XXXXXX BRODSKY ET AL . : RAYLEIGH - TAYLOR IN FAULTS 3 of 14 XXXXXX

6 [1] The absolute value of stress on a fault during slip is a critical unknown quantity in 7 earthquake physics. One of the reasons for the uncertainty is a lack of geological 8 constraints in real faults. Here we calculate the slip rate and stress on an ancient fault 9 in a new way based on rocks preserved in an unusual exposure. The study area consists of 10 a fault core on Kodiak Island that has a series of asymmetrical intrusions of ultrafine11 grained fault rock into the surrounding cataclasite. The intrusive structures have ductile 12 textures and emanate upward from a low-density layer. We interpret the intrusions as 13 products of a gravitational (Rayleigh-Taylor) instability where the spacing between 14 intrusions reflects the preferred wavelength of the flow. The spacing between intrusions is 15 1.4 ± 0.5 times the thickness of the layer. This low spacing-to-thickness ratio cannot be 16 explained by a low Reynolds number flow but can be generated by one with moderate 17 Reynolds numbers. Using a range of density contrasts and the geometry of the outcrop 18 as constraints, we find that the distance between intrusions is best explained by moderately 19 inertial flow with fluid velocities on the order of 10 cm/s. The angle that the intrusions 20 are bent over implies that the horizontal slip velocity was comparable to the vertical rise 21 velocity, and therefore, the fault was slipping at a speed of order 10 cm/s during 22 emplacement. These slip velocities are typical of an earthquake or its immediate afterslip 23 and thus require a coseismic origin. The Reynolds number of the buoyant flow requires a 24 low viscous stress of at most 20 Pa during an earthquake.