Induced ionization damage preferentially affects the cell boundaries, consistent with them comprising more hydrous material. Cell boundaries are defined by variations in scattering contrast with no extended defects imaged. Anhedral silica grains exhibit coherent electron scattering typical of long-range order and contain elliptical to hexagonal cells of slightly different crystallographic orientation ( Fig. These observations confirm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions.Īmorphous silica exists as blebs surrounded by anhedral crystalline silica grains ( Fig. The layer therefore represents a relict silica gel that formed during fault motion, and which could have resulted in frictional instability. Kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fluid during fault slip. The layer is composed of ∼100 nm to 1 µm grains of quartz, hydrous crystalline silica, and amorphous silica, with 10–100 nm inclusions of Fe oxides and ellipsoidal silica colloids. Microstructures in this layer show flow banding, armored clasts, and extreme comminution compared to adjacent cataclasites. The Corona Heights fault slickenside in San Francisco, California, is covered by a shiny layer of translucent silica. Here we report on the first documented occurrence of a natural silica gel coating a fault surface. However, few have been observed in nature. Many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments, including silica gel lubrication. Dynamic reduction of fault strength is a key process during earthquake rupture.