What allows seismic events to grow big?: Insights from fault roughness and b-value analysis in stick-slip experiments T. H. W. Goebel 1, G. Kwiatek 2, T. W. Becker 3, G. Dresen 2, E.E. Brodsky 1 1 UC Santa Cruz; 2 GFZ-Potsdam; 3 University of Texas at Austin
How are observations on exhumed fault structures connected to in-situ processes at seismogenic depth? Corona Heights Fault Candela et al, 2012; Schorlemmer et al. 2008; Chester and Chester 1998
Physical controls on statistical seismicity distributions such as Gutenberg-Richter distribution: 1) Geometric effects 2) Dynamic effects and stress Burridge & Knopoff, BSSA 1967 Tchalenko and Ambraseys 1970; King 1983 Schorlemmer et al. 2005; Candela et al., 2011
Research Objectives 1. Identify similarities in seismicity statistics between lab and nature underlying self-similar processes 2. What controls changes in magnitude distributions in stick-slip experiments? fault roughness, stress heterogeneity, spatial localization 3. Examples of geometrically controlled magnitude distributions in nature
Experimental set-up and loading conditions
Fractured granite sample Specimen and AE sensors Pressure vessel and loading rig
1. Fracture 2. Fault locking 3. Stick-slip σ 1 σ 1 Principle Slip Surfaces Confining Pressures: 75 MPa 150 MPa
Stick-slip cycles on fractured surfaces
Research Objectives 1. Identify similarities in seismicity statistics between lab and nature
log Seismicity Density log Seismicity Density Seismicity distributions across natural and lab faults controlled by damage and roughness Fall-off controlled by roughness Fault Normal Distance [km] Fault Normal Distance [mm] Powers & Jordan, JGR, 2010 Goebel et al., GJI, 2014
Magnitude distributions show Gutenberg-Richter-type relationship e.g. Scholz 1968, Weeks et al. 1978, Amitrano et al. 2003; Goebel et al. 2012
Event-event triggering of AEs that give rise to Omori decay and productivity relation Goebel et al. (in prep) Davidsen et al., PRL, in rev.
Some differences between lab stick-slip and natural earthquakes
Finite fault dimension inflates stress drops and alleviates stress transfer processes
Research Objectives 1. Identify similarities in seismicity statistics between lab and nature underlying self-similar processes 2. What controls changes in magnitude distributions in stick-slip experiments? fault roughness, stress heterogeneity, spatial localization 3. What controls magnitude distributions in nature?
Seismic events preferably grow to larger sizes when differential stresses are high Goebel et al. GRL, 2013
Change in b value is high when stress change during slip is high Goebel et al. GRL, 2013
The influence of fault roughness on seismic events statistics in stick-slip experiments
Different initial conditions, same loading procedure Cut, Polished Cut, Roughened Fractured
Surface roughness and power-spectral-density Polished Roughened Fractured H ~ 0.5
Applied stress and acoustic emission activity
Polished Roughened Fractured Surface roughness controls spatial distribution of acoustic emissions during stickslip sliding
Slip planes of AEs during deformation on polished surface are aligned with remotely applied stress-field M = M ISO + M DEV = M ISO + M DC + M CLVD tr(m DEV ) = 0 σ 1 σ 1
Stress fields are highly heterogenous at small-scales for rough faults Polished Roughened Fractured Roughened Fractured Polished -7.0-6.5-6.0-5.5-5.0-4.5-4.0
Roughness controls spatial and magnitude distributions roughened fractured polished A Fractured Roughened Polished C B D
Magnitude distribution controlled by fault roughness and spatial localization rough polished
Magnitude distribution and spatial localization during fracture of intact rocks Lockner et al. Nature, 1991 see also Meredith et al. 1989; Main et al. 1989; Main 1992
Geometry and b-values on pre-existing faults Gutenberg-Richter Distribution log 10 (N) = a - bm w M 0 ~ L 3 (constant stress drop) log 10 (N) ~ b log 10 (L 3/c ) Assuming recurrence interval is independent of size (or choose short time intervals) D = 3/c b = 2b Fault-Size Distribution log 10 (N) ~ D log 10 (L) e.g. Aki, 1967; Hanks, 1979; King 1983; Frankel 1991, Wyss et al. 2004 Parkfield Segment San Andreas Fault Wyss et al. 2004
Research Objectives 1. Identify similarities in seismicity statistics between lab and nature underlying self-similar processes 2. What controls changes in magnitude distributions in stick-slip experiments? fault roughness, stress heterogeneity, spatial localization 3. Examples of geometrically controlled magnitude distributions in nature?
I. Seismic activity near volcanically-active regions
Spatial localization and low b-value during periods of swarm activity Shelly et al. JGR 2016
II. Fluid-injection induced seismicity near Fairview Oklahoma
Goebel et al. EPSL (2017)
map_cross_sec_pref Goebel et al. EPSL (2017)
Decreasing b-values with onset of spatial localization along Fairview fault in early 2015 Start of Woodward Start of Fairview Goebel et al. EPSL (2017)
III. Magnitude distribution as a function of distance from faults in Southern California
Smaller b-values for seismic events that occur closer to large faults in Southern California Page et al. JGR, 2011
Conclusion: What promotes larger seismic ruptures? Smoother faults More homogeneous stress field Localized deformation Higher proportion of large-magnitude events, i.e. seismic events preferably grow to large sizes
Thank You Reference T.H.W. Goebel, G. Kwiatek, T.W. Becker, E.E. Brodsky, G. Dresen (2017). What allows seismic events to grow big?: Insights from b-value and fault roughness analysis in laboratory stick-slip experiments, Geology (in press) T.H.W. Goebel, M. Weingarten, J. Haffener, X. Chen & E.E. Brodsky (2017). The 2016 Mw5.1 Fairview, Oklahoma earthquakes: Evidence for long-range poroelastic triggering at >40 km from fluid disposal wells, Earth Planetary Science Lett., 472, 50-61. Supported by Alexander von Humboldt Foundation. http:/ / www.humboldt-foundation.de
- Additional Slides -
Waveforms of a large slip event and typical AE Typical AE event Large stress drop event
AE location uncertainty Hypocenter uncertainty from active source measurements: σ = 1.5 mm.
Applied stress and acoustic emission activity
Seismicity distributions across strike-slip faults are influenced by fault roughness log Seismicity Density log Seismicity Density Fault Normal Distance [km] Powers & Jordan, JGR, 2010 Fault Normal Distance [mm] Goebel et al., GJI, 2014