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We performed a numerical investigation of dynamic ruptures on a bimaterial interface in 3D with regularized slip-weakening friction and a heterogeneous initial shear stress and discussed the resulting strong ground motion. We showed that for many parameter sets the dynamics of rupture propagation are significantly influenced by the broken symmetry due to the material dis- continuity during rupture propagation. Large differences in peak ground motion (PGV & PGA) are possible when changing the orientation of the material contrast even when slip-distribution of the individual events are very similar.

Wrinkle-like slip pulse specific to the bimaterial mechanism nucleates naturally from ini- tially crack-like propagation of rupture when the involved parameters allowed for large propa- gation distances. Once such a pulse is generated it appears to have high potential to overcome large distances within areas of relatively low initial shear stress. It also appears that the existence of a wrinkle-like slip pulse impedes the initiation of supershear propagation in the preferred di- rection. For those cases where supershear is nevertheless initiated, a secondary superimposed wrinkle pulse propagating at the generalized Rayleigh velocity is often nucleated behind the rupture front. By contrast, supershear propagation seems to be promoted in the unfavored di- rection.

The dynamic weakening of the fault due to the normal stress alteration during slip is also efficient in the range of small propagation velocities. In such cases, secondary events are trig- gered and the orientation of the material contrast determines rupture extent and the size of the earthquake, potentially by orders of magnitude.

The variety of propagation modes investigated in this study is consistent with laboratory experiments, thorough numerical investigation, as well as with crustal earthquake observations. Therefore our main conclusions are:

in this study and envisioned by Andrews and Harris (2005) – the effect on the surface ground motion and earthquake hazard can be substantial.

2. Our simulations contradict the conclusion of previous studies (‘The wrinkle-like slip pulse is not important in earthquake dynamics’ Andrews and Harris, 2005). We find that for a broad range of realistic parameters the wrinkle-mode of propagation is an attractive propagation mode of rupture. When such pulses are generated, earthquake rupture dynamics is strongly influenced by the wrinkle-like slip pulse characteristic of bimaterial interfaces. The resulting effect on ground motion may be very large.

3. It is not necessary that a wrinkle-like pulse (or Weertman pulse) be generated to signifi- cantly alter slip history. The bimaterial mechanism can affect rupture dynamics throughout the entire range of seismic propagation velocity (from sub-Rayleigh to supershear) and might even support very slow modes of propagation, the latter obviously needing deeper investigation.

Chapter 6

Discussion, Conclusion and Future Work

6.1

Discussion and Conclusion

The results presented here may have important implications to a number of issues of earthquake and fault physics and ground motion associated with large geological structures that have well- developed material interfaces.

Plate boundaries and other major faults, the sites of the large earthquakes, often have promi- nent bimaterial interfaces that separate different media. Such media contrasts are generated progressively by the creation of damage during the faulting process and the juxtaposition of different rocks across large displacement faults. Large faults also tend to nucleate and grow along pre-existing geological sutures that separate different blocks.

The numerical investigations of dynamic ruptures on bimaterial interfaces in this study demonstrate, that in the presence of material discontinuity over significant ranges of parameter sets, the dynamics of rupture propagation on individual faults, and rupture migration patters in systems of multiple faults are significantly influenced by the broken symmetry of stress during rupture propagation. The bimaterial interfaces are mechanically favored surfaces for rupture propagation, introducing directionality of dynamic fault rupture propagation and seismic radia- tion. Changing the orientation of shearing on a heterogenously loaded bimaterial fault can lead to large differences in peak values of ground motion even when the distribution of slip remains essentially unaffected.

The findings presented in chapters 3, 4, and 5 and other complementary studies (e.g., Shi and Ben-Zion, 2006; Dalguer and Day, 2007b; Ampuero and Ben-Zion, 2008) show that ruptures on a bimaterial interface tend to evolve during propagation from initial symmetric cracks with relatively low mechanical efficiency to slip pulses with preferred propagation direction and high mechanical efficiency, for significant ranges of parameters. Such pulses can overcome large areas of relatively low initial shear stress, or (at high stress level) impede supershear propagation in the favored direction, support supershear propagation in the unfavored direction. The generality of bimaterial specific modes of rupture propagation (chapter 4), the large variety

of propagation modes emerging (chapter 5), as well as the patterns of localization of fault slip in structures with velocity contrasts (chapter 3) is consistent with laboratory experiments (e.g., Xia et al., 2005), with thorough numerical investigation (e.g., Ampuero and Ben-Zion, 2008; Dunham and Rice, 2008), with crustal earthquake observations (e.g., Lewis et al., 2005, 2007; Rubin and Ampuero, 2007), as well as with geological studies (e.g., Dor et al., 2006b, 2008).

Main Conclusions The results provided in chapter 3, chapter 4, and chapter 5 and the fore- going discussion supports the following as the main conclusions of this study:

1 For comparatively broad ranges of realistic parameters the wrinkle-mode of propagation is an attractive propagation mode of rupture, the earthquake rupture dynamics is strongly influ- enced by the wrinkle-like slip pulse characteristic of bimaterial interfaces.

2 The bimaterial mechanism can affect rupture dynamics throughout the entire range of seismic propagation velocity (from very slow, over sub-Rayleigh, to supershear).

3 Spontaneous migration of ruptures to the material interfaces implies that the dynamic phe- nomena associated with the wrinkle-like pulses are not limited to the set of hypocenters lo- cated directly on the material interfaces. Hence, material interfaces provide a mechanism for a positive feedback between structure and rupture properties that can lead to progressive reg- ularization of geometrical heterogeneities with cumulative slip and potentially suppression of dynamic branching from large fault zone structures.

4 The influence of a material contrast across a fault on the ground motion and its directivity and earthquake hazard can be substantial.

Contradictory but fragile conclusions exist in the literature (‘The wrinkle-like slip pulse is not important in earthquake dynamics’ Andrews and Harris, 2005), (‘Material contrast does not predict earthquake rupture propagation direction’ Harris and Day, 2005). Placing such rigorous statements may be a catalyzing style of research since they provoked a considerable debate recently. Yet they hold the risk of loosing unbiasedness and are likely misleading since those studies involve models that incorporate simultaneously multiple ingredients letting the models appear “realistic”. However, the number of simulations provided by Andrews and Harris (2005) and Harris and Day (2005) does not suit the high dimensionality (less than 5 simulations in both studies altogether). By contrast, the parameter studies presented here (chapters 3 and 5) contain several hundreds of simulations for comparatively few degrees of freedom (in the sense of few parameters specific to dynamic rupture). Different potential key ingredients should be the target of separate detailed parameter-space studies. Published results on the problem at hand indicate that properties of ruptures on material interfaces in cases incorporating slip-weakening friction (e.g., chapter 5) and mixed-mode rupture (e.g., chapter 4) evolve for wide ranges of conditions toward results of cases without these ingredients (e.g., Andrews and Ben-Zion, 1997; Harris

and Day, 1997; Shi and Ben-Zion, 2006; Brietzke and Ben-Zion, 2006; Brietzke et al., 2007; Rubin and Ampuero, 2007; Ampuero and Ben-Zion, 2008; Brietzke et al., 2009).

However, the physical basis for many additional observed phenomena remain to be explored in future times, making it challenging to coming to definitive conclusions (compare chapter 2). Therefore discussions on the problems at hand should be conducted in an unprepossessed candid way.