William R. Lettis (Lettis, Barka, Bachhuber, Brankman)

1. Influence of Releasing Stepovers on Fault Segmentation William R. Lettis (Lettis, Barka, Bachhuber, Brankman) William Lettis & Associates, Inc.

2. Segmentation • Behavioral Features (Quasi dynamic) - Slip Rate - Paleoseismic Data - Sense of slip, etc. • Structural/Geometric Features (Quasi static) - Fault intersection - Fault bend - Stepover, etc.

3. Relationship between SS Fault Segmentation and Rupture Dynamics Empirical Data -Aydin and Nur, 1982 -Barka and Kadinsky-Cade, 1988 -Knuepfer, 1989 Physical Analog Models -Reches, 1987 -Dooley and McClay, 1997 Numerical Models -Segall and Pollard, 1980 -Crider and Pollard, 1998 -Harris and Day, 1993, Harris et al, 2000 -Stein, 1999, etc.

4. Knuepfer, 1989

8. Conclusions: The variety of end-point characteristics clearly indicates the rupture ends do not follow any single, simple model... For example, not all releasing features of a certain size terminate strike-slip fault ruptures, nor do a large percentage of strike-slip ruptures actually terminate at releasing features. The major implication is that faults behave in complex fashions, that segment boundaries can and are caused by numerous different geologic and geometric features. Equally important, the study indicates that features that might be expected to act as segment boundaries, like steps in range fronts along normal fault, discontinuities along reverse faults, or even many releasing steps on strike-slip faults, often are ruptured through by earthquakes... Ultimately, the study suggests that, in many cases, rupture endpoints are controlled more by the rupture dynamics than by static characteristics of the fault zone. An increased understanding and utilization of segmentation awaits more study of the role static characteristics really do play in controlling ruptures. Knuepfer, 1989

9. Releasing Stepovers - a few observations • North Anatolian Fault – Case History • ·1999 Izmit Earthquake • ·1999DuzceEarthquake • Worldwide Database • ·Wells & Coppersmith (1994) • ·Updated through 2002 • Total: 30 Strike Slip Earthquakes with 59 Stepovers • - Ruptured through • - Arresting

10. North Anatolian Fault

18. Evaluation of Transient Stress Poly3D Numerical Model

22. Conclusion from Data Base Releasing Stepovers < 2 km wide ·Generally rupture through regardless of slip amount Releasing Stepovers > 4 - 5 km wide ·Generally arrest rupture regardless of slip amount Releasing Stepovers 2 - 4 km wide ·Increasing slip will rupture through stepovers of progressively greater width ·Ratio of width (km) to displacement (m) is 2.5:1 to 1:1

23. Summary ·Releasing stepovers of < 2 km width should not be used as segmentation boundary unless coupled with other structural or behavioral features. · Releasing stepovers of > 4-5 km width form effective barriers to rupture whether or not coupled with other structural or behavioral features. ·Caveat – earthquake nucleation within a large stepover (> 4-5 km) may (?) produce bilateral rupture, although such cases are not known.

24. Summary (continued) ·Increasing lapse of time (i.e.: increasing elastic strain) therefore leads to increasing probability of multi-segment fault ruptures. ·Earthquake recurrence models that assume fixed Mmax (with uncertainty) for segmented strike-slip faults may need revision.

28. McCalpin, 1996