Imaging fault geometry to learn about earthquake mechanics Phillip G Resor Wesleyan University Vanessa Meer, Giulio Di Toro, Ashley Griffith

Fault zones record an integrated history of fault slip Fault geometry and earthquakes: the seismological record Geometry and kinematics of exposed fault surfaces Looking inside rocks to see the frozen record of ancient earthquake

Slip during earthquake rupture is heterogeneous Observed in surface ruptures And inversions of seismological and geodetic data Mai and Beroza, 2002, JGR Treiman et al., 2002, BSSA

Slip heterogeneity is attributed to asperities and barriers Shipton and Cowie, 2001, JSG Rheological: fault zone materials Geometrical: Slip surface geometry Fault Analysis Group

Fault geometry effects earthquake dynamics Km-scale discontinuities: earthquake initiation, termination, near fault stress Wesnousky, 2006, Nature Small-scale slip-perpendicular roughness: frictional properties, near fault stress Sagy and Brodsky, 2009, JGR

Fault surfaces are characterized by a suite of slip-parallel features Hancock and Barka, 1987, JSG Corrugation axis Gutter Tool Track Slickenside lineations

Fault surface roughness is fractal (self-affine) in nature Power et al, 1987, EPSL Dixie Valley Fault Candela et al, 2011, BSSA dx -> dx dz ->  dz 0 ≤ H < 1 linear decay of the power spectrum with slope of 1+2H dz

TLS and other technologies have led to a renaissance in fault surface analysis Sagy et al, 2007, GeologyBrodsky et al, 2011, EPSL Photo: Emily Brodsky  =1

Can we document effects of fault topography on fault kinematics? Resor and Meer, 2009 EPSL

Surface Morphology Striation Orientation Data Collection

Visualizing Surface Morphology

Slip perpendicular profiles Slip parallel profiles Quantifying Morphology Spectral Analysis Resor and Meer, 2009 EPSL

Incremental slip varies across the corrugated surface Resor and Meer, 2009 EPSL

Fault normal and striation orientation are correlated across the entire exposure Spherical correlation, significant at 99% probability Striations Fault Normals Resor and Meer, 2009 EPSL

Observations from creeping faults corroborate role of slip-parallel features Rubin et al., 1999, Nature Earthquake streaks on creeping faults are slip-parallel independent of local geology

Summary of work on exposed fault surfaces Fault surface exhibit self-affine scaling over many orders of magnitude Faults evolve (slowly) toward smoother profiles in the slip direction Faults roughness perturbs fault slip direction But, these results are derived from faults from <~5 km depth

Dynamic Rupture experiments reveal a number of lubrication processes Di Toro et al., 2011, Nature Pseudotachylyte (frictional melt) is the only unequivocal evidence of earthquake rupture velocities preserved in fault zones

Pseudotachylyte-bearing fault surfaces are not widely exposed Bistacchi et al, 2011, Pageoph TLS and photogrammetry used to image 3D geomtry of fault trace

Roughness of pseudotachylyte-bearing faults appears similar to small-slip faults Self-affine scaling over 3-5 orders of magnitude Smoother in slip direction

Micro-roughness is critical to dynamic slip and melt lubrication CT scanning allows us to image micro-scale roughness of fault surface Experiments reveal that weakening distance decreases with increasing normal stress

L05_07 core from extensional fault bend Griffith et al, 2010, JGR

Extensional bend surface roughness due to fracture and wear Griffith et al, 2010, JGR

L05_06 core from contractional fault bend Griffith et al, 2010, JGR

Contractional bend surface roughness dominated by differential melting Griffith et al, 2010, JGR

Conclusions and future directions Exposed fault surfaces exhibit self-affine roughness Evolves with slip Perturbs incremental slip direction How is roughness generated? Ferril et al., 1999, JSG Sagy and Brodsky, 2009 JGR Pseudotachylyte-bearing fault traces also exhibit self- affine roughness at larger scales Fault zone geometry may be different Micro-roughness is controlled by melting Tie fault observations to experimental work

Slow to High Velocity Apparatus (SHIVA), INGV “Rocks [melt] like butter in Rome” G. Di Toro

Corrugation axes are correlated with long wavelength shape Spherical correlation significant at 99% probability Corrugation Axes Fault Normals