High localization of primary slip zones in large earthquakes from paleoseismic trenches: Observations and implications for earthquake physics

[1] Paleoseismic exposures excavated across relatively straight sections of major faults in southern California display a high degree of localization at depths of only a few meters below the surface. In some cases, the width of the slip zone in events with multimeter displacement is on the order of 1–2 mm to a cm, which is the resolution of the observations. Repetitive slip events in the same zone increase the observed width of the faulting, as expressed at trench depths, but the superposed slip again tends to be highly localized. These observations are probably representative of >80% of the length of the faults studied. Based on these results, combined with the expected tendency for narrower slip zone with depth, observations from exhumed faults, and high localization of seismicity along large faults, we hypothesize that the majority of slip carried by large faults occurs in very narrow zones. If correct, the emerging integrated view of high slip localization during earthquake ruptures places important constraints on many issues of earthquake dynamics.