letters to nature 12. Spicer, C. W., Kenny, D. V., Shaw, W. J., Busness, K. M. & Chapman, E. G. A laboratory in the sky: New geodetically observed contraction is accommodated on the prin- frontiers in measurements aloft. Environ. Sci. Technol. 28, 412A–420A (1994). 13. Chapman, E. G., Kenny, D. V., Busness, K. M., Thorp, J. M. & Spicer, C. W. Continuous airborne cipal thrust systems across the Los Angeles region. We integrate measurements of gaseous formic and acetic acids over the western North Atlantic. Geophys. Res. Lett. the most recent geological, geodetic and seismological data to 22, 405–408 (1995). 14. Spicer, C. W., Kenny, D. V., Chapman, E. G., Busness, K. M. & Berkowitz, C. M. Observations of assess the spatial distribution of strain across the Los Angeles dimethyl sulfide over the western North Atlantic Ocean using an airborne tandem mass spectrometer. metropolitan region. We then demonstrate that a significant J. Geophys. Res. 101, 29137–29147 (1996). component of seismic moment release and shortening in this 15. Fast, J. D. & Berkowitz, C. M. A modeling study of boundary-layer processes associated with ozone layers observed during the 1993 North Atlantic Regional Experiment. J. Geophys. Res. 101, 28683– region is accommodated by east–west crustal escape ‘extrusion’ 28699 (1996). along known strike-slip and oblique-slip faults. 16. Lurmann, F. W., Lloyd, A. C. & Atkinson, R. A chemical mechanism for use in long-range transport/ The Los Angeles metropolitan region lies within a transitional acid deposition computer modeling. J. Geophys. Res. 91, 10905–10936 (1986). 8 17. Yin, F., Grosjean, D. & Seinfeld, J. H. Photooxidation of dimethyl sulfide and dimethyl disulfide. I: zone where predominantly strike-slip rigid-block tectonics to the Mechanism development. J. Atmos. Chem. 11, 309–364 (1990). south gives way to east-west-trending folding and contractional 18. Zaveri, R. A. Development and Evaluation of A Comprehensive Tropospheric Chemistry Model for Regional and Global Applications. Thesis, Virginia Polytechnic Inst. (1997). faulting to the north (Fig. 1; ref. 10). The structural framework of 19. Gery, M. W., Whitten, G. Z., Killus, J. P. & Dodge, M. C. A photochemical kinetics mechanism for this region is a product of polyphase deformation that includes urban and regional scale computer modeling. J. Geophys. Res. 94, 12925–12956 (1989). 20. Ragains, M. L. & Finlayson-Pitts, B. J. Kinetics and mechanism of the reaction of Cl atoms with 2- Miocene extension and clockwise rotation, Pliocene contraction, 11–13 methyl-1,3-butadiene (isoprene) at 298K. J. Phys. Chem. 101, 1509–1517 (1997). and Plio-Quaternary transpression (oblique compression) . 21. Bonsang, B., Polle, C. & Lambert, G. Evidence for marine production of isoprene. Geophys. Res. Lett. Many faults that are currently recognized as active show a complex 19, 1129–1132 (1992). 22. DeMore, W. B. et al. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling (Rep. history of movement; in some cases, varied senses of slip are 97-4, Jet Propulsion Lab., Pasadena, CA, 1997). evident14. Consequently, simple mechanical models of fault nuclea- 23. Atkinson, R. Gas phase tropospheric chemistry of volatile organic compounds: 1. Alkanes and alkenes. J. Phys. Chem. Ref. Data 26, 215–290 (1997). tion based on Coulomb fracture criteria do not readily explain the 24. Oum, K. W., Lakin, M. J., DeHaan, D. O., Brauers, T. & Finlayson-Pitts, B. J. Formation of molecular orientation of known faults with respect to presently observed chlorine from photolysis of ozone and aqueous sea salt particles. Science 279, 74–77 (1998). maximum principal stresses (approximately north–south) defined 25. Chameides, W. L. & Stelson, A. W. Aqueous-phase chemical processes in deliquescent sea-salt 15 16 aerosols: A mechanism that couples the atmospheric cycles of S and sea salt. J. Geophys. Res. 97, by seismicity and borehole breakout studies . Therefore, the 20565–20580 (1992). activity and sense of slip on many active faults in the Los Angeles 26. Keene, W. C. et al. Comment on ‘‘Aqueous-phase chemical processes in deliquescent sea-salt aerosols: A mechanism that couples the atmospheric cycles of S and sea salt’’ by W. L. Chameides and A. W. region are dictated by their pre-existing orientation relative to the Stelson. J. Geophys. Res. 98, 9047–9049 (1993). current state of stress. Recent studies show that contemporary crustal strain, regional Acknowledgements. We thank pilots R. V.Hannigan and M. J. Warren for their invaluable help during the field studies; X. Bian for graphics preparation; and W. C. Keene, R. D. Saylor, J. A. Shorter and W. R. north–south shortening and east–west extension in south Califor- Barchet for discussions. We acknowledge the financial support of the US Department of Energy’s nia are being accommodated by rotations and by strike-slip, Atmospheric Chemistry Program. oblique-slip and conjugate faulting7,11–13,17,18. Molnar19 compared Correspondence and requests for materials should be addressed to C.W.S. (e-mail: [email protected]). the