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Full Moment Tensor Variations and Isotropic Characteristics of Earthquakes in the Gulf of California Transform Fault System

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Abstract

The full moment tensor is a mathematical expression of six independent variables; however, on a routine basis, it is a common practice to reduce them to five assuming that the isotropic component is zero. This constraint is valid in most tectonic regimes where slip occurs entirely at the fault surface (e.g. subduction zones); however, we found that full moment tensors are best represented in transform fault systems. Here we present a method to analyze source complexity of earthquakes of different sizes using a simple formulation that relates the elastic constants obtained from independent studies with the angle between the slip and the fault normal vector, referred to as angle \( \theta \); this angle is obtained from the full moment tensors. The angle \( \theta \), the proportion of volume change \( \left( k \right) \) and the constant volume (shear) component \( \left( T \right) \) are numerical indicators of complexity of the source; earthquakes are more complex as \( \theta \) deviates from \( \pi /2 \) or as T and k deviate from zero as well. These parameters are obtained from the eigensolution of the full moment tensor. We analyzed earthquakes in the Gulf of California that exhibit a clear isotropic component and we observed that the constant volume parameter T is independent of scalar moments, suggesting that big and small earthquakes are equally complex. In addition, simple models of one single fault are not sufficient to describe physically all the combinations of \( \theta \) in a source type plot. We also found that the principal direction of the strike of the Transform Fault System in the Gulf of California is following the first order approximation of the normal surface of the full moment tensor solution, whereas for deviatoric moment tensors the principal direction does not coincide with the strike of the Transform Fault System. Our observations that small and large earthquakes are equally complex are in agreement with recent studies of strike-slip earthquakes.

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Acknowledgments

This work was funded by CONACYT Mexico, under Grant Number 133910. Additionally, we would like to thank Sean Ford and an anonymous reviewer for their insightful comments.

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Authors and Affiliations

  1. Unidad La Paz, Centro de Investigación Científica y de Educación Superior de Ensenada, Miraflores 334, Col. Bellavista, 23050, La Paz, BCS, Mexico

    Roberto Ortega

  2. Instituto de Geofísica, Universidad Nacional Autónoma de México, Circuito de la Investigación Científica, 04510, Coyoacán, D.F., Mexico

    Luis Quintanar

  3. Institute de Physique du Globe de Strasbourg, 5 rue Descartes, 67084, Strasbourg Cedex, France

    Luis Rivera

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  1. Roberto Ortega
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  2. Luis Quintanar
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  3. Luis Rivera
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Correspondence to Roberto Ortega.

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Ortega, R., Quintanar, L. & Rivera, L. Full Moment Tensor Variations and Isotropic Characteristics of Earthquakes in the Gulf of California Transform Fault System. Pure Appl. Geophys. 171, 2805–2817 (2014). https://doi.org/10.1007/s00024-013-0758-7

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