Nonlinear Rheology of the Shallow Crust Inferred From Multi-Year Borehole Strain Time Series

Date: 4/26/2019

Time: 02:15 PM

Room: Vashon

The largest strains that borehole strainmeters record are caused by slow creep of the formation around the borehole and grouted-in strainmeter. These strains, which are amplified by the stress concentration around the borehole, dwarf most signals of tectonic or magmatic origin, so are usually removed and ignored. However, these long-term strains can be interpreted to characterize the nonlinear rheology of the shallow crust at the borehole depths of 150-200 m. Long-term (10-13 year) records from four-component Gladwin Tensor Strainmeters (GTSMs) of the Earthscope Plate Boundary Observatory (PBO) yield horizontal strain tensors whose principal strain rates decrease with time toward non-zero steady rates, often with superimposed seasonal signals. After 10-13 years of operation, typical strain rates are several microstrain/year, one to two orders of magnitude higher than strain rates inferred from GNSS. The azimuths of the principal strains and maximum shear are approximately constant in time except for seasonal variation. For a large subset of the PBO strainmeters, the long-term strains can be closely fit by the sum of a secular strain rate plus a fractional-power dependence on time. Such a time history is consistent with a fractional Maxwell rheology, a generalization of a Maxwell material whose viscous component is replaced by a component in which stress is a fractional derivative of strain rate. This characterization can provide rock mass parameters of interest to geotechnical engineers who are concerned with long-term stability of underground structures such as tunnels or fractures in unconventional hydrocarbon reservoirs. The nonlinear, inelastic rheology is not apparent in strain recordings of earth tides, but may influence measured time histories of deformation in response to large tectonic stress steps, such as those imposed by earthquakes, and therefore may need to be taken into account in interpreting post-seismic geodetic information.

Presenting Author: Evelyn Roeloffs

Authors