DEPARTMENT OF THE INTERIOR U. S. GEOLOGICAL SURVEY PROCEEDINGS OF CONFERENCE XLV Fault Segmentation and Controls of Rupture Initiation and Termination Palm Springs, California Sponsored by U.S. GEOLOGICAL SURVEY NATIONAL EARTHQUAKE-HAZARDS REDUCTION PROGRAM Editors and Convenors David P. Schwartz Richard H. Sibson U.S. Geological Survey Department of Geological Sciences Menlo Park, California 94025 University of California Santa Barbara, California 93106 Organizing Committee John Boatwright, U.S. Geological Survey, Menlo Park, California Hiroo Kanamori, California Institute of Technology, Pasadena, California Chris H. Scholz, Lamont-Doherty Geological Observatory, Palisades, New York Open-File Report 89-315 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1989 TABLE OF CONTENTS Page Introduction and Acknowledgments i David P. Schwartz and Richard H. Sibson List of Participants v Geometric features of a fault zone related to the 1 nucleation and termination of an earthquake rupture Keitti Aki Segmentation and recent rupture history 10 of the Xianshuihe fault, southwestern China Clarence R. Alien, Luo Zhuoli, Qian Hong, Wen Xueze, Zhou Huawei, and Huang Weishi Mechanics of fault junctions 31 D J. Andrews The effect of fault interaction on the stability 47 of echelon strike-slip faults Atilla Ay din and Richard A. Schultz Effects of restraining stepovers on earthquake rupture 67 A. Aykut Barka and Katharine Kadinsky-Cade Slip distribution and oblique segments of the 80 San Andreas fault, California: observations and theory Roger Bilham and Geoffrey King Structural geology of the Ocotillo badlands 94 antidilational fault jog, southern California Norman N. Brown and Richard H. Sibson Segmentation of basin-and-range normal faults: 110 examples from east-central Idaho and southwestern Montana Anthony J. Crone and Kathleen M. Haller Historical basin and range province surface faulting 131 and fault segmentation Craig M. dePolo, Douglas G. Clark, D. Burton Slemmons, and William H. Aymard The character of faulting processes of earthquakes 163 in the intermountain region Diane L Doser Effects of restraining bends on the rupture of strike-slip earthquakes 181 Katharine Kadinsky-Cade and A. Aykut Barka Implications of the characteristics of end-points of historical 193 surface fault ruptures for the nature of fault segmentation Peter L.K. Knuepfer Segmentation models and Holocene movement history 229 of the Wasatch fault zone, Utah Michael N. Machette, Stephen F. Personim, Alan R. Nelson, David P. Schwartz, and William R. Lund Formation of segmented strike-slip fault zones, 246 Mount Abbot quadrangle, California Stephen J. Markel Numerical model studies of dynamic rupture processes 260 Paul G. Okubo The geologic and seismic expression of the Calaveras fault, 283 central California: a lack of coincidence David H, Oppenheimer Behavior of individual fault segments along the Elsinore-Laguna Salada 288 fault zone, southern California and northern Baja California: implications for the characteristic earthquake model Thomas Rockwell Surface rupture in a fault stepover on the Superstition Hills 309 fault, California Michael J. Rymer Fault segmentation and earthquake occurrence in 324 the strike-slip San Jacinto fault zone, California Christopher O. Sanders Comments on models of earthquake recurrence 350 C. H. Scholz Paleoseismicity, persistence of segments, and temporal clustering 361 of earthquakes Examples from the San Andreas, Wasatch, and Lost River fault zones David P. Schwartz Structural duplexing in the strike-slip environment 376 Mark T. Swanson Earthquake fault slip estimation from geologic, geodetic, 386 and seismologic observations: implications for earthquake mechanics and fault segmentation Wayne T hatcher and Manuel G. Bonilla Fault-plane segmentation in brittle crust and anistropy in loading system 400 Robert E. Wallace Seismicity and structural evolution of strike-slip faults 409 Steven G. Wesnousky Persistent segment boundaries on basin-range normal faults 432 Russell L. Wheeler INTRODUCTION TO WORKSHOP ON FAULT SEGMENTATION AND CONTROLS OF RUPTURE INITIATION AND TERMINATION David P. Schwartz Richard H. Sibson U.S. Geological Survey Department of Geological Sciences MenloPark,CA 94025 U.C.Santa Barbara Santa Barbara, CA 93106 On March 6-9, 1988, a USGS workshop on Fault Segmentation and Controls of Rupture Initiation and Termination was held in Palm Springs, California. The purpose of the workshop was to bring together a diverse group of geologists, seismologists and geophysicists to discuss the status and future direction of fault segmentation, an emerging interdisciplinary field of earthquake