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

Abstract

The Southern California San Jacinto fault is geometrically complex, consisting of several major strands with smaller scale complexity within each strand. The two northernmost strands, the Claremont and the Casa Loma-Clark, are separated by a 25-km-long extensional stepover with an average of 4 km separation between the strands. We use a combined modeling method to assess probable rupture and groundmotion behaviors for this stepover. First, dynamic rupture modeling on geometrically complex fault strands embedded in a state-of-the-art 3D crustal velocity model is used to generate a series of scenario earthquakes. We then use the resulting near-fault lowfrequency (< 1 Hz) ground-motion time histories to generate broadband synthetic seismograms with a hybrid approach. These synthetics are then compared with a distribution of precariously balanced rocks (PBRs) near the fault to constrain our results and assess shaking hazard for the region surrounding the fault. Our dynamic models produce sources between M w 5.4 and 6.9, with rupture limits imposed by sharp contrasts in fault stress or by geometrical barriers. The main stepover serves as a primary barrier to rupture in our model, producing event sizes that are consistent with the historical behavior of the San Jacinto fault. The largest broadband synthetics are a good match to leading ground-motion prediction equations and are generally consistent with the distribution of PBRs, none of which experience accelerations that produce toppling probabilities significantly higher than zero. Thus, although the PBRs do not rule out any of our model scenarios, they confirm that our models produce realistic rupture extents and shaking.Online Material: Figures of total slip for additional rupture models, low-frequency intensity plots, synthetic seismograms, and comparison with ground-motion prediction equations.

Simulation of broad-band strong ground motion for a hypothetical Mw 7.1 earthquake on the Enriquillo Fault in Haiti

“…Low-frequency dynamic rupture simulations conducted by Douilly et al (2015) on the Léogâne Fault predicted the PGA in Port-au-Prince to be ∼0.1 g. This PGA estimate is low compared to other studies because dynamic rupture simulations with simplistic stress complexity and based on 1-D velocity models do not account for higher frequencies that are necessary to adequately resolve the PGA at a site. Dynamic rupture studies have shown that complexities, such as fault roughness or stochastic stress asperities, can lead a rupture to reach higher fre-quencies (Oglesby & Day 2002;Shi & Day 2013, Lozos et al 2015. Furthermore, Hough et al (2012) used a rigid body displacement technique and inferred a PGA value of ∼0.2 g in Port-au-Prince, not too far off from that of Douilly et al (2015).…”

“…S2, Supporting Information). The critical role of rupture directivity and fling step on ground motion has been observed both in ground motion simulations (Aki 1968;Haskell 1969;Mavroeidis & Papageorgiou 2010;Lozos et al 2015) and in numerous actual earthquakes (Somerville et al 1997;Mavroeidis & Papageorgiou 2003;Bray & Rodriguez-Marek 2004).…”

“…Since peak ground accelerations (PGAs) and spectral accelerations are primarily associated with high frequencies, hybrid simulation approaches that combine deterministic and stochastic simulation techniques are commonly used in practice to generate broad-band ground motions for engineering applications and advance our understanding of seismic hazard for a particular site or region (e.g. Mavroeidis et al 2008;Graves & Pitarka 2010;Mai et al 2010;Mavroeidis & Scotti 2013;Lozos et al 2015). For example, the Southern California Earthquake Center (SCEC) has recently put together the SCEC Broadband Platform, an open-source software system that can generate broad-band ground motions for historical and scenario earthquakes using physics-based models (Goulet et al 2015).…”

A case for historic joint rupture of the San Andreas and San Jacinto faults

“…I think it is unlikely that the 1800 earthquake ruptured both the Claremont and Clark strands of the SJF for several reasons. First, I have previously conducted dynamic rupture models on several different interpretations of the Claremont-Clark stepover geometry and have been unable to achieve model ruptures that propagate through the stepover while also producing slip values that are consistent with paleoseismic observations ( 24 – 26 ). The stepover is a persistent barrier.…”

“…The stepover is a persistent barrier. Second, these models indicate that rupture toward the stepover raises the chance of toppling a cluster of precariously balanced rocks (PBRs) near the stepover ( 26 ). A full-SJF 1800 rupture would propagate toward the stepover regardless of whether it nucleated in the north or in the south.…”