Julian C. Lozos

The Effects of Double Fault Bends on Rupture Propagation: A Geometrical Parameter Study

We use the 2D finite element method to determine how geometrical parameters determine whether rupture will propagate across a linked stepover in a strike-slip fault. The end segments of the fault system are aligned in the direction of maximum shear, and the length and angle of the linking segment are allowed to vary. We observe that ruptures propagate through extensional stepovers with steeper angles and longer linking segments than otherwise equivalent compressional step- overs. These different rupture behaviors form distinct regions in angle-stepover-length parameter space; the boundary between these regions takes the shape of an asymptotic curve in both the extensional and compressional cases. Models in which the size of the entire fault system was made larger or smaller revealed that the location of the bound- aries between regions of different rupture behavior do not scale linearly with the system size; it was easier to rupture steeper and relatively longer stepovers in fault systems that were larger overall. A separate set of models in which the stress field is rotated so that the parallel end segments were optimally aligned for rupture significantly altered the rupture behavior curves; in this stress field, it was easier to rupture compressional stepovers with steeper angles and longer linking segments than it was to rupture equivalent extensional stepovers. In both the case in which the end segments are aligned with the direction of maximum shear and the case in which the end segments are optimally oriented for rupture, the angles at which rupture could no longer propagate through the entire fault corresponded with peaks in the faults S value.

Rupture Propagation and Ground Motion of Strike‐Slip Stepovers with Intermediate Fault Segments

Field studies of historic rupture traces show that fault stepovers commonly serve as endpoints to earthquake ruptures. This is an effect that is corroborated by past dynamic modeling studies. However, field studies also show a great deal of complexity in fault‐zone structure within a stepover, which is often simplified out of modeling studies. In the present study, we use the 3D finite‐element method to investigate the effect of one type of smaller‐scale complexity on the rupture process: a smaller fault segment positioned between the two primary strands of a strike‐slip fault stepover. We find that such small faults can have a controlling effect on whether or not a rupture is able to jump the stepover and on the resulting ground motions from these ruptures. However, this effect is neither straightforward nor linear: the length of the intermediate segment and its basal depth, as well as whether the stepover is extensional or compressional, all contribute to the rupture behavior and ground‐motion distribution. These results have important implications for assessing the probability of a rupture propagating through small‐ and large‐scale discontinuities in faults, as well as for evaluating ground‐motion intensities near fault stepovers. Because of the sensitivity of results to so many parameters, these results also suggest that modeling studies on idealized fault geometries may not be sufficient to describe the rupture behaviors of specific complex fault systems. Site‐specific modeling studies, where possible, will provide better inputs and constraints for probabilistic rupture length assessments as well as for ground‐motion estimates.

Laboratory Observations of Fault Strength in Response to Changes in Normal Stress

Changes in fault normal stress can either inhibit or promote rupture propagation, depending on the fault geometry and on how fault shear strength varies in response to the normal stress change. A better understanding of this dependence will lead to improved earthquake simulation techniques, and ultimately, improved earthquake hazard mitigation efforts. We present the results of new laboratory experiments investigating the effects of step changes in fault normal stress on the fault shear strength during sliding, using bare Westerly granite samples, with roughened sliding surfaces, in a double direct shear apparatus. Previous experimental studies examining the shear strength following a step change in the normal stress produce contradictory results: a set of double direct shear experiments indicates that the shear strength of a fault responds immediately, and then is followed by a prolonged slip-dependent response, while a set of shock loading experiments indicates that there is no immediate component, and the response is purely gradual and slip-dependent. In our new, high-resolution experiments, we observe that the acoustic transmissivity and dilatancy of simulated faults in our tests respond immediately to changes in the normal stress, consistent with the interpretations of previous investigations, and verify an immediate increase in the area of contact between the roughened sliding surfaces as normal stress increases. However, the shear strength of the fault does not immediately increase, indicating that the new area of contact between the rough fault surfaces does not appear preloaded with any shear resistance or strength. Additional slip is required for the fault to achieve a new shear strength appropriate for its new loading conditions, consistent with previous observations made during shock loading.

