The impact of static stress change, dynamic stress change, and the background stress on aftershock focal mechanisms

Journal of Geophysical Research B: Solid Earth
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Abstract

The focal mechanisms of earthquakes in Southern California before and after four M ≥ 6.7 main shocks provide insight into how fault systems respond to stress and changes in stress. The main shock static stress changes have two observed impacts on the seismicity: changing the focal mechanisms in a given location to favor those aligned with the static stress change and changing the spatial distribution of seismicity to favor locations where the static stress change aligns with the background stress. The aftershock focal mechanisms are significantly aligned with the static stress changes for absolute stress changes of ≥ 0.02 MPa, for up to ~20 years following the main shock. The dynamic stress changes have similar, although smaller, effects on the local focal mechanisms and the spatial seismicity distribution. Dynamic stress effects are best observed at long periods (30–60 s) and for metrics based on repeated stress cycling in the same direction. This implies that dynamic triggering operates, at least in part, through cyclic shear stress loading in the direction of fault slip. The background stress also strongly controls both the preshock and aftershock mechanisms. While most aftershock mechanisms are well oriented in the background stress field, 10% of aftershocks are identified as poorly oriented outliers, which may indicate limited heterogeneity in the postmain shock stress field. The fault plane orientations of the outliers are well oriented in the background stress, while their slip directions are not, implying that the background stress restricts the distribution of available fault planes.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title The impact of static stress change, dynamic stress change, and the background stress on aftershock focal mechanisms
Series title Journal of Geophysical Research B: Solid Earth
DOI 10.1002/2014JB011533
Volume 119
Issue 11
Year Published 2014
Language English
Publisher American Geophysical Union
Contributing office(s) Earthquake Science Center
Description 28 p.
First page 8239
Last page 8266
Country United States
State California
Online Only (Y/N) N
Additional Online Files (Y/N) N