Page 263 - Global Tectonics

248 CHAPTER 8

segments of the fault or to the different methods of imity to these repeating earthquakes to obtain down-
inferring stresses. To resolve these problems indepen- hole measurements of the physical and chemical
dent measurements of principal stress orientations and conditions under which earthquakes occur and to

magnitudes from within large, tectonically active faults exhume rock and fluid samples for laboratory analyses
are needed. The San Andreas Fault Observatory at (Hickman et al., 2004). Although there is still consider-
Depth (SAFOD) drilling program involves such mea- able uncertainty in the preliminary estimates of hori-
surements. This program involves drilling into the zontal stress magnitudes, stress observations near the
hypocentral zone of repeating M ≈ 2 earthquakes on a bottom of a 2.2-km-deep pilot hole (Fig. 8.25b)
creeping segment of the San Andreas Fault near Park- (Hickman & Zoback, 2004) and heat fl ow measure-

field, California (Fig. 8.1) at a depth of about 3 km. The ments (Williams et al., 2004) suggest a locally weak San
goals include establishing an observatory in close prox- Andreas Fault in an otherwise strong crust.