Borehole Breakouts Versus Earthquake Focal Mechanisms as Stress Field Orientation Indicators in Southern California: Should We Agree to Disagree?

Karen M. Luttrell, & Jeanne L. Hardebeck

Published August 14, 2017, SCEC Contribution #7642, 2017 SCEC Annual Meeting Poster #208

One of the SCEC5 priorities is the development of an internally consistent group of Community Models (CXM), including the maturation of the Community Stress Model (CSM). Many of the geodynamic models that integrate multiple physical processes into estimates of lithosphere stress state rely heavily of 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 investigations comparing existing borehole observations and regional focal mechanism stress inversions indicate strong disagreement between the two, primarily due to the necessity of smoothing in regional -scale focal mechanism inversions that obscures small-scale heterogeneity.

To assess the robustness of these conclusions, we conduct new local-scale inversions of earthquake focal mechanisms for local stress field orientation in the vicinity of existing borehole observations of stress orientation. Borehole observations, drawn from Wilde and Stock [1997], are divided into six subregions: Santa Barbara, the upper Ojai Valley, the central Ventura Basin, the east Ventura Basin and central Transverse Range, the west Los Angeles Basin, and the east Los Angeles Basin. Focal mechanisms in each of these subregions are derived from the YSH catalog [Yang et al., 2012], with 10s to 1000s of earthquakes in each subregion. Mechanisms for each subregion are then 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 [Vavryčuk, 2014], and the uncertainty is determined by bootstrap resampling.

Preliminary results suggest that disagreement between borehole- and focal mechanism-derived stress orientations persist at local scale (few kilometers). This may indicate that stress in the top 2 km is fundamentally more heterogeneous than stress at seismogenic depth. Alternatively, it could indicate rotations in the stress field between the time borehole observations were made and the occurrence of earthquakes included in the catalog subsets. Further study is required to identify the primary cause of disagreement between these two types of stress orientation indicators.

Key Words
Stress, Community Models, Borehole, Focal Mechanism

Citation
Luttrell, K. M., & Hardebeck, J. L. (2017, 08). Borehole Breakouts Versus Earthquake Focal Mechanisms as Stress Field Orientation Indicators in Southern California: Should We Agree to Disagree?. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)