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

Transcription

1 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

2 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

3 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

4 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

5 Experimental set-up and loading conditions

6 Fractured granite sample Specimen and AE sensors Pressure vessel and loading rig

7 1. Fracture 2. Fault locking 3. Stick-slip σ 1 σ 1 Principle Slip Surfaces Confining Pressures: 75 MPa 150 MPa

8 Stick-slip cycles on fractured surfaces

9 Research Objectives 1. Identify similarities in seismicity statistics between lab and nature

10 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

11 Magnitude distributions show Gutenberg-Richter-type relationship e.g. Scholz 1968, Weeks et al. 1978, Amitrano et al. 2003; Goebel et al. 2012

12 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.

13 Some differences between lab stick-slip and natural earthquakes

14 Finite fault dimension inflates stress drops and alleviates stress transfer processes

15 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?

16 Seismic events preferably grow to larger sizes when differential stresses are high Goebel et al. GRL, 2013

17 Change in b value is high when stress change during slip is high Goebel et al. GRL, 2013

18 The influence of fault roughness on seismic events statistics in stick-slip experiments

19 Different initial conditions, same loading procedure Cut, Polished Cut, Roughened Fractured

20 Surface roughness and power-spectral-density Polished Roughened Fractured H ~ 0.5

21 Applied stress and acoustic emission activity

22 Polished Roughened Fractured Surface roughness controls spatial distribution of acoustic emissions during stickslip sliding

23 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

24 Stress fields are highly heterogenous at small-scales for rough faults Polished Roughened Fractured Roughened Fractured Polished

25 Roughness controls spatial and magnitude distributions roughened fractured polished A Fractured Roughened Polished C B D

26 Magnitude distribution controlled by fault roughness and spatial localization rough polished

27 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

28 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 Parkfield Segment San Andreas Fault Wyss et al. 2004

29 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?

30 I. Seismic activity near volcanically-active regions

31 Spatial localization and low b-value during periods of swarm activity Shelly et al. JGR 2016

32 II. Fluid-injection induced seismicity near Fairview Oklahoma

33 Goebel et al. EPSL (2017)

34 map_cross_sec_pref Goebel et al. EPSL (2017)

35 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)

36 III. Magnitude distribution as a function of distance from faults in Southern California

37 Smaller b-values for seismic events that occur closer to large faults in Southern California Page et al. JGR, 2011

38 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

39 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, Supported by Alexander von Humboldt Foundation. /

40 - Additional Slides -

41 Waveforms of a large slip event and typical AE Typical AE event Large stress drop event

42 AE location uncertainty Hypocenter uncertainty from active source measurements: σ = 1.5 mm.

43 Applied stress and acoustic emission activity

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45 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