Geodetic Evidence for Distributed Shear Below the Brittle Crust of the
Walker Lane, Western United States
Abstract
The predominant approach for modeling faults in the Earth’s crust
represents them as elastic dislocations, extending downdip into the
lower crust, where the faults slip continuously. The resulting surface
deformation features strain accumulation concentrated across locked
faults during the interseismic period. An alternative model proposes
faults confined to the elastic crust, with surface deformation driven by
a wide zone of distributed shear underneath. Using high-precision GPS
data, we analyze deformation profiles across the Walker Lane (WL), USA.
The WL is a transtensional region of complex faulting, which delineates
the western edge of the Basin and Range province and accommodates a
significant portion of the Pacific-North American plate boundary
deformation budget. Despite a dense geodetic network surveyed
collectively for nearly 20 years, horizontal velocities reveal no
evidence of localized strain rate accumulation across fault surface
expressions. Instead, deformation within the shear zone is uniformly
linear, suggesting that the surface velocities reflect distributed shear
within the ductile crust rather than discrete fault deformation. This
implies no downdip fault extension below the seismogenic layer. The
shear zone, bound by the Sierra Nevada crest in the west, is 172±6 km
wide in the northernmost WL narrowing to 116±4 km in the central WL.
This study’s conclusion challenges the assumption of the presence of
dislocations in the lower crust when estimating geodetic slip rates,
suggesting that slip rates are instead controlled by the fault’s
position and orientation within the shear zone. This has important
implications for quantifying seismic hazards in regions with complex
fault systems.