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

Abstract

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), 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.

Broadband Ground Motions from Dynamic Models of Rupture on the Northern San Jacinto Fault, and Comparison with Precariously Balanced Rocks

“…As mentioned above, and described at length in the companion article to this study (Lozos et al, 2015), rupture propagation remains confined to high-stress asperities, because the energy budget of the rupture becomes so skewed toward fracture when it reaches the edge of the asperity that it is not able to propagate far into the unfavorable portions of the fault. The shape of these asperities in our models is defined by the combination of the regional stress field with stochastic stresses.…”

“…The Claremont strand is more planar than the Casa Loma in this interpretation of the fault geometry. It is also more favorably aligned within the current regional stress field (see the companion article [Lozos et al, 2015] for more discussion of this effect). However, as mentioned previously, the paleoseismic record for both strands includes events with an average of ∼3 m of surface slip and does not capture smaller ruptures that do not reach the surface.…”

“…In addition to localized stress change patterns associated with rupture on a single fault, rotations of regional stresses have been observed and documented after large earthquakes in southern California (Hauksson, 1994;Hardebeck and Hauksson, 2001). Rotations of 10°in either direction from N7°E produced very different rupture patterns on this same fault geometry in lower-resolution models (Lozos et al, 2015). It is possible that the average stress field at the time of the last large Casa Loma-Clark event was oriented differently from the current regional stress orientation of N7°E.…”

“…Seismicity relocation shows a regional stress orientation of N7°E for the northern SJF (Hardebeck and Hauksson, 2001). We use a N7°E-oriented stress tensor that produces a stress drop of 5.5 MPa and a strength parameter S of 0.6 (defined as the difference between the yield stress and the initial shear stress, divided by the dynamic stress drop) when resolved onto a planar fault of the average strike of the SJF, as in the lower-resolution models of the Claremont-Casa Loma stepover in the companion article to this study (Lozos et al, 2015). This low S value would result in supershear rupture velocities on a planar fault in a homogeneous setting, but the complexity of our model prevents the rupture front from exceeding the shear-wave velocity.…”

“…Because of the pattern of these asperities, the majority of the ruptures do not even approach the stepover region itself and thus do not address the issue of the overall ability of the rupture to jump. However, the Lozos et al (2015) companion study of this stepover, using a larger grid size, included similar models using only the regional stress field. These also did not allow the rupture front to reach the stepover region in most cases, suggesting that features of the fault geometry and assumed regional stress field, independent of any random stress distribution, still discourage jumping rupture.…”