research based on the observation that fault zones, particularly long ones, do not rupture along their entire length during a single earthquake. Increasingly, geological and seis- mological studies are indicating that the location of rupture is not random, that there are recog­ nizable physical properties of fault zones that control the nucleation point and lateral extent of rupture and divide a fault into segments, that ruptures with the same characteristics often repeat in the same location, and that independent rupture segments can persist through several seismic cycles. The workshop was attended by 51 geologists, seismologists, and geophysicists represent­ ing Federal and state agencies, universities, and the private sector. The two days of presentations and panel discussions focused on three major topics: a) observations of segmentation including the extent of historical ruptures, geology and seismology of rupture end points, and scale; b) mechanical, structural, and rheological controls of segmentation and rupture; and c) long-term segmentation and seismic hazards. A one-day field trip to view deformation associated with compressional stepovers in the Ocotillo Badlands and the surface faulting from the November 22-23, 1987 Superstition Hills earthquake sequence was sandwiched between the two days of presentations. The field trip provided an important hands-on focus for many of the discussion topics. Historically the first fault-specific segmentation model can be traced to Alien (1968). He suggested a division of the San Andreas fault zone into five major segments (Figure la) on the basis of differences in historical seismic behavior and on the distribution of different rock types that could affect fault mechanics. Without the benefit of paleoseismicity data, but with strong geologic intuition, he suggested that the historical behavior remains relatively constant with time. Wallace (1970) defined seven San Andreas segments (Figure Ib). This segmentation model was also based on differences in the historical behavior of the fault, particularly seismicity and creep. Wallace (1970) suggested that these might be permanent segments. Using the very preliminary ^ 1906 Breok i \ \ Central Calif. \ Vactive area \ HOII.Ster \ \ fPa'Kfiel Figure 3. Areas of contrasting seismic behavior along the San Andreas fault zone in California. BEHAVIOR CATEGORY MAXIMUM MAGNITUDE 7-8 + 6-7 5-6 MAXIMUM STRIKE SLIP (m) 1.2-10 0.3-1.2 0.1-0.3 =0 I RECURRENCE INTERVAL (yrs) 00-1000 10-100 CREEP RATE, PERCENT OF SECULAR clO 10-30 30-50 SURFACE BREAK Figure 6. Map of California thowing behavior of different tegmenli of San Andreas syite Figure 1. Initial segmentation models of the San Andreas fault. A) Alien (1968). B) Wallace (1970). data on plate tectonic rates available at that time, he estimated recurrence intervals, slip per event, and maximum earthquakes for each segment. Also, fundamental ideas relating earthquake rupture to the occurrence of persistent structural or geometric features of fault zones, either barriers to rupture propagation (Aki, 1979) or heterogeneities (asperities) that nucleate rupture (Kanamori, 1978), form an important part of the foundation for present research. Fault segmentation can provide an important framework for increasing our understanding of the mechanics of earthquake generation and for quantifying seismic hazards. During the past decade the growth of paleoseismology, the use of both strong motion seismology and improved geodetic modeling to quantify the amount and distribution of coseismic slip, high precision microearthquake monitoring to image fault planes and define the locations of locked and creep­ ing patches, and an increasing knowledge of physical and chemical conditions and processes at seismogenic depth have all been providing new observations and techniques for better under­ standing the deep structure of fault zones and how faults work in space and time. Ideally, we would like to look at a fault zone prior to an earthquake and be able to identify the part of the fault that will rupture as an independent segment and, within the segment, identify the nucleation point of the earthquake. This has implications for: a) short-term earthquake prediction experiments, particularly in selecting sites of potential rupture nucleation for deployment of instrument arrays; b) long-term earthquake forecasting, especially probabilistic hazard assessment, which requires a fault segmentation model as well as information on recurrence interval, elapsed time, slip per event, and slip rate for a specific fault segment; c) estimates of ground motion that reflect the relationship between dynamic