THE STRUCTURE AND KINEMATICS OF AN EXHUMED STRIKE-SLIP FAULT TERMINATION; IMPLICATIONS FOR EARTHQUAKE RUPTURE PROCESSES

Structural complexity is commonly present at the terminations of earthquake surface ruptures, as well as around shallow exhumed faults. Similar deformation may therefore be expected at the end zones of earthquake ruptures at depth. The Glacier Lakes fault (GLF) in the central Sierra Nevada, CA, is an 8.2km long, left-lateral strike slip fault with a maximum observed displacement of ~150m. Empirical scaling relations (Wells and Coppersmith, 1994, Bulletin of the Seismological Society of America, 84, 974-1002) suggest that if the entire mapped trace length of the GLF had slipped during an earthquake, the fault could have experienced an earthquake rupture up to magnitude Mw ~ 5.7. Field observations and microscope analyses of fault rocks from the fault zone show that pseudotachylyte is present in veins that cross-cut cataclasites. Displacement on the GLF was therefore accommodated at least partly during seismic events, and the physical properties of the fault will have affected the earthquake rupture process. The western termination of the GLF is defined by a gradual decrease in the slip on the main fault, accompanied by a broad (~1.3 km wide) zone of secondary faulting in the dilational quadrant of the GLF. The secondary faults splay from the main fault trace in a counter-clockwise sense forming average dihedral angles of 39° with the main fault trace. Slip vectors defined by slickenlines are more oblique for these splay faults than for the GLF. The GLF termination structure shows that structural complexity is present at the terminations of faults at seismogenic depths, therefore ruptures that propagate beyond fault terminations, or through stepovers between two faults, will be interacting with complex secondary fault structures. The geometry of the GLF termination matches off-fault damage geometries predicted by both quasi-static and dynamic models of fault processes. Models of dynamic rupture propagation beyond fault terminations, or across other structural discontinuities, must account for the effect of pre-existing structures on the elastic properties of the host rock. Additionally, aftershock distributions and focal mechanisms may be controlled by the size, geometry and kinematics of structures present at fault terminations.