Extreme Slip Heterogeneity and Near-Fault Ground Motions

Field observations indicate extreme earthquake slip heterogeneity, that is, several meters of variation within tens of meters along-strike. Up to several parts in ten of strain can occur within a matter of seconds. Such variation in slip implies that, similarly, highly variable stress and strain occur within a fault zone. Field geologists' observations tend to emphasize the coherent several-kilometer wavelength slip signal, and typically treat the short-wavelength slip variations as noise. Observations of variation in slip, however, can be important, even if they are not structurally or tectonically significant. We interpret these data to indicate that, at least during the brief time when a rupture is propagating along a fault, the fault reaches a condition in which it must be both extremely strong and very weak, in rapid succession. Two possible explanations for this are considered; 1) geometrical irregularities in the fault zone, and 2) sharp momentary decrease in compressional lithostatic stress normal to the fault zone. In the first case, the fault appears weak when slip occurs on thin planes with low dynamic sliding friction, but strong when the rupture encounters geometric irregularities. Alternatively, while the fault zone is experiencing a transitory virtual decompression between the wall rock on the sides of the fault, friction drops to a low sliding value. Immediately upon passage of the rupture front, however,the decompression would abruptly halt, locking the extreme slip and stress heterogeneity into the fault. Highly heterogeneous slip also implies that as the rupture front propagates along the fault, slipping patches accelerate and decelerate on a scale of a few meters. The energy radiation patterns emanating from such a rough rupture process would therefore contain much high-frequency noise. When the friction drops to a low sliding value, however, the source of high-frequency energy noise radiation would cease. This may occur in patches where pre-existing stress is relatively homogeneous over a large area. Upon encountering such a smooth patch, rupture propagation could momentarily reach super-shear velocity in this near-frictionless condition, only to be strongly decelerated once slip rate again drops and friction rises abruptly. Such a model appears to be consistent with the large near-field ground motion recorded at Pump Station #10 (PS10) from 2002 Denali fault earthquake, in which we interpret that high-frequency radiation decreased during a brief episode of supershear conditions. This model is also consistent with the relative lack of pseudotachylites and heat flow anomalies along major faults.