Stress Field

The Los Angeles region is the site of much active deformation, observable in the seismicity patterns, geodetic measurements, and neotectonic fault displacements. The regional stress field plays an important role in controlling which faults are likely to move, and in placing constraints on the dynamic processes controlling the recent evolution of the area. We are analyzing the regional stress field, and its variations, using data on borehole ellipticity in non-vertical drill holes (information provided by the oil industry). The degree of ellipticity is a function of the orientation and magnitudes of the principal stresses in the region, and is also dependent on the strength of the rock in the borehole.

So far, we have found generally N to NNE direction of compression in the southern California region, except for several anomalous areas (eastern San Fernando valley and Santa Susana mountains; near the Whittier Fault) which may be affected by local structural complexities, such as lateral ramps in the frontal fault system of the San Gabriel Mountains.

Click here to see the web page with the stress results.

Click here to see some results of inversions of specific smaller datasets for the southern California region.

Results were published in these articles:

Kerkela, Stacy and J. M. Stock, Compression directions north of the San Fernando Valley determined from borehole breakouts, Geophysical Research Letters, 23, No. 23, 3365-3368, 1996.

Wilde, Melita, and J. M. Stock, Compression directions in southern California (from Santa Barbara to Los Angeles Basin) obtained from borehole breakouts, J. Geophys. Res., 102, No. B3, 4969-4983, March 1997.

Zajac, B., and J. M. Stock, Using borehole breakouts to constrain the complete stress tensor: Results from the Sijan Deep Drilling Project and offshore Santa Maria Basin, California, J. Geophys. Res., 102, No. B5, 10,083-10,100, 1997.

Funding for this project came from the Southern California Earthquake Center and from Unocal Corporation.