INSIGHTS INTO EARTHQUAKE RUPTURE AND RECOVERY FROM STUDIES OF PALEOSEISMIC FAULTS CHRISTIE ROWE, MCGILL UNIVERSITY

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1 INSIGHTS INTO EARTHQUAKE RUPTURE AND RECOVERY FROM STUDIES OF PALEOSEISMIC FAULTS CHRISTIE ROWE, MCGILL UNIVERSITY

high) 3.Radiates short period seismic waves (10 Hz) 4.

2 DEFINITION OF AN EARTHQUAKE 1.Fast fault slip (~1 m/s) generates heat 2.Slip propagates dynamically (inertia is significant and stress can be transiently very high) 3.Radiates short period seismic waves (10 Hz) 4.Stress drop (1-10 MPa) across all magnitudes 5.Initiation and cessation of rupture are controlled by stress conditions on the fault Cowan, 1999 JSG

3 Sibson, 1975 CO-SEISMIC HEATING FORMS PSEUDOTACHYLYTES ΔT = τ f v Faults get HOT if strong, fast Earthquakes = fastest fault motion c p ρ t s temperature rise = shear stress * velocity heat capacity * density * thickness deep = strong If temp reaches rock melting temp, layer of melt forms in fault

4 PSEUDOTACHYLYTES MARK EARTHQUAKE RUPTURE PLANES Record energy budget of earthquake slip Preserve geometry of co-seismic faulting Quenching and overprinting textures record fault healing

5 RARELY RECOGNIZED 90% of reported pseudotachylytes occur in light-coloured host rocks Most geologists have never seen in the wild - may not recognize or report

6 RARELY PRESERVED Pseudotachylytes quench to glass or fine-grained textures Easily hydrated (e.g. epidote alteration, right) Easily broken during post-seismic slip or creep Easily recrystallized back to metamorphic textures Preferentially mylonitize due to fine grain size Price et al., 2012; Kirkpatrick & Rowe, 2013

7 Most earthquakes occur in ambient temps ~ C. Most rocks melt at C. So, pseudotachylytes occur when dt ~ C. WHAT ABOUT COSEISMIC HEATING BELOW MELTING POINT? Photo Sketch Depth (m) Magnetic susceptibility (10 5 SI) Sr (p.p.m.) Ba Li Rb Cs TiO 2 La Sm Pb 87 Sr/ 86 Sr (p.p.m.) (p.p.m.) (p.p.m.) (p.p.m.) (wt%) (p.p.m.) (p.p.m.) (p.p.m.) 206 Pb/ 204 Pb BGZ BrZ GGZ GGZ BrZ FDZ Ishikawa et al., 2008

8 Temperature rise at JFAST borehole Fulton et al., 2013, Science M w 7.4 Local Earthquake A B 01 Aug Oct Dec 2012 Depth (mbsf) o C 0.5 Depth (mbsf) Feb Apr Aug 2012 Oct 2012 Dec 2012 Feb 2013 Apr Residual Temperature ( o C)

9 ORGANIC MOLECULE MATURITY Sensitive to temp. rise ~ Disrupted strata BFR Mélange Cataclasite Pseudotachylyte Cataclasite 0 Not to scale Off-fault Rocks Diamondoids/ n-alkanes (%) Distance from Pseudotachylyte (m) Shear Stress (MPa) Possible Displacement Range 1 10 Displacement (m) Estimated Range of Shear Stress Savage et al Geology

10 SINCE 1999, ~13 INDICATORS OF CO- SEISMIC HEATING HAVE BEEN DESCRIBED Not Just Pseudotachylyte!

11 C. t 2 : v r = 0.2c s Stress at rupture tip increases with faster rupture y 2 < Stressing RATE t 3 : v r = 0.8c s σ 1 /σ 1 0 comparable to propagation rate y >70-90 Di Toro et al., 2005; Rowe & Griffith, 2015 x

12 Closely spaced tensile cracks on one side of the fault Griffith et al. 2009

13 DYNAMIC FRACTURING: TWO SIGNATURE PATTERNS Propagation Direction Rupture Front θ = Principle Slip Surface Griffith et al., 2009 Rosakis, 2002 Sharon and Fineberg, mm Melosh et al. 2014

14 POFADDER SHEAR ZONE, S.AFRICA-NAMIBIA Paul Macey, Conrad Groenwald, Chris Lambert (SA-CGS), Jodie Miller, Dey Cisneros Lazaro (Stellenbosch), Chris Gerbi (U. Maine) Ben Melosh, Erik Young, Noah Phillips, Jamie Kirkpatrick (McGill U.)

15 >70-90 Rupture Front Propagation Direction Slip Surface Slip Surface Griffith et al., 2009 Rosakis, 2002 Melosh et al Melosh et al. 2016

16 Melosh et al EPSL

17 POROSITY=15% CHAOTIC BRECCIA MYLONITE CRACKLE BRECCIA 4 cm PS194 Melosh et al., 2016 JSG

18 MYLONITE MICROBRECCIA MYLONITIZED BRECCIA POROSITY=1-5% 4 cm PS196 Melosh et al., 2016 JSG

19 Small circle apex Approx. flow plane Fold Axis Quartz Stretching Lineation Pole to Foliation Melosh et al. (submission imminent, I am told)

21 breccia amphibolite boudin ultramylonite breccia ultramylonite pseudotachylyte orthogneiss orthogneiss ultramylonite pseudotachylyte slickenline stretching lineation

22 BRECCIATION-HEALING CYCLES Porous pathways along faults - preferential sites of post-seismic creep/healing? Rupture planes rarely reused / or / don t persist very long in terms of earthquake cycles Folding of slip planes caused by viscosity gradients expected in banded rock - probably common.

23 TAKE AWAY THOUGHTS Signatures of frictional heating are common Structures recording dynamic stress around faults also record seismic slip Slip surfaces are reused, but not too many times (prevented by pseudotachylyte welding, folding, mylonitization) Rock record can reveal aspects of earthquake slip (geometry, mechanisms of deformation, and energy) which seismology cannot

FRICTIONAL HEATING DURING AN EARTHQUAKE. Kyle Withers Qian Yao

FRICTIONAL HEATING DURING AN EARTHQUAKE Kyle Withers Qian Yao Temperature Change Along Fault Mode II (plain strain) crack rupturing bilaterally at a constant speed v r Idealize earthquake ruptures as shear