Investigate fault zone hydrogeologic architectures by using water level tidal and barometric response

Lian Xue, Emily E. Brodsky, Vincent Allegre, Patrick M. Fulton, L. Parker L. Beth, & John A. Cherry

Submitted August 15, 2016, SCEC Contribution #6994, 2016 SCEC Annual Meeting Poster #105

Fault zone hydrogeologic architecture is critical to faulting processes; however, they are not well understood and difficult to measure in situ. Water levels inside conventional water wells can tap an artesian aquifer response to pressure head disturbances caused by the Earth tides and surface atmospheric loading. The fluctuation of water levels can measure the hydrogeologic properties of the formation surrounding these wells. Specifically, the amplitude of water level oscillation is mainly determined by formation specific storage, and the phase shift between the water level oscillation and the pressure head disturbance is mainly determined by formation permeability. Utilization of wells at diffident distances to faults is able to map fault zone hydrogeologic architectures. We investigate fault zone hydrogeologic architectures in two locations: Logan Quarry and Southern Simi Valley, by applying water level tidal response. The San Andreas Fault near Logan Quarry has a surprising uniform diffusivity structure, while with higher specific storage and larger permeability in a localized zone near the fault (within 40 m of the fault). This change of properties might be related to the fault zone fracture distribution. The faults in the southern Simi Valley also have a uniform diffusivity structure but no significant fault-guided permeability or compliance structures. Such homogenous by fault zone damage is possible in a region of multiple strands and copious secondary faulting. The observed uniform diffusivity structure may suggest that the permeability contrast might not efficiently trap fluids during the interseismic period. Both of these two sites have a diffusivity of 10-2 m2/s which is also comparable to the post-earthquake hydraulic diffusivity measured on the Wenchuan Earthquake Fault. These two studies hint that hydraulic diffusivity may evolve to a narrow range of values 10-2 m2/s in fault zones.

Key Words
Lian Xue, Emily E. Brodsky, Vincent All├Ęgre, Patrick Fulton, Beth L. Parker, and John A. Cherry

Xue, L., Brodsky, E. E., Allegre, V., Fulton, P. M., Beth , L. L., & Cherry, J. A. (2016, 08). Investigate fault zone hydrogeologic architectures by using water level tidal and barometric response. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology