BEM Model of slip on the Channel Islands Thrust, CA

BEM Model of slip on the Channel Islands Thrust, CA

Credit Where Credit is Due: Michele Cooke Michele Cooke UMass Amherst Has been training students/postdocs to work with and remesh the CFM since at least 2003 Michele and I have developed slightly different tools/methods, but our overall approach and goals are the same Main meshing software Move suite (Midland Valley Exploration) www.mve.com Available to academic institutes for a small annual fee ~ 200/yr We also use several home-grown Perl scripts for various tasks First* publication using CFM-based mesh: Griffith & Cooke (2004)

Publications Using Various Remeshed CFM s 1. Griffith, W. A., and M. L. Cooke (2004), Mechanical validation of the threedimensional intersection geometry between the Puente Hills blind-thrust system and the Whittier fault, Los Angeles, California, Bulletin of the Seismological Society of America, 94(2), 493-505. 2. Griffith, W. A., and M. L. Cooke (2005), How sensitive are fault slip rates in the Los Angeles Basin to tectonic boundary conditions?, Bulletin of the Seismological Society of America, 95(4), 1263-1275. 3. Olson, E. L., and M. L. Cooke (2005), Application of three fault growth criteria to the Puente Hills thrust system, Los Angeles, California, USA, Journal of Structural Geology, 27, 1765-1777. 4. Cooke, M. L., and S. T. Marshall (2006), Fault slip rates from threedimensional models of the Los Angeles metropolitan area, California, Geophysical Research Letters, 33(L21313), 1-5. 5. Marshall, S. T., M. L. Cooke, and S. E. Owen (2008), Effects of non-planar fault topology and mechanical interaction on fault slip distributions in the Ventura Basin, California, Bulletin of the Seismological Society of America, 98(3), 1113-1127. 6. Meigs, A. J., M. L. Cooke, and S. T. Marshall (2008), Using vertical rock uplift patterns to infer and validate the three-dimensional fault configuration in the Los Angeles basin, Bulletin of the Seismological Society of America, 98(2), 106-123. 7. Dair, L., and M. L. Cooke (2009), San Andreas fault geometry through the San Gorgonio Pass, California, Geology, 37(2), 119-122. 8. Marshall, S. T., M. L. Cooke, and S. E. Owen (2009), Interseismic deformation associated with three-dimensional faults in the greater Los Angeles region, California, Journal of Geophysical Research, 114(B12403), 1-17. 9. Cooke, M. L., and L. C. Dair (2011), Simulating the recent evolution of the southern big bend of the San Andreas fault, southern California, Journal of Geophysical Research, 116(B-44-5). 10. Herbert, J. W., and M. L. Cooke (2012), Sensitivity of the southern San Andreas fault system to tectonic boundary conditions and fault configurations, Bulletin of the Seismological Society of America, 102(5), 2046-2062. 11. Madden, E. H., and D. D. Pollard (2012), Integration of surface slip and aftershocks to constrain the 3D structure of faults involved in the M7.3 Landers earthquake, southern California, Bulletin of the Seismological Society of America, 102(1), 321-342. 12. Marshall, S. T., G. J. Funning, and S. E. Owen (2013), Fault slip rates and interseismic deformation in the western Transverse Ranges, CA, Journal of Geophysical Research, 118, 4511-4534. 13. Fattaruso, L. A., M. L. Cooke, and R. J. Dorsey (2014), Sensitivity of uplift patterns to dip of the San Andreas fault in the Coachella Valley, California, Geosphere, 10(6), 1235-1246. 14. Herbert, J. W., M. L. Cooke, and S. T. Marshall (2014a), Influence of fault connectivity on slip rates in southern California: Potential impact on discrepancies between geodetic derived and geologic slip rates, Journal of Geophysical Research: Solid Earth, 119(3), 2342-2361. 15. Herbert, J. W., M. L. Cooke, M. Oskin, and O. Difo (2014b), How much can off-fault deformation contribute to the slip rate discrepancy within the eastern California shear zone?, Geology, 42(1), 71-75. 16. Fattaruso, L. A., M. L. Cooke, R. J. Dorsey, and B. A. Housen (2016), Response of deformation patterns to reorganization of the southern San Andreas fault system since ca. 1.5 Ma, Tectonophysics, 693, Part B, 474-488. 17. Marshall, S. T., G. J. Funning, H. E. Krueger, S. E. Owen, and J. P. Loveless (2017), Mechanical models favor a ramp geometry for the Ventura-pitas point fault, California, Geophysical Research Letters, 44(3), 1311-1319. **Apologies if I left yours out!

