Using surface characteristics to infer the permeability structure of an active fault zone

Studies of fault hydraulic architecture have shown faults to be spatially and temporally heterogeneous, with the potential to act as conduits, barriers, or a combination of conduit and barrier to subsurface fluid flow. Here we present a model for the distribution of permeability in an active normal fault based on a geostatistical analysis of 702 spring and ground temperature measurements. The temperatures were measured at the land surface along the trace of a normal fault that cross-cuts alluvium and weakly lithified sediments in the Alvord Basin of Oregon, USA. For flow parallel to the plane of the fault our analysis shows that the fault zone is dominated by broad areas of low to moderate permeability, interspersed with a number of spatially discrete, high-permeability flow paths. These observations are in agreement with conceptual models for faults in crystalline and well-lithified sedimentary rocks, but diverge from expectations for faults in alluvium and weakly lithified sediments. Our analysis demonstrates the potential for faults to develop hydraulic architectures intermediate between the idealized conceptual models currently available, and illustrates the need to test hypothesized behavior against observations in active systems.