Evidence for a prehistoric multifault rupture along the southern Calico fault system, Eastern California Shear Zone, USA

Abstract

Geomorphic mapping and paleoseismologic data reveal evidence for a late Holocene multifault surface rupture along the Calico-Hidalgo fault system of the southern Eastern California Shear Zone (ECSZ). We have identified ~18 km of continuous surface rupture along the combined Calico and Hidalgo faults in the vicinity of Hidalgo Mountain in the southern Mojave Desert. Based on the freshness of geomorphic fault features and continuity of surface expression, we interpret this feature to reflect a simultaneous paleorupture of both faults. Displacement along the paleorupture is defined by 39 field measurements to be generally pure right-slip with a mean offset of 2.3 m. Scaling relationships for this offset amount imply that the original surface rupture length may have been ~82 km (corresponding to a M7.4 earthquake) and that much of the rupture trace was erased by subsequent erosion of sandy and unconsolidated valley alluvium. Eight luminescence ages from a paleoseismic trench across the paleorupture on the Hidalgo fault bracket the timing of the most recent rupture to 0.9–1.7 ka and a possible penultimate event at 5.5–6.6 ka. This timing is generally consistent with the known earthquake clusters in the southern ECSZ based on previous paleoseismic investigations. The ages of these earthquakes also overlap with the age brackets of the most recent events on the Calico fault 42 km to the north and the Mesquite Lake fault 40 km to the south from earlier work. Based on these age constraints and the expected surface rupture length, we propose that the Calico fault system experienced a major, multifault rupture that spanned the entire length of the fault system between the historical Landers and Hector Mine ruptures but preceded these events by ~1–2 k.y. Coulomb stress change modeling shows that the Calico paleorupture may have delayed the occurrence of the Landers-Hector Mine cluster by placing their respective faults in stress shadows and may have also prevented a triggered event from occurring on the Calico fault following the historic events. This work implies that closely spaced ruptures in complex shear zones may repel each other and thereby stretch out the duration of major earthquake clusters. These results also suggest that complex multifault ruptures in the ECSZ may not follow simple, repeatable patterns.

What Controls Early Restraining Bend Growth? Structural, Morphometric, and Numerical Modeling Analyses From the Eastern California Shear Zone

“…As a note, we limit our interpretations and choose to present only fault mapping and exhumed sediment distribution for the composite bends on the Calico-Hidalgo faults (bends 20, 21, and 22) (Figures 3g, 5f, and 5g), because they exhibit several distinct characteristics that set them apart in the SECSZ. First, the Calico and Hidalgo faults show sharp geomorphic features (Figure 4e) with widespread deformation and disruption of younger alluvial deposits that have been linked to a late-Holocene paleoseismic event (Vadman et al, 2023). Second, the exposed and folded pre-uplift deposits within and around the restraining bends are much more extensive compared to other bends in the SECSZ.…”

“…From north to south, the number of active faults increases due to the southward bifurcation of some principal faults (e.g., Lenwood and Old Woman Springs faults, Emerson and Homestead Valley faults, Calico and Hidalgo faults) and the addition of other faults (e.g., Johnson Valley, Lavic Lake, and Mesquite Lake faults). Numerous zones of fault intersection facilitate structural interaction and translate to complex earthquake rupture, as highlighted by two historic large earthquakes (1992 M w 7.3 Landers and 1999 M w 7.1 Hector Mine) and one prehistoric rupture (Vadman et al, 2023) that occurred in the SECSZ, each of which involved rough, sinuous surface ruptures across multiple (∼3-5) faults and numerous stepovers (Figure 1b) (Sieh et al, 1993;Treiman et al, 2002). The zones of fault intersection tend to occur in areas of high relief and increased fault segmentation indicating inefficient transfer of strike-slip motion between faults and local contraction, which influences earthquake rupture dynamics and may lead to growing geometric complexity over iterative earthquake cycles.…”

“…This mapping was guided by previous fault mapping in the area, including of previous fault ruptures (Dibblee, 1964a(Dibblee, , 1964b(Dibblee, , 1964c(Dibblee, , 1966(Dibblee, , 1967a(Dibblee, , 1967b(Dibblee, , 1967cSieh et al, 1993;Spotila & Garvue, 2021;Spotila & Sieh, 1995;Treiman et al, 2002;U.S. Geological Survey & California Geological Survey, 2020;Vadman et al, 2023;Wesoloski & Marquis, 2022;Zachariasen & Sieh, 1995). We also mapped exhumed, eroding, and deformed sediments that highlight faulting and are often folded around the ends of restraining bends (Figures 4 and 5; Table S4 in Supporting Information S1).…”

“…Because these presumably Pleistocene and older deposits indicate low-energy fluvial deposition, we interpret them as pre-uplift valley deposits that record the growth and migration of restraining bends (e.g., Burkett et al, 2016;Wakabayashi et al, 2004). Field mapping was supplemented with high-resolution satellite imagery from Google Earth Pro, 1-m resolution LiDAR available from OpenTopography.org (EarthScope Southern & Eastern California LiDAR Project, 2007; U.S. Geological Survey, 2022), 2-m resolution from the EarthDEM Project (Porter et al, 2022), and UAV-derived Structure-from-Motion DEMs along specific faults (Vadman et al, 2023).…”