What allows seismic events to grow big?: Insights from b-value and fault roughness analysis in laboratory stick-slip experiments

Thomas H. Goebel, Grzegorz Kwiatek, Thorsten W. Becker, Emily E. Brodsky, & Georg Dresen

Published August 9, 2017, SCEC Contribution #7463, 2017 SCEC Annual Meeting Poster #226

Estimating the expected size of the largest earthquake on a given fault is complicated, for example, by dynamic rupture interactions in addition to geometric and stress heterogeneity. However, a statistical assessment of the potential of seismic events to grow to larger sizes may be possible based on variations in magnitude distributions. Such variations can be described by the b-value which quantifies the proportion of small to large magnitude events. The values of b appear to systematically vary with stress, but additional factors such as geometric heterogeneity may affect the growth-potential of seismic ruptures. We examined the influence of fault roughness on b-values, focal mechanisms and spatial localization of laboratory acoustic emission (AE) events during stick-slip experiments. We created three types of roughness on Westerly granite surfaces and studied AE event statistics during triaxial loading of the lab-faults. Since both roughness and stress variations are expected to influence b, we isolated roughness contributions by analyzing AEs at elevated stresses close to stick-slip failure. Our results suggest three characteristics of seismicity on increasingly rough faults: 1) seismicity is spatially more distributed, 2) b-values are higher and 3) focal mechanisms are more heterogeneous, likely caused by underlying stress field heterogeneity along the fault zones. Localized deformation on smooth faults, on the other hand, promotes larger rupture sizes within the associated homogeneous stress field which is aligned with the macroscopic stress orientation. The statistics of earthquake magnitude distributions may help quantify these fault states and expected rupture sizes in nature.

Citation
Goebel, T. H., Kwiatek, G., Becker, T. W., Brodsky, E. E., & Dresen, G. (2017, 08). What allows seismic events to grow big?: Insights from b-value and fault roughness analysis in laboratory stick-slip experiments. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)