Scientific drilling of active faults: Past and future

Drilling into active faults has become a major scientific endeavor during the last decade and it appears as a most promising approach to resolve long-standing questions in earthquake and faulting processes. The first boreholes were drilled into the Nojima Fault following the 1995 Kobe earthquake. Since then, drilling into active faults has begun or been planned in a wide range of tectonic settings, such as a strike-slip plate boundary (San Andreas Fault, California), a thrust zone in an active orogenic belt (Chelungpu Fault, Taiwan), a normal fault in an active rift zone (Aigion Fault, Greece), a reactivated Archean fault (Pretorius Fault, South Africa), and a major subduction thrust (Nankai Thrust, Japan). These projects have already revealed many details on in-situ stresses, fault-zone structure, fault-rock composition, mechanical properties, heat flow, and near-field seismicity. Furthermore, most of these projects will continue to serve as observatories for monitoring fault deformation, fluid pressure and near-field earthquake source processes for a decade or two. Future drilling projects will focus on near-field observations and long-term monitoring of time-dependent processes and in-situ experimentation in active fault zones. Collaboration with industry and government will address practical issues pertaining to petroleum and geothermal energy, radioactive waste disposal, and urban seismic hazards. The outcome of these international efforts in drilling active faults will revolutionize our understanding of the processes controlling faulting and earthquakes and lead to a stronger scientific basis for earthquake hazard mitigation.