DO FAULTS PRESERVE RECORDS OF THE SEISMIC CYCLE? 14 YEARS OF ADVANCEMENTS FROM FIELD OBSERVATIONS AND LABORATORY EXPERIMENTS

Earthquakes are dynamic slip pulses that propagate along faults, radiating seismic waves. The majority of energy released is expended in heating and breaking rocks along the fault. Much of the fracture damage is linked to transient, high amplitude stress conditions near the rupture tip. Frictional heating occurs because high velocity frictional slip behind the rupture tip generates heat energy faster than it can be dissipated to the surrounding rock. The extreme stress and temperature fields associated with earthquake rupture should thus be linked to damage and alteration unique to these conditions, leaving an unambiguous trace in the geological record. As recently as 1999, however, the rare fault rock pseudotachylyte (solidified melt formed during frictional heating) was the only accepted unequivocal evidence (Cowan 1999 J. Str. Geol. v. 22, 995-1001) for past seismic slip.

Recently, more indicators of “fossilized earthquakes” have been proposed. Even when coseismic heating does not lead to melting, it may cause other reactions. Mineral dissociation, increase in thermal maturity of organic matter, and trace element partitioning are possible signatures. Distinctive fracture patterns and extreme fracture density have been linked to dynamic rupture propagation by analog experiments and field observations. Fluidization of gouge might also record fast slip. While several of the proposed signatures of seismic slip are promising, they are based on highly idealized models that need to be ground-truthed in observations of the natural system. Future advancements must integrate seismological, experimental, geodetic, and mechanical studies of earthquakes into predictions about field and microstructural signatures that can be tested observationally.

Concurrently, the recognition of widespread fault tremor, low frequency earthquakes, and aseismic slip transients has broadened the scope of observed fault motions. As the roles of friction, fluid flow, lithology, depth, temperature and timing of these events become better understood, the structural geology community will be challenged to identify geological signatures covering the range of slip behavior throughout the seismic cycle.