The Raw CFM: The Good The Raw CFM Mesh Is highly irregular mesh density/resolution triangle shape/size The Northridge Thrust (CFM5.0) Many triangles have poor aspect ratios The Good The mesh is carefully constructed based on all available data TONS of work! Mesh density shows where there is data and where there is not Packaged as ascii files (tsurfs) Easy to read/write Relocated aftershocks from Carena & Suppe (2002)

The Raw CFM: Potential Issues Mesh Quality The mesh is has too much variation in triangle size/shape to be numerically stable Ideal: Equilateral triangles (not possible for nonplanar/irregular faults) The Santa Monica Bay Thrust (CFM5.0) Some faults have small gaps/holes Difficult to visually detect Can write scripts do check this I can share mine, if anyone is interested

Subsurface Fault Intersections The Raw CFM: Potential Issues The CFM mesh does not intersect faults along triangle edges BEM models calculate values at Elt centroids Interpenetrating Elts can cause stress singularities Kinematic models are OK with intersections Faults must be remeshed to intersect along triangle edges Subsurface intersections are typically unconstrained The Newport-Inglewood fault (blue) intersects the Compton Thrust (gray) in CFM5.0

The Raw CFM: Potential Issues CFM Surfaces only cover Seismogenic Depths Modeling interseismic deformation requires extending faults to depth Must extend faults beyond CFM depths Marshall et al. (2009, JGR) provides a computationally-efficient method Many faults many intersect when extended Unconstrained; requires arbitrary geometric decisions Should small faults be extended? How far? Gray: The Imperial Fault (CFM5) Blue: Remeshed & Extended

The Raw CFM: Potential Issues Fault Traces Follow Topography Z-values are elevation, not depth Fault traces go up to their actual surface elevation Most faults don t occur at high elevations Total surface area above z=0 is typically small Offshore faults don t go to z=0 (are they blind?) Modeling Challenges Most BEM models use a halfspace Could make a surface topography mesh Shear traction-free Surface would be complex (many elts) Would need to fix fault intersections with surface (very time consuming) Increase computation time The Santa Susana Fault (CFM5)

Flattening Topography: Two Choices Choice #1 Project the surface trace down to some datum (z=0) The Good Fault trace is in correct location The Bad Fault is at least one of the following: Mislocated in the subsurface Incorrect dip and mislocated Must curve/kink to merge with correct fault location at depth Surface area changes These cause significant changes in mechanics Z = 0 Actual Fault Trace

Flattening Topography: Two Choices Choice #2* *this is what I do Remove portion of fault surface above the datum The Good Fault orientation is unchanged Correct location in subsurface The Bad Fault trace is mislocated (for nonvertical faults) Surface area changes Less significant changes in compared to choice #1 Z = 0 Actual Fault Trace Model Fault Trace

Software: 3DMove from Midland Valley www.mve.com

Unnecessary Triangles Poor aspect ratios

Creating good fault meshes is a combination of science and art Science: must fit the data Art: how good is good enough? Meshing: A Bit of an Art VERY time consuming Each fault: 1-3 hrs Depends on number of intersections Blind faults are easier Is this possible to automate? Maybe I have tried and failed many times Transverse Ranges Mesh Based on CFM5.0 Marshall et al. (2017, GRL) 79 Faults; 21,054 Elements Numerous subsurface fault intersections

Determination of Slip Rates/Rakes Such a dataset would almost certainly need to be model-derived Our groups have been doing this since Cooke & Marshall (2006, GRL) For recent examples, see Dorsett et al. SCEC Poster #218 Beyer et al. SCEC Poster #219 Getting pushy with the San Gorgonio Pass: Investigating active fault geometries with crustal deformation models Mechanical Models of Fault Slip Rates in the Imperial Valley, CA

Stuff to Share Scripts Useful for Meshing and Visualization ts2matlab.pl : converts t-surf to a MATLAB-friendly format plotmesh.m : plots a fault surface used with ts2matlab.pl ts2facet.pl : converts to facet format (FEM models) fixts.pl : makes a grouped surface into a single surface remtrgl.pl : used to find holes, and fix intersections checkfaulttop.pl : checks to make sure faults go to z=0 checkfaultbottom.pl : checks the maximum depth of faults mesh.pl : a simple planar or sinusoidal mesh generator Contact me if you think these would be of use