Fault Zone Segmentation A Geophysicist's Perspective

1. Fault Zone Segmentation A Geophysicist's Perspective Ruth A. Harris U.S. Geological Survey March 2006 WGCEP Workshop

2. Two Big Questions: *What causes earthquakes to start in specific locations? *What causes earthquakes to stop in specific locations? Three Big Possible Answers: *Fault Zone Rheology (Friction, Deformation Style) *Fault Zone Stress Conditions (Tectonics, Stress Triggers/Shadows, Pore-pressure) *Fault Zone Geometry (Fault Continuities and Discontinuities) March 2006 WGCEP Workshop

3. *Fault Zone Rheology (Friction, Deformation Style) We currently use fault zone rheology as a basis for delimiting rupture extent in at least one case, the San Andreas fault. It is thought that the creeping section of the central San Andreas is not capable of sustaining large earthquakes, and that earthquakes nucleating either north or south of the creeping section cannot propagate through this section. Alternatively, other faults that just partially relieve their strain accumulation aseismically have been known to produce M6-M7 earthquakes, and we probably can’t assume that their creep will limit rupture extent on those faults. At least two sets of researchers in the 1980’s formulated earthquake predictions partly based on places where faults transition from creeping to locked fault behavior. March 2006 WGCEP Workshop

4. *Fault Zone Stress Conditions(Tectonics, Stress Triggers/Shadows, Pore-pressure) We sometimes use fault zone stress/strain conditions as a basis for estimating or at least understanding rupture locations. E.g., Some have suggested that Vp and Vp/Vs images from seismic tomography can be used to understand locations of rupture nucleation and extent (inferences of pore-pressure conditions). E.g., We commonly use the assumption of the stress shadow. I.e. We do not think that a second M7.9 will occur on the same parts of a fault that just slipped in an M7.9. March 2006 WGCEP Workshop

5. *Fault Zone Geometry (Fault Continuities and Discontinuities) At least since 1980, many researchers have viewed fault geometry as playing a key role in earthquake nucleation and arrest. Some of the original players are on the next slide. March 2006 WGCEP Workshop

6. Some Famous Segment Observers and Modelers of the Past March 2006 WGCEP Workshop

7. *Fault Zone Geometry Some years we have been so good at segmenting faults that we have (almost) forgotten about the potential of multi-fault rupture. Then the earth reminds us about multi-fault rupture. Some years we have been so good at cascades that we have (almost) forgotten about the potential of single segment rupture. Then the earth reminds us about segments. Landers: multi-fault example Parkfield: single segment example March 2006 WGCEP Workshop

8. *Segmentation and Fault Zone Geometry Earthquake and Fault Observations are the key. But sometimes we don’t have all of the observations that we need, or, we want to understand the physical basis of what happened. So we use Models. Some people use physical simulations (lab experiments). Some people use computer simulations. Some people use both. Spontaneous rupture models are computer simulations of dynamic rupture. They include what we know about Fault Geometry Fault Friction Initial Stresses Fault-surrounding Materials March 2006 WGCEP Workshop

9. *Spontaneous Rupture Simulations of the Effects of Fault Geometry March 2006 WGCEP Workshop

10. *Spontaneous Rupture Simulations of the Effects of Fault Geometry March 2006 WGCEP Workshop

11. *Spontaneous Rupture Simulations of the Effects of Fault Geometry Example - the effect of a narrow stepover in a strike-slip fault Results of Simulations: If there are no connecting faults, then 5 km appears to be upper limit for jumping stepovers Harris et al., GRL, 1991, Harris & Day, JGR, 1993, Harris & Day, GRL, 1999 March 2006 WGCEP Workshop

12. *Spontaneous Rupture Simulations of the Effects of Fault Geometry Example: Simulation of the 1999 Izmit quake = stepovers + bend in a strike-slip fault Harris, Dolan, Hartleb and Day (BSSA, 2002) March 2006 WGCEP Workshop

13. *Fault Zone Geometry (Fault Continuities and Discontinuities) Some famous late 1990’s - 2000’s people who have used spontaneous rupture simulations to study the effects of fault geometry on earthquakes Bouchon and Streiff (bends in strike-slip faults) Kase and Kuge (stepovers and bends in strike-slip faults) FEATURED Workshop Participant Magistrale and Day (stepovers in thrust faults) Oglesby, Duan et al. (stepovers and bends in strike-slip and thrust faults) FEATURED Workshop Participant Aagaard et al. (geometry changes in strike-slip and thrust faults) FEATURED Workshop Participant Rice/Dmowska group: Dmowska, Rice, Poliakov, Bhat, Fliss, Templeton (bends, stepovers) FEATURED Workshop Participant Aochi, Madariaga et al. (complex geometry in strike-slip faults) Kame and Yamashita, Kame et al. (bends in strike-slip and thrust faults) March 2006 WGCEP Workshop

14. *Fault Zone Geometry (Fault Continuities and Discontinuities) Some of the complex geometry earthquakes that have been modeled Kase and Kuge (1997 Kagoshima) Harris et al. (1934/1966 Parkfield, 1999 Izmit) Oglesby et al. (1999 Chi-Chi, 1999 Hector Mine, 2002 Denali) Aagaard et al. (1906 San Francisco, 2002 Denali) Bhat et al. (2002 Denali) Fliss et al. (1992 Landers) Aochi et al. and Madariaga et al. (1992 Landers, 1999 Izmit) March 2006 WGCEP Workshop

15. *Fault Zone Geometry (Fault Continuities and Discontinuities) Things to think about: What does the fault geometry really look like at depth (see talks by Legg, Plesch, Simpson)? And, Some new ‘big picture’ findings about the effects of geometry: Manighetti et al., JGR, 2005 compiled slip and rupture distributions for many earthquakes. Their conclusions: Earthquakes often start at fault discontinuities Earthquakes are abruptly stopped by abrupt changes in fault geometry, but earthquake ruptures also “fade away” where there is no obvious change in fault geometry. March 2006 WGCEP Workshop

16. Two Big Questions: *Where will earthquakes start? * Where will earthquakes stop? Possible Answers: Both initiation and termination are likely caused by *Fault Zone Rheology (Friction, Deformation Style) *Fault Zone Stress Conditions (Tectonics, Stress Triggers/Shadows, Pore-pressure state) *Fault Zone Geometry (Fault Continuities and Discontinuities) It is probably best to assume that earthquakes can start anywhere (on faults). Large earthquakes can be stopped by 1) Encountering large creeping sections of a fault 2) A Stress shadow from a recent large earthquake on the same fault 3) Big changes in fault geometry. E.g. inter-fault distances of >5 km March 2006 WGCEP Workshop

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