THE SAN-ANDREAS FAULT ZONE DRILLING PROJECT - SCIENTIFIC OBJECTIVES AND TECHNOLOGICAL CHALLENGES

We are leading a new international initiative to conduct scientific dr illing within the San Andreas fault zone at depths of up to 10 km. Thi s project is motivated by the need to understand the physical and chem ical processes operating within the fault zone and to answer fundament al questions about earthquake generation along major plate-boundary fa ults. Through a comprehensive program of coring, fluid sampling, downh ole measurements, laboratory experimentation, and long-term monitoring , we hope to obtain critical information on the structure, composition , mechanical behavior and physical state of the San Andreas fault syst em at depths comparable to the nucleation zones of great earthquakes. The drilling, sampling and observational requirements needed to ensure the success of this project are stringent. These include: 1) drilling stable vertical holes to depths of about 9 km in fractured rock at te mperatures of up to 300 degrees C; 2) continuous coring and completion of inclined holes branched off these vertical boreholes to intersect the fault at depths of 3, 6, and 9 km; 3) conducting sophisticated bor ehole geophysical measurements and fluid/rock sampling at high tempera tures and pressures; and 4) instrumenting some or all of these incline d core holes for continuous monitoring of earthquake activity, fluid p ressure, deformation and other parameters for periods of lip to severa l decades. For all of these tasks, because of the overpressured clay-r ich formations anticipated within the fault zone at depth, we expect t o encounter difficult drilling, coring and hole-completion conditions in the region of greatest scientific interest.