deformation within the western Transverse Ranges to extrusion tectonics in China; however, he focused on active, continuing rotations as a more accurate description of the deformation. Humphreys20 proposed westward escape of the San Gabriel block, Escape tectonics in the accommodated by left lateral faulting on its southern margin, as a mechanism to avoid convergence associated with the Big Bend of Los Angeles metropolitan the San Andreas fault. However, recent kinematic and seismic hazard models for the Los Angeles metropolitan region have region and implications emphasized that most of the geodetically determined north– south shortening rates are accommodated on the principal thrust for seismic risk systems, such as the Sierra Madre, Elysian Park and Santa Monica Christian Walls*†‡, Thomas Rockwell*†, Karl Mueller§†, faults, and have largely overlooked the role of strike-slip and Yehuda Bockk†, Simon Williamsk†, John Pfanner*†, oblique-slip faults as potentially hazardous seismic sources and as James Dolan¶† & Peng Fangk† contractional accommodation structures. High rates of slip (3.8– −1 * Department of Geological Sciences, San Diego State University, 5.5 mm yr ; refs 6, 21) have been inferred for the Sierra Madre- California 92182, USA Cucamonga fault zone in part because of the substantial relief in its † Southern California Earthquake Center, University of Southern California, hanging wall and magnitude of deformation in Quaternary sedi- 22 Los Angeles, California 90089-0740, USA ments on the foot wall . However, geological observations west of § Department of Geological Sciences, University of Colorado, Boulder, the Cucamonga strand do not support high late Quaternary rates of Colorado 80309, USA slip. Our recent work shows that the rate of dip slip decreases from −1 −1 k IGPP, Scripps Institution of Oceanography, La Jolla, California 92093, USA ,3–5mmyr along the Cucamonga fault to ,1mmyr for the − ¶ Department of Geological Sciences, University of Southern California, central Sierra Madre fault zone and 1.5–3.0 mm yr 1 for the San − Los Angeles, California 90089, USA Fernando segment23. The resulting shortening rate of ,1mmyr 1 ......................................................................................................................... along the central Sierra Madre fault zone is thus far less than is − Recent damaging earthquakes in California, including the 1971 required to account for a large percentage of the 7–9 mm yr 1 of San Fernando1, 1983 Coalinga2, 1987 Whittier Narrows3 and 1994 north–south shortening across the Los Angeles region estimated Northridge4 events, have drawn attention to thrust faults as both from our analysis of the past three years of continuous Global potentially hazardous seismic sources and as a mechanism for Positioning System (GPS) measurements from the Southern accommodating shortening in many regions of southern Califor- California Integrated GPS Network (SCIGN)24,25. nia. Consequently, many geological studies5,6 have concluded that The San Jose, Raymond and other sinistral oblique-slip faults thrust faults in Southern California pose the greatest seismic extend west-southwest away from the central section of the Sierra − − hazard, and also account for most of the estimated 5–7 mm yr 1 of Madre fault zone where the slip rate is ,1mmyr 1. Their tectonic contraction across the greater Los Angeles metropolitan area7,8 geomorphic expression26 complements left lateral focal mechanisms indicated by Global Positioning System geodetic measurements9. and aftershock patterns27,28 from the 1988 and 1990 Upland earth- Our study demonstrates, however, that less than 50% of the quakes (ML ¼ 4:6 and ML ¼ 5:2) on the San Jose fault, and the 1988 Pasadena earthquake (ML ¼ 4:9) on the Raymond fault (ML is ‡ Present address: Earth Consultants International, 2522 N. Santiago Blvd, Suite B, Orange, California Richter magnitude). The Cucamonga and San Fernando segments 92867, USA. have significantly greater slip rates as they extend east and west of Nature © Macmillan Publishers Ltd 1998 356 NATURE | VOL 394 | 23 JULY 1998 letters to nature the sinistral faults. This suggests that the conjugate strike-slip faults Table 1). Analysis of site velocities estimated from the past three bounding the Pomona, Verdugo and Santa Monica blocks accom- years of SCIGN continuous GPS data were used to calculate strain modate shortening across the Sierra Madre fault zone as the rates across structures in the Los Angeles region. Relative velocities intervening crustal blocks rotate clockwise, and escape westward. were computed with respect to several different sites, and hence We assessed the strain distribution in the Los Angeles region different perspectives, and synthesized to determine strain rates using two independent methods; geology and geodesy (Figs 2 and 3, across known faults. In some cases the far-field strain across two or 120 119 118 GF 35 8 8 mm/yr SYF 22-34 mm/yr 8 mm/yr SCF 117 ORF SMZ C 34 SCIF 5 mm/yr SRF SAF 1 mm/yr PV W 34 Los Angeles NI SAFZ 116 22 mm/yr Metrop.