Fault Zones from Top to Bottom: A Geophysical Perspective

Abstract

We review recent geophysical insights into the physical properties of fault zones at all depths in the crust and subcrustal lithosphere. The fault core zone, where slip occurs, is thin (tens of centimeters) and can mainly be studied in trenches and in borehole well logs. The fault damage zone is wider (tens to hundred of meters) and can be measured by the analysis of fault zone-trapped waves. Such studies indicate that the damage zone extends to a depth of at least 3–5 km, but there is no agreement on the maximum depth limit. The damage zone exhibits a seismic velocity reduction (with respect to the neighboring country rock) as high as 20–50%. Significantly, this velocity reduction appears to have a temporal component, with a maximum reduction after a large rupture. The fault damage zone then undergoes a slow healing process that appears to be related to fluid-rock interactions that leads to dissolution of grain contacts and recrystallization. Deep seismic reflection profiles and teleseismic receiver functions provide excellent images of faults throughout the crust. In extensional environments these profiles show normal faulting in the upper crust and ductile extension in the lower crust. In compressional environments, large-scale low-angle nappes are evident. These are commonly multiply faulted. The very thin damage zones for these low angle faults are indicative of high pore-fluid pressures that appear to counteract the normal stresses, thereby facilitating thrusting. The presence of fluids within fault zones is also evidenced by geo-electrical studies in such diverse environments as the Himalayan and Andean orogens, the San Andreas fault, and the Dead Sea Transform. Such studies show that the fault can act as a fluid conduit, barrier, or combined conduit-barrier system depending on the physical properties of the fault core zone and damage zone. The ge ometry of active fault zones at depth is revealed by precise microearthquake hypocentral locations. There is considerable geometric diversity, with some strike-slip faults showing a very thin (less then 75 m wide) fault plane and others showing wider, segmented planes and/or parallel strands of faulting. A new discovery is slip-parallel, subhorizontal streaks of seismicity that have been identified on some faults. Such streaks may be due to boundaries between locked and slipping parts of the fault or lithologic variations on the fault surface. Measurements of seismic anisotropy across strike-slip faults are consistent with localized fault-parallel shear deformation in the uppermost mantle, with a width that varies between 20 and 100 km. In addition to shear deformation zones, seismic reflection profiles have imaged discrete faults in the uppermost mantle, mainly associated with paleo-continent/continent collisions. Looking deeper, the lithosphere-asthenosphere boundary may be considered as a major shear zone, considering the horizontal movement of lithospheric plates. This shear zone can be imaged with newly developed seismic receiver function methods.