SCEC Award Number 11057 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title How does damage affect rupture propagation across a fault stepover?
Investigator(s)
Name Organization
Michele Cooke University of Massachusetts Amherst Heather Savage Lamont-Doherty Earth Observatory
Other Participants
SCEC Priorities A7, A9, A10 SCEC Groups FARM, Geology
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 We investigated the potential for fault damage to influence earthquake rupture at fault stepovers using a mechanical numerical model that explicitly includes the generation of cracks around faults. We compared the off-fault fracture patterns and slip profiles generated along faults with a variety of step-over widths and distances between fault tips. We also compared the damage patterns around restraining versus releasing steps as well as the rupture behavior for models with and without damage. This work has illuminated the interesting interplay of damage and rupture across fault steps. Because conditions under which rupture produces damage area far more likely than those that would not produce damage, we conclude that ruptures should more easily propagate through releasing steps than restraining steps (should all other factors like total slip and stress drop be similar). Furthermore, we expect wider damage zones around restraining steps due to the propagation of fractures away from the step over, which is borne out in the literature on fault damage zones.
Intellectual Merit In releasing stepovers, splay cracks near the ends of the two faults grow towards one another and into the releasing step. These cracks eventually form hard linkages between the faults and facilitate rupture propagation from one fault to the other. In contrast, restraining steps form cracks that propagate away from and outside of the stepover. Rupture is not as easily passed from the first fault to the second across the restraining stepover; however, this trend may only occur in nature during the first few earthquakes through the stepover. More importantly from the perspective of fault zone growth, restraining steps will create wider damage zones per unit of displacement than releasing steps. Damage zone width correlates with fault slip up to about 200 m of displacement [Savage and Brodsky, 2011]. Variations in this trend may owe to the differences in damage width at releasing or restraining stepovers.

The ability of the rupture to pass from one fault to another also depends on whether damage is allowed to grow. In releasing steps, we compared models where cracks were allowed to form versus those without damage. The hard linkage that develops from the growth of damage zone cracks allows slip to develop on the second fault, whereas slip in the model without damage terminates at the end of the first fault.

The opposite situation can occur along restraining steps. Models without damage allow for rupture to propagate across the step, whereas rupture within a model with the same boundary and initial conditions that grows damage will stop at the stepover. The production of damage and slip along splay cracks serves to dissipate the rupture tip stress concentration and reduce the propensity for the rupture to pass to the second fault.
Broader Impacts By investigating the detailed rupture behavior at stepovers with explicit damage, we can provide constraints for the parameterizations of damage at stepovers for regional fully dynamic earthquake rupture models.

This work resulted in presentations at both the 2011 SCEC annual meeting and the Fall AGU 2011 meeting in San Francisco. The project supported two women scientists, one of whom is early career and the other is hearing impaired. The results of the project have enhanced our model repertoire giving us and others idea of specific damage features to look for in the field in future studies.
Exemplary Figure Figure 2: Example of damage patterns around releasing step (left) and restraining step (right). The positions of the faults are shown in the gray box. The faults are shown in blue and green, cracks that are generated during slip are shown in red. The slip profiles for each fault are shown above. In the releasing step, the damage grows between the two faults leading to facilitation of rupture, whereas in the restraining step damage grows away creating wider damage zones.