Heterogeneity of Shallow Crustal Stress Estimated From Borehole Breakouts and Local Earthquake Focal Mechanism Inversions in the Los Angeles Basin

Date: 4/26/2019

Time: 01:30 PM

Room: Vashon

Many models of stress state rely heavily on the observations of stress field orientation provided by earthquake focal mechanisms and borehole breakouts to constrain their estimates. However, these two types of observations necessarily sample different locations within the 3-D crust, and it is unclear how they should be jointly interpreted and incorporated into ongoing modeling efforts. The goal of this research is to constrain the conditions 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, 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 stress field orientation is most consistent when considering earthquakes that are shallow (< 5 km depth) and nearby the borehole breakout (< 10 km horizontal distance). Spatial variation in misfit between subregions suggests that underlying crustal structure may mitigate the characteristic length scale of stress field heterogeneity within the region. Overall, the results indicate that stress orientations from borehole breakouts are primarily representative of very nearfield stresses and should not in general be interpreted to represent the broader tectonic crustal stresses.

Presenting Author: Karen Luttrell

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