Synthesis of Creep Measurements from Strainmeters and Creepmeters along the San Andreas Fault: Implications for Seismic vs. Aseismic Partitioning

Abstract:

Strainmeters and creepmeters have been operated along the San Andreas Fault, observing creep events for decades. In particular, the EarthScope Plate Boundary Observatory (PBO) has added a significant number of borehole strainmeters along the San Andreas Fault (SAF) over the last decade. The geodetic data cover a significant temporal portion of the inferred earthquake cycle along this portion of the SAF. Creepmeters measure the surface displacement over time (creep) with short apertures and have the ability to capture slow slip, coseismic rupture, and afterslip. Modern creepmeters deployed by the authors have a resolution of 5 µm over a range of 10 mm and a dynamic sensor with a resolution 25 µm over a range 2.2 m. Borehole strainmeters measure local deformation some distance from the fault with a broader aperture. Borehole tensor strainmeters principally deployed as part of the PBO, measure the horizontal strain tensor at a depth of 100-200 m with a resolution of 10^-11 strain and are located 4 – 10 km from the fault with the ability to image a 1 mm creep event acting on an area of ~500 m² from over 4 km away (fault perpendicular). A single borehole tensor strainmeter is capable of providing broad constraints on the creep event asperity size, location, direction and depth of a single creep event. The synthesis of these data from all the available geodetic instruments proximal to the SAF presents a unique opportunity to constrain the partitioning between aseismic and seismic slip on the central SAF. We show that simple elastic half-space models allow us to loosely constrain the location and depth of any individual creep event on the fault, even with a single instrument, and to image the accumulation of creep with time.