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Intellectual Merit
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The Elsinore fault is one of the three primary strands of the onshore San Andreas fault system in southern California and is thought to accommodate ~10% of the total relative plate motion between the Pacific and North American plates. Understanding the hazard posed by the Elsinore fault is critically important not only because it has the potential to generate Mw≥7 earthquakes in the densely developed Inland Empire (Millman and Rockwell, 1986; Rockwell et al., 1986, 2016; Field et al., 2017), but also because this major dextral strike-slip fault feeds slip directly into several of the major faults of the Los Angeles metropolitan region, including the Whittier fault and the Puente Hills blind thrust fault, which directly underlies much of the northern urban area, including downtown Los Angeles itself (Dolan et al., 1995; Shaw et al., 2002; Plesch et al., 2007; Gath et al., 2017) (Figure 1). The slip rate of the northern Elsinore fault has long been accepted to be on the order of 5 mm/yr (Millman and Rockwell, 1986; Vaughan, 1987; Millman, 1988; Vaughan et al., 1999; Rockwell et al., 2000) based primarily on offset geomorphic features with ages constrained by soil age estimates. Only the Rockwell et al. (2000) study applied 14C dating of charcoal from a buried channel offset by 9.8±0.5 m, and that rate is only for the past ≤2 ka, which is too short to assess the actual long-term rate. Moreover, the soil ages that underpin our current understanding of the Elsinore fault slip rate are estimated on the basis of a geochronologically constrained soil chronosequence from the Ventura region (Rockwell, 1983; Millman, 1988). Aside from the fact that the slip rate of the northern Elsinore fault is only poorly constrained by geochronologic data, the soils of the Ventura region developed under coastal conditions, including the presence of abundant Na+ cations that act as a deflocculent, leading to more rapid soil development than would be seen in the drier, inland setting of the Elsinore fault (Rockwell et al., 2000). Thus, the currently accepted long-term slip rate of the Elsinore fault is not directly constrained by geochronologic data and may, in fact, be inaccurate. Moreover, there are currently no incremental slip-rate records available for the Elsinore fault with which to gauge the relative constancy or non-constancy of the fault slip rate through time. This issue is of critical importance for the use of geological slip rates in seismic hazard assessment and in studies of geodetic-geologic rate comparisons, particularly in the case of the Elsinore fault, because recent work by Gauriau and Dolan, (2021) suggests that faults such as the Elsinore, which lie within structurally complex fault networks, are more likely to exhibit highly variable slip rates through time, thus potentially complicating the use of slip rates averaged over different cumulative displacements in probabilistic seismic hazard assessment (PSHA). Interestingly, this northern section of the Elsinore fault was called out in the UCERF-3 model (Field et al., 2015, 2017) as a region where the model results did not match the geological slip-rate estimates. Such incremental slip-rate data at the scales of several to several tens of earthquakes are necessary to ground-truth earthquake simulator results and to assess potential spatial and temporal fault interactions amongst the major faults of the southern California plate boundary system.
In order to address these issues, we have begun research to construct a geochonologically constrained incremental slip-rate record for the Elsinore fault. Specifically, in this project we have mapped and sampled for dating two fluvial terraces that have been offset by the Elsinore fault. The riser between the Qf3 and younger Qf1a (active stream deposits) and Qf1b (very young, potentially active stream deposits) has been offset by 44±5 m, whereas the riser between the Qf3 and Qf5 terraces has been offset by 190+25/-15 m. We collected four samples for post-IR IRSL luminescence dating from each of the Qf3 and Qf5 deposits (Figures 2 & 3). The Qf3 samples were collected from a pit we excavated into the wide Qf3 deposit (Figure 3), which is a cobble-small boulder gravel with a sandy matrix. We were able to sample this deposit by pounding sample tubes between the large clasts. The Qf5 deposit is even coarser grained, consisting of a predominantly small boulder to cobble gravel with a sandy matrix. Fortuitously, there was an old road cut that extend perpendicular to the Qf5 deposit, so we were able to clean off and sample this exposure. When final dating of the luminescence samples with the post-IR IRSL technique (Rhodes, 2015) is complete (final dates are pending as of this writing), we will be able document the slip rate of the Elsinore fault at two different displacement intervals. These two slip rates, together with the previously published 2 ka, 10 m slip rate documented by Rockwell et al. (2000), will form the beginnings of an incremental slip-rate record for this key fault, providing not only a more robust estimate of the slip rate of the Elsinore fault, but also a point of comparison with similar records from other nearby faults in the search for constraints on possible plate-boundary fault system-level behavior.
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Broader Impacts
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The Broader Impacts for this work fall into two categories – improved understanding of the slip rate of the Elsinore fault for use in seismic hazard assessment, and student education and training. The Elsinore fault is a major seismic hazard in its own right, as well as a key structure that feeds slip directly onto major faults in the Los Angeles metropolitan region (e.g., Whittier fault, Puente Hills blind thrust fault), yet its slip rate is not well constrained. Our research will provide two geochronologically constrained slip rates at different displacement scales that will form the beginnings of a robust incremental slip-rate record for this important fault. In terms of student training and engagement, this project forms part of USC PhD Judith Gauriau’s dissertation research. She has taken the leading role in all phases of this effort, from geomorphic remapping of the study sites using lidar data, vintage aerial photos, and field work, to choosing sample sites and sampling for luminescence dating, to participating in writing the original SCEC proposal. Judith will also take the lead role in writing up the results for peer-reviewed publication. These efforts are all part of her training as a modern active tectonicist as part of her PhD research. In addition to Judith Gauriau, three other USC PhD students (Christoffel Anthonissen, Dannielle Fougere, and Caje Kindred Weigandt) and one USC undergraduate student (Luke Gordon) participated in the field research. All of these students have also participated at various times in analysis of lidar digital topographic data and have used these data to help constrain measurements of offset geomorphic features in related projects. These experiences have been integral to their training in state-of-the-art field and laboratory techniques as part of their overall training to become skilled researchers in Active Tectonics. |
