High-frequency imaging of elastic contrast and contact area with implications for naturally observed changes in fault properties

Kohei Nagata, Brian Kilgore, Nick Beeler, & Masao Nakatani

Published July 2, 2014, SCEC Contribution #1952

During localized slip of a laboratory fault we simultaneously measure the contact area and the dynamic elastic fault normal stiffness. One objective is to determine conditions where stiffness may be used to infer changes in area of contact during sliding on non-transparent fault surfaces. Slip speeds between 0.01 and 10 microns/s and normal stresses between 1 and 2.5 MPa were imposed during velocity-step, normal stress-step and slide-hold-slide tests. Stiffness and contact area have a linear interdependence during rate stepping tests and during the hold portion of slide-hold-slide tests. So long as linearity holds, measured fault stiffness can be used on non-transparent materials to infer changes in contact area. However, there are conditions where relations between contact area and stiffness are non-linear and non-unique. A second objective is to make comparisons between the laboratory- and field-measured changes in fault properties. Time-dependent changes in fault zone normal stiffness made in stress relaxation tests imply post-seismic wavespeed changes on the order of 0.3% to 0.8% per year in the 2 or more years following an earthquake; these are smaller than post-seismic increases seen within natural damage zones. Post-seismic fault normal contractions could be accommodated with a few decimeter wide fault core. Changes in the stiffness of laboratory shear zones exceed 10% per decade and might be detected in the field.

Citation
Nagata, K., Kilgore, B., Beeler, N., & Nakatani, M. (2014). High-frequency imaging of elastic contrast and contact area with implications for naturally observed changes in fault properties. Journal of Geophysical Research: Solid Earth, 119(7), 5855-5875. doi: 10.1002/2014JB011014.