stress magnitude and orientation come from borehole experiments that use hydrofracturing techniques. Inflatable rubber packers are used to isolate a section of a vertical borehole, which is then pressurized with fluids until a tensile fracture is induced. If one of the principal stresses is vertical, a vertical fracture will form at the azimuth of the greatest horizontal principal stress. With knowledge of the pressure-time history of the borehole, the magnitude of the stress can be calculated (148).

Almost all extant data on earthquake processes have been collected in the laboratory or from surface-based measurements. A number of key quantities, such as fluid pressures, cannot be directly measured or accurately inferred from surface measurements alone (149). For example, it is difficult to assess the importance of fluids in earthquake generation and rupture based solely on studies of exhumed fault zones, because the complex history of uplift and denudation severely alters, or even destroys, the evidence on deformation mechanisms, fault-zone mineralogy, and fluid compositions during the actual faulting. Drilling holes to relatively shallow seismogenic depths (less than 5 kilometers) is feasible, however, and the means have been developed to sample fault-zone materials and pore fluids, to make a variety of down-hole measurements, and to conduct in situ experiments related to the physics of faulting. Several nations, particularly Germany, have mounted ambitious programs to explore the physical properties and mechanical state of the Earth’s crust through deep drilling (150). Fault-zone drilling has received high priority in several recent scientific drilling programmatic assessments, both on continents and in the oceans (151).

Drilling into active fault zones, whether in the ocean basins or on continents, presents a number of technological and programmatic challenges. Nevertheless, measurements in the Kontinentale Tiefbohrprogramm (KTB) borehole in Germany have provided critical in situ data on crustal processes and the physics of faulting. Moreover, drilling projects to depths of 4 kilometers, such as that proposed for SAFOD at Parkfield, California, have the potential to provide the types of data on fault-zone composition, structure, mechanical behavior, and physical properties that are needed to address the question of why plate-bounding faults are anomalously weak.