Broadside interferometric and reverse-time imaging of the San Andreas Fault at depth

The San Andreas Fault Observatory at Depth provides the most comprehensive set of data on the structure and dynamics of the San Andreas fault. We use two independent experiments recorded by the seismometer arrays of the SAFOD Pilot and Main Holes to resolve the localized structure of the San Andreas fault zone and of an intermediate fault zone at depth. From Pilot Hole recordings of the drilling noise coming from the Main Hole, we reconstruct the waves that propagate between the Pilot Hole sensors and use them to image the fault zone structure. The use of correlated drilling noise leads to a high-resolution image of a major transform fault zone. Another independent image is generated from the detonation of a surface explosive charge recorded at a large 178-sensor array placed in the Main Hole. The images reveal the San Andreas fault as well as an active blind fault zone that could potentially rupture. This is confirmed by two independent methods. The structure and the activity of the imaged faults is of critical importance in understanding the current stress state and activity of the San Andreas fault system.