The influence of testing apparatus stiffness on the source properties of laboratory stick-slip
Abstract
Stick-slip experiments were performed to determine the influence of the testing apparatus stiffness on source properties, to develop methods to relate stick-slip to natural earthquakes, and to examine the hypothesis of McGarr [2012] that the product of unloading stiffness, k, and slip duration, T, is both scale-independent and approximately constant for both laboratory and natural earthquakes. A double-direct shear load frame was used with Sierra White Granite samples at 2 MPa normal stress, and a remote loading rate of 0.2 µm/s. The stiffness of the test apparatus was varied by more than an order of magnitude by inserting disk springs into the shear loading column adjacent to the granite samples. Servo-controlling slip at a point between the forcing ram and the shear force load cell, produced repeatable slip events.
Slip and slip duration decrease as k increases, as they do for natural earthquakes. In contrast to earthquakes, stress drop and slip rate decrease with increasing k, and the product kT for these experiments is not constant, but decreases with k. These data, collected over a range of k, do not conform to McGarr's [2012] hypothesis. However, analysis of stick-slip studies from other testing apparatuses is consistent with McGarr's hypothesis; kT is scale-independent, similar to that of earthquakes, equal to the ratio of static stress drop to average slip velocity, and similar to the ratio of shear modulus to wavespeed of rock. These properties result from conducting experiments over a range of sample sizes, using rock samples with the same elastic properties as the Earth, and using testing machines whose stiffnesses decrease, and characteristic periods increase with scale. A consequence of our experiments and analysis is that extrapolation of lab scale earthquake source properties to the Earth is more difficult than previously thought, requiring an accounting for the properties of the testing machines and additional research beyond that reported here.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.S41B2772K
- Keywords:
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- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGYDE: 7209 Earthquake dynamics;
- SEISMOLOGYDE: 7215 Earthquake source observations;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS