Compilation of data on the present stress field in southern California has given us a good background database of compression directions in the region. This includes previously published data on stress directions and magnitudes, and new data that we obtained by study of breakout orientations in drill holes around LA, using oil or gas industry well logs from the public domain. Wherever feasible we included information on the relative magnitudes of the principal stresses, from hydraulic fracturing measurements, inversion of focal mechanisms, and inversion of breakout orientations from boreholes (Zajac and Stock, 1997). We have now analyzed all of the useable wells in the Division of Oil and Gas archives, and used these to map compression directions in the southern California area (Wilde and Stock, 1997; Kerkela and Stock, 1996). These show a generally N to NNE direction of maximum horizontal compressive stress, except locally near the Whittier fault and in the NE San Fernando Valley, where the direction of maximum horizontal compression strikes NW (Figure 1). We recently acquired a few more datasets from oil companies for offshore wells which still need to be analyzed. Major results for 1997 include Ö modification of our inversion code to minimize the stress differences between the observed and expected breakout orientations. This gives more physically realistic results than the inversion based on observed angular differences which was previously been used by most authors. It also gives smaller (and probably more realistic) uncertainty regions on the directions of the principal stress axes; Ö inversion of selected breakout data sets from variably oriented, deviated boreholes in the LA region, in order to place constraints on the complete stress tensor. We find that over even small subregions of southern California, the variations in the stress field at shallow depths (the upper 3 km) can be quite significant (Figure 2).
Kerkela, S., and J. M. Stock, Compression directions north of the San Fernando Valley determined from borehole breakouts, Geophys. Res. Lett., v. 23, no 23, p. 3365-3368, 1996. SCEC publication number 342.
Wilde, M., and J. M. Stock, Compression directions in southern California (from Santa Barbara to the Los Angeles Basin) obtained from borehole breakouts, J. Geophys. Res., v. 102, no. B3, p. 4969-4983, 1997. SCEC publication number 320.
Zajac, B., and J. Stock, Using Borehole Breakouts to constrain the complete stress tensor, J. Geophys. Res., v. 102, p. 10,083-10,100, 1997.
Publications resulting from work in 1997:
Wilde, M., and J. M. Stock, Compression directions in southern California (from Santa Barbara to the Los Angeles Basin) obtained from borehole breakouts, J. Geophys. Res., v. 102, no. B3, p. 4969-4983, 1997. SCEC publication number 320.
