Geologic evidence for multiple slip weakening mechanisms during seismic slip in crystalline rock

We examine exhumed seismogenic faults to investigate the mechanisms that may
have achieved dynamic fault weakening during ancient ruptures. Field and microscope observations imply more than one weakening mechanism must have been active during slip events on the faults. Pseudotachylytes that are continuous over the scale of field exposures are indicative of melt lubrication. A fault breccia crosscutting earlier formed cataclasites was mobilized during faulting and possibly represents a pressurized fault rock that resulted from thermal pressurization or elastohydrodynamic lubrication. In some faults, pseudotachylytes are developed in patches several meters long. Cataclasites in
which there is no evidence for melting are present immediately along strike from the pseudotachylyte patches. By considering the energy required for melting, we show that the pseudotachylytes must have formed during ruptures larger than the patches, implying that the cataclasites also accommodated seismic slip. The distribution of fault rock types shows that the frictional response to slip during a single event was spatially variable. Reworked pseudotachylytes also indicate that coseismic processes change over time at a point on a fault. These observations emphasize that macroscopic dynamic fault weakening is a function of multiple coeval processes at microscales to mesoscales. Detailed observations of the discontinuous pseudotachylytes show that slip zone thickness is the critical parameter that controls active coseismic processes. The frictional response
of a fault to slip is therefore dependent on the internal structure of faults; given the along-strike heterogeneity of most mapped fault zones, the coexistence of multiple slip weakening mechanisms in a single earthquake will be common.