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1 Forum Reply 10.1130/G39439Y.1 doi: Persistent slip rate discrepancies in the eastern California (USA) shear zone address with total variation regula rization (TVR): does including many Eileen L. Evans U.S. Geological Survey. Menlo Park, California 94025, USA d geodetic agreement? faults improve geologic an We thank John Platt for his interest (his Comment: Platt, 2017) in our paper (Evans et al., 2016) and for bringing our attention to Platt and Becker (2013), which explores the hypothesis that large-scale rotations may contribute to geologic-geodeti c slip rate discrepancies on the USA. We agree with his Comment Garlock fault, Southern California, (also stated by Platt and Becker, 2013) that the Garlock fault appears to ervations, which is surprising given be nearly invisible to geodetic obs that the Garlock fault (1) is a conti nuous morphological feature that is not right-lateral eastern California obviously disrupted by through-going shear zone (ECSZ) faults, and (2) appears have slipped 4–11 mm/yr ., McGill et al., 2009). during the late Holocene (e.g The degree to which wholesale cl ockwise rotation of the region contributes to the geologic-geodetic discrepancy seems to depend on the two approaches. The conceptual differing modeling assumptions in our model of Platt and Becker (2013) postulates clockwise rotation in a region around the Garlock fault that ca ncels sinistral shear due to buried slip on the Garlock. The degree to which dextral shear occurs in the Platt and Becker (2013) model is depend ent on the applied sphericity Figure 1. Toy model of slip rate estimation within a block model, in the correction of 26% of Pacific–North America relative plate motion and is rd velocity field due to relative presence of clockwise rotation. A: Forwa sensitive to this particular choice. block motions equivalent to a long-term slip rate of 7 mm/yr on a vertical left- from the surface to 15 km depth. B: lateral strike-slip fault that is locked is using block modeling is that it An advantage of our geodetic analys Forward velocity field due to relative block motions equivalent to a long-term explicitly accounts for the kinematics of intersecting fault sets. Block slip rate of 10 mm/yr on a vertical righ t-lateral strike-slip fault that is locked ates that are bounded by faults and models divide the crust into micropl ward velocity field due to clockwise from the surface to 15 km depth. C: For rotation about an Euler Pole near 0 Long, 0 Lat. D: Combined velocity field estimate block rotations and elastic st rain accumulation, constrained by (sum of A–C), plus noise. Fault locati ons shown in gray. E: Estimated strike- geodetic observations. Fault slip rates are then determined by the relative slip rates (left-lateral positiv e), and modeled velocity field . rotations of adjacent blocks (e.g., Meade and Hager, 2005). l estimation in the presence of As a demonstration of block mode The TVR strategy applied to the block model in our paper may add wholesale rotation, we generate a sy nthetic forward velocity field across additional complications to the block model’s ability to resolve left- four blocks separated by a left-later al east-west–striking fault and a right- lateral slip on the Garlock fault (although this would not be a factor in ockwise rotation nd superimpose a cl lateral north-south–striking fault a previous geodetic block models). Th e sparse spatial distribution of se randomly drawn from a normal (Figs. 1A–1C). We then add noi geodetic observations, the potential presence of many additional strike distribution with standard devia tion 1.5 mm/yr in east and north slip faults, and the termination of right-lateral ECSZ faults into the ons on a spherical shell. Relative velocities, and estimate block rotati Garlock fault may all influence estimated slip rates and limit the ability block rotations determine estimated slip rates. In this experiment, the of the current geodetic data to resolve left-lateral slip on the Garlock lip on the east-west fault (Figs. 1D block model does resolve left-lateral s demonstrates that the riment described above fault. However, the expe and 1E) (without added noise, slip rates are recovered exactly). Garlock fault, even in the presen ce of wholesale regional rotation as Another advantage of the block model is that it fits the full surface described in Platt and Becker (2013), is not necessarily invisible in impossible when considering one- velocity field in a way that is geodetic analysis with a block model. dimensional profiles alone, ensuring that the Garlock study area is rmation in the rest of Southern kinematically consistent with defo REFERENCES CITED Becker (2013) do not consider the California. In contrast, Platt and Evans, E.L., Thatcher, W.R ., Pollitz, F.F., and Murray, J.R., 2016, Persistent slip deformation that must necessarily occur beyond the boundaries of the ornia (USA) shear zone: Geology, v. 44, rate discrepancies in the eastern Calif p. 691–694, doi:10.1130/G37967.1. small region to which their rotation wa s applied. In particular, how does K., Kuzma, H.A., and McGill, J.D., 2009, McGill, S.F., Wells, S.G., Fortner, S. transition to dextral shear to the the postulated local clockwise rotation Slip rate of the western Garlock fault, at Clark Wash, near Lone Tree Canyon, north in the Walker Lane fault or to the south in the central Mojave and Mojave Desert, California: Geological Society of America Bulletin, v. 121, southern San Andreas faults? p. 536–554, doi:10.1130/B26123.1. Consideration of the larger region is especially important in the Meade, B.J., and Hager, B.H., 2005, Block models of crustal motion in southern context of the Platt’s assumption that faulting may be represented as California constrained by GPS measur ements: Journal of Geophysical Re- search, v. 110, B03403, doi:10.1029/2004JB003209. simple shear at a local scale. Interseismic strain accumulation only screpancies in the eastern California (USA) Platt, J.P., 2017, Persistent slip rate di produces simple shear very close to a locked fault (within ~1 locking shear zone: Comment: Geology, v. 45, p. exxxx, doi:10.1130/Gxxxxx.1. depth, where the velocity gradient is linear), and the magnitude of simple s of rotating panels of E-W faults in Platt, J.P., and Becker, T.W., 2013, Kinematic shear decays with distance from the fault (e.g., Savage and Burford, the San Andreas system: What can we tell from geodesy?: Geophysical 1973). Therefore a left-lateral fau lt in the presence of a clockwise Journal International, v. 194, p. 1295–1301, doi:10.1093/gji/ggt189. rotation would only be invisible very close to an intersection with a right- determination of relative plate motion Savage, J., and Burford, R., 1973, Geodetic lateral fault. Simple shear across the wider region would require many in central California: Journal of Geophysical Research, v. 78, p. 832–845, strike slip faults with very through-going, closely spaced (<15 km) doi:10.1029/JB078i005p00832. similar slip rates. This is exactly the question that we (Evans et al., 2016) © 2017 Geological Society of America. For permission to copy , contact Copyright Pe rmissions, GSA, or [email protected] | September 2017 | www.gsapubs.org e426 GEOLOGY FORUM

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