Earthquake source complexity? Near-fault velocity spectra from laboratory failures and their relation to natural ground motion

N. M. Beeler, Brian D. Kilgore, & David A. Lockner

Published August 15, 2016, SCEC Contribution #7014, 2016 SCEC Annual Meeting Poster #029 (PDF)

Natural earthquake sources have characteristic spectra, for example at high frequency the displacement amplitude spectrum of a body wave depends on frequency as 1 / f^2 [e.g., Hanks and Wyss, 1972] and velocity amplitude as 1/f. Such natural observations made at the Earth's surface are far removed from the source and it is unknown whether these characteristic spectra are due to the source or in part due to alteration along the source to station path. Well-instrumented laboratory experiments may provide some guidance in understanding the source contributions to the frequency content of ground motion, especially in instances where the surface and rheological properties of the fault and the physical processes in the source are well characterized and fault slip velocity is measured directly. Here we report preliminary results on the velocity spectra of laboratory failures: stick-slip on flat fault surfaces unconfined (normal stress of 2 MPa), at intermediate confining pressure (100 MPa), at high confinement where shear melting occurs (300 MPa), slip on fractured surfaces at high confinement (300 MPa), and rock fracture at intermediate confining pressure (50 MPa).

In contrast to a real earthquake, the theoretical on-fault velocity amplitude spectrum of a laboratory failure event with constant sliding friction follows 1/f^2. We compare our experiments to this reference. The reference represents a laboratory faulting experiment in which the fault properties make no contribution to the observed spectrum at high frequencies. That is, the reference is a fault that has no high frequency component: its strength does not change with time during the event, thus does not to not contribute to increasing or decreasing the slip speed, and which contains no intrinsic source of high frequency energy, e.g., no wear of the surfaces, no comminution of the existing shear zone which can produce fracturing or other sources of small scale rapid displacement that give rise to elastodynamic waves. Among the notable experimental results are that flat fault surfaces and intact rock failures are enriched in high frequency motion relative to the reference and show the same characteristic 1/f decay as the velocity amplitude of natural earthquakes. However, slip on rough fracture surfaces are depleted in high frequencies relative to natural earthquakes, and are very similar to the reference, essentially following the 1 / f^2 decay. Shear melting shows evidence of mixed behavior, a 1/f decay at intermediate frequencies and 1/f^2 at greater than 100 khz as if the presence of melt inhibits very rapid changes in slip rate.

Citation
Beeler, N. M., Kilgore, B. D., & Lockner, D. A. (2016, 08). Earthquake source complexity? Near-fault velocity spectra from laboratory failures and their relation to natural ground motion. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)