulf of Source-to-receiver distances are between 40 California and the oceanic crust is reported to and 500 km. Assuming a power law of the be thinner in the southern region. form = a, values were averaged QC (f) QO f QC and value of = 83±3 and a frequency- a QO Key words: Coda attenuation, attenuation- dependence α value of 1.06±0.03 in the frequency, Gulf of California, Mexico. frequency range from 1 to 7 Hz was obtained. Introduction Data from 6 stations of the NARS-Baja network (Network of Autonomously Recording The Gulf of California, Mexico, is a complex Seismographs of Baja California) installed tectonic zone where the peninsula of Baja by Utrecht University, CALTECH and CICESE California is separating from the continent. This (Centro de Investigación Científica y Educación active zone has generated moderate earthquakes Superior de Ensenada Baja California, México) which have affected cities on both sides of the during 2002 were used. This region was chosen gulf. In 2012, an M6.0 earthquake affected due to its implication in terms of civil protection the city of Los Mochis, where damages in the for the cities on both sides of the Gulf. The regional hospital were reported. Last earthquake correlation of the results with the morphology occurred on September, 2015 (MW6.7). and tectonics of the region is also presented. Social and economic effects resulting from Tectonic Setting and Seismicity earthquakes can be reduced through seismic risk analysis. The elaboration of good quality The Gulf of California forms an oblique- hazard maps is required. To do this, studies divergent boundary between the Pacific and of source characteristics and wave propagation North America plates. Relative motion between are essential. Attenuation is an important the peninsula of Baja California and North factor to such studies. America plate in the southern gulf is about 46 mm/yr (Plattner et al., 2007). It consists of Attenuation of seismic waves has been a system of linked transform faults and short widely studied in different regions around the spreading centers forming basins (Lonsdale, world since Aki and Chouet (1975) and Sato 1989). Deformation accommodates in oblique (1977) introduced their theories on coda waves faults to the north and transform faults to attenuation (Q). Different tectonic regions the south (Fenby and Gastil, 1991; Nagy and around the world (e.g., volcanic, active, stable) Stock, 2000). Northern basins are shallower have been characterized by their Q-1 values than those to the center and southern basins (Pulli, 1984; Jin et al., 1985; Wiggins-Grandison (Curray et al., 1982; Lonsdale, 1989; Persaud and Havskov, 2004, among others). In Mexico et al., 2003). there are attenuation studies mostly for the south and southeastern for the subduction In the north zone of the Gulf, an incipient zones of the Rivera and Cocos plates (Castro spreading center begins in the Wagner basin and Munguía, 1993; Ordaz and Singh 1992; (Persaud et al., 2003) developed from Cerro Prieto Domínguez et al., 2003, among others) and for and Wagner transform faults, and ending in the the northwestern zone of Mexico for the Gulf south with the East Pacific Rise which corresponds of California region (Domínguez et al., 1997; to the northern limit of the Rivera plate. Castro et al., 2008). Lonsdale (1989) suggested the presence of In the present study, Sato’s formulation a transitional crust at the Guaymas basin in the (Sato, 1977) was used to estimate at the middle sector of the Gulf of California. In this QC central zone of the Gulf of California. location, there is a high rate of sedimentation, 138 VOLUME 56 NUMBER 2 GEOFÍSICA INTERNACIONAL Figure 1. Tectonic setting and location of used seismic stations (triangles) and analyzed earthquakes (circles). White squares indicate main cities in the area. Red circles indicate events with Mw >5. TAFZ (Tosco-Abreajos fault zone), EPR (East Pacific Rise). which promoted the development of a basin of scatterers in an infinite volume and cross- containing sills and altered sediments. sectional area s. The density of energy in terms of root mean square (rms) amplitude, scattered Most of the seismicity is distributed in the by the inhomogeneities on the surface of an NW–SE direction along the axis of the Gulf ellipsoid whose foci are the source, and the of California, following a linear trend that, receiver can be expressed as from north to south, steps southward near the main basins (Wagner, Delfin, Guaymas, 12/ 12/ 1 Ω()ω ∆f []K()α −ω tQ/ 2 Carmen, Farallon, Pescadero, and Alarcon) A(,rt\)ω = e , and spreading centers. Seismicity in the ω []2πσL r northern zone has low magnitude (Castro et (1) al., 2007) but increases to the south reaching magnitudes of M>6 such as the 2010 and where A is the rms amplitude of the coda 2015 earthquakes that occurred near (south) wave, W(w) is the total energy radiated by parallel 25º N, or the 1992 earthquake (M7.0), the source within a frequency band, r is the which is the greater earthquake ever recorded distance between the source and the receiver, in this zone (Pacheco and Sykes, 1992). 1 (a) = [(a+1)/(a−1)] and a = , t is the K a ln t/ts Only good quality data (high signal/noise elapsed time of the coda wave, is the elapsed ts ratio) was used from five years recording which time of the S wave, both measured from the included 4 moderate events (M>5) and their earthquake origin time.