Estimates of Shallow Crustal Stress Heterogeneity Length Scale from Borehole Breakouts and Local Earthquake Focal Mechanism Inversions in the Los Angeles Basin

Karen M. Luttrell, & Jeanne L. Hardebeck

Submitted August 15, 2018, SCEC Contribution #8803, 2018 SCEC Annual Meeting Poster #143

Many contributions to the SCEC Community Stress Model rely heavily on the observations of stress field orientation provided by earthquake focal mechanisms and borehole breakouts to constrain their estimates. However, earthquake focal mechanisms and borehole breakouts necessarily sample different locations within the 3-D crust, and it is unclear how these observations should be jointly interpreted and incorporated into ongoing modeling efforts. Previous comparisons between existing borehole breakout observations and local earthquake focal mechanism inversions indicate strong disagreement between the two, even when considering only shallow earthquakes. The goal of this research is to constrain the length scale over which borehole breakouts and earthquake focal mechanisms indicate consistent stress field orientations. We consider 60 published observations of SHmax from borehole breakouts throughout the Los Angeles basin area. We identified subsets of earthquakes nearby each borehole, using a range of maximum depth criteria and maximum distance criteria, and inverted each subset of earthquake focal mechanisms for the orientation of the local stress state. Focal mechanisms in each subset are derived from the YSH catalog [Yang et al., 2012] and independently inverted using the method of Michael [1984, 1987]. The nodal plane with the greater instability in the stress field is selected as the preferred plane [Vavryńćuk, 2014], and the uncertainty is determined by bootstrap resampling. We then compared the SHmax from each local subset with SHmax from each borehole to identify the optimal length scale of earthquakes consistent with borehole observations. Preliminary results suggest the best agreement between SHmax derived from borehole breakouts and SHmax derived from earthquake focal mechanisms is achieved when considering earthquakes that are shallow (< 5 km depth) and nearby the borehole breakout (< 1 km horizontal distance). This may indicate that stress orientations from borehole breakouts are primarily representative of very nearfield stresses and should not be interpreted to represent the broader tectonic crustal stresses.

Citation
Luttrell, K. M., & Hardebeck, J. L. (2018, 08). Estimates of Shallow Crustal Stress Heterogeneity Length Scale from Borehole Breakouts and Local Earthquake Focal Mechanism Inversions in the Los Angeles Basin. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
SCEC Community Models (CXM)