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The 2006 Bahía Asunción Earthquake Swarm: Seismic Evidence of Active Deformation Along the Western Margin of Baja California Sur, Mexico

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Abstract

The study of the Bahía Asunción earthquake swarm is important for two reasons. First, the earthquakes are clear evidence of present activity along the zone of deformation on the Pacific margin of Baja California. The swarm, with earthquakes of magnitude M w of up to 5.0, occurred on the coastline of the peninsula, showing that the Tosco-Abreojos zone of deformation is wider than previously thought. Second, the larger earthquakes in the swarm caused some damage and much concern in Bahía Asunción, a small town located in the zone of epicenters. We relocated the larger earthquakes with regional and/or local seismic data. Our results put the earthquake sources below the urban area of Bahía Asunción, at 40–50 km to the north of the teleseismically determined epicenters. In addition, these new locations are in the area of epicenters of many smaller events that were located with data from local temporary stations. This area trends in an E–W direction and has dimensions of approximately 15 km by 10 km. Most earthquakes had sources at depths that are between 4 and 9 km. A composite focal mechanism for the smaller earthquakes indicated right-lateral strike–slip motion and pure-normal faulting occurred during this swarm. Interestingly, the ANSS earthquake catalog of the United States Geological Survey (USGS) reported each one of these faulting styles for two large events of the swarm, with one of these earthquakes occurring 2 days before the other one. We associate the earthquake with strike–slip mechanism with the San Roque Fault, and the earthquake with the normal faulting style with the Asunción Fault. However, there is need of further study to verify this possible relation between the faults and the earthquakes. In addition, we recorded peak accelerations of up to 0.63g with an accelerometer installed in Bahía Asunción. At this site, an earthquake of M w 4.9 produced those high values at a distance of 4.1 km. We also used the acceleration dataset from this site to estimate the linear response of sediments lying beneath the station. The resulting average amplification function has a fundamental resonance frequency of about 5 Hz and shows amplification factors of 2–4 for motions at frequencies in the range 2–8 Hz. A comparison of this amplification function with the response of soils to shaking from larger events shows that above 20 Hz the amplification of motion in the larger events decreases relative to the weak-motion response.

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Acknowledgments

Financial support for this research was provided by Centro de Investigación Científica y de Educación Superior de Ensenada, B. C. (CICESE). We also want to express our gratitude to Guillermo Gutiérrez de Velasco, of CICESE at La Paz, Baja California Sur, Pedro G. Osuna López of H. XII Ayuntamiento de Mulegé, Luis Martínez Pozo, Subdelegate of Bahía Asunción, Mario Ramade of Federación de Cooperativas de Baja California, Managing personnel of Cooperativa Leyes de Reforma, Cesar Redona Camacho of Sociedad Cooperativa California en San Ignacio, Ramón Villa Villegas, Armando Naranjo Rivera of XIII District of Mulegé, David A. Maraver Romero of Servicio Geológico Mexicano (SGM), Antonio de Jesús Ceseña Moyrón of Seguridad Pública y Tránsito Municipal of Bahía Asunción, and to personnel of Civil Protección of Baja California Sur. We acknowledge Carlos Gutiérrez and technicians of Centro Nacional de Prevención de Desastres for providing some seismic data they collected in the epicentral area with an analog seismograph and two weak-motion digital stations. We appreciate the field service provided by Martín Romero to our analog stations. Many other people supported our study in different forms (technical support, economic funds, transportation vehicle, materials and accessories, etc.). Our deepest thanks to all. The authors would also like to acknowledge the work of Dr. William Bandy and two anonymous reviewers. Their in-depth reviews and constructive comments and suggestions were helpful in improving the manuscript.

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Authors and Affiliations

  1. División de Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada, B. C., Carretera Ensenada-Tijuana No. 3918, Zona Playitas, C. P. 22860, Ensenada, Baja California, Mexico

    Luis Munguía, Ignacio Méndez, Mario González-Escobar & Manuel Luna

  2. Centro de Investigación Científica y de Educación Superior de Ensenada, B. C., Unidad La Paz, Baja California Sur, Miraflores 334, Fraccionamiento Bellavista, C. P. 23050, La Paz, Baja California Sur, Mexico

    Sergio Mayer & Alfredo Aguirre

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  1. Luis Munguía
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  2. Sergio Mayer
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  3. Alfredo Aguirre
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Correspondence to Luis Munguía.

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Munguía, L., Mayer, S., Aguirre, A. et al. The 2006 Bahía Asunción Earthquake Swarm: Seismic Evidence of Active Deformation Along the Western Margin of Baja California Sur, Mexico. Pure Appl. Geophys. 173, 3615–3629 (2016). https://doi.org/10.1007/s00024-015-1184-9

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