The Effects of Fault Stepovers on Ground Motion

Abstract Using 3D dynamic models, we investigate the effect of fault stepovers on near‐source ground motion. We use the finite‐element method to model the rupture, slip, and ground motion of two parallel strike‐slip faults with an unlinked overlapping stepover of variable width. We model this system as both an extensional and a compressional stepover and compare the results to those of single planar faults. We find that, overall, the presence of a stepover along the fault trace reduces the maximum ground motion when compared to the long planar fault. Whether the compressional or extensional stepover exhibits higher ground motion overall depends on the width of the separation between the faults. There is a region of reduced ground motion at the end of the first fault segment, when the faults are embedded in a homogeneous material. We also experiment with stress fields leading to supershear and subshear rupture velocities, and with different stress drops within those conditions. We find that subshear rupture produces stronger motions than supershear rupture, but supershear ruptures produce that maximum over a larger area than subshear areas, even though the overall area that experiences any shaking at all is not drastically different between the two cases. Lastly, we experiment with placing realistic materials along and around the faults, such as a sedimentary basin in an extensional stepover, a damage zone around the fault, and a soft rock layer on top of bedrock through the entire model area. These configurations alter the pattern of ground motion from the homogeneous case; the peaks in ground motion for the bimaterial cases depend on the materials in question. The results may have implications for ground‐motion prediction in future earthquakes on geometrically complex faults. Online Material: MPEG‐4 movies of models of dynamic rupture of fault stepovers embedded in heterogeneous material settings.

Rupture and Ground‐Motion Models on the Northern San Jacinto Fault, Incorporating Realistic Complexity

We use the 3D finite‐element method to conduct dynamic models of rupture and resulting ground motion on the Claremont–Casa Loma stepover of the northern San Jacinto fault. We incorporate complex fault geometry (from the U.S. Geological Survey [USGS] Quaternary Faults Database; see [Data and Resources][1]), a realistic velocity structure (the Southern California Earthquake Center Community Velocity Model‐S), a realistic regional stress field with an orientation taken from seismicity relocation literature, and several stochastic self‐similar shear stress distributions. As we incorporate more types of complexity, the specific effects of any individual factor become less apparent within the overall rupture behavior. We also find that the distribution of high and low shear stress that arises from combining regional and stochastic stress fields has the strongest control over where the rupture terminates. Using a regional stress field alone, as well as with the combined regional and stochastic stress realization, we find that the stepover presents a significant barrier to rupture, regardless of our choice of initial nucleation point and that it is difficult for rupture to propagate the full length of either fault segment. Greater heterogeneity of stresses tends to produce shorter ruptures. Within this result, we find that the Claremont strand is more favorable for long ruptures than the Casa Loma–Clark strand. Low‐frequency ground‐motion intensity and distribution are controlled largely by the velocity structure rather than by stress heterogeneity. The strongest motions produced in these models are in the San Bernardino basin. Although directivity effects do contribute to the low‐frequency ground‐motion distribution, particularly in the near field, they are secondary to the effects of the velocity structure. Online Material: Figures of ground motions from models used to calibrate the stress conditions for dynamic rupture propagation. [1]: #sec-18

Rock friction under variable normal stress

This study is to determine the detailed response of shear strength and other fault properties to changes in normal stress at room temperature using dry initial bare rock surfaces of granite at normal stresses between 5 and 7 MPa. Rapid normal stress changes result in gradual, approximately exponential changes in shear resistance with fault slip. The characteristic length of the exponential change is similar for both increases and decreases in normal stress. In contrast fault normal displacement and the amplitude of small high frequency elastic waves transmitted across the surface follow a two stage response consisting of a large immediate and a smaller gradual response with slip. The characteristic slip distance of the small gradual response is significantly smaller than that of shear resistance. The stability of sliding in response to large step decreases in normal stress is well-predicted using the shear resistance slip length observed in step increases. Analysis of the shear resistance and slip-time histories suggest nearly immediate changes in strength occur in response to rapid changes in normal stress; these are manifest as an immediate change in slip speed. These changes in slip speed can be qualitatively accounted for using a rate-independent strength model. Collectively the observations and model show that acceleration or deceleration in response to normal stress change depends on the size of the change, the frictional characteristics of the fault surface and the elastic properties of the loading system.