Prior to the early 1990s, nearly all surface faults recognized in south Louisiana were faults of the Baton Rouge system. Since then, the number of surface fault traces interpreted in the region has increased dramatically, owing to a combination of (1) application of traditional analysis of cues on topographic maps and aerial- photographic imagery over increasingly large areas, particularly in southwest Louisiana, (2) the employment of geophysical surveying techniques in the Holocene delta plain where surface scarp relief is negligible, and (3) the advent of light detection and ranging (LiDAR) digital elevation models (DEMs). Like faults of the Baton Rouge system, newly recognized surface faults of the Tepetate system show distinctive depth-displacement relations in Quaternary and pre-Quaternary strata indicating that they are active and are the surface expressions of deep-subsurface older Cenozoic growth faults that have been reactivated following extended periods of quiescence. The differential displacement of older relative to younger Pleistocene terrace surfaces characteristic of individual Tepetate–Baton Rouge system faults also characterizes surface faults of the other systems, suggesting they may share similar movement histories. Commonplace recognition of active surface faults throughout south Louisiana now suggests that many of the known deep-subsurface growth-fault systems have surface expression reflecting their reactivation in the late Cenozoic. The recently amplified picture of surface faulting in this region highlights an important aspect of coastal tectonics in the northern Gulf of Mexico setting, and can provide useful constraints for modeling tectonics in this and other coastal settings characterized by reactivation of growth faults following lengthy intervals of quiescence.

Abstract In 1886, a large earthquake (∼M6.9–M7.3) rocked the Summerville-Charleston South Carolina area along the southeastern coast of North America. The largest east coast earthquake in North America, the earthquake caused massive damage to the cities and left ∼100 people dead. No surface rupture has ever been located; however, ongoing seismicity and damage from the 1886 earthquake has helped scientists to locate the active faults at depth and to identify potential surface offsets. The first day of the field trip will look at the damage from the earthquake as a means of understanding more about the mechanics of the earthquake. As the field trip moves into downtown Charleston, the damage will be examined as a proxy for how earthquakes cause buildings to fail and the type of damage a future earthquake could cause. The ongoing seismic activity along the suspected causal faults suggests that the earthquake risk in the Summerville-Charleston area remains high, and so the second day of the field trip will focus on the potential effects of a moderate to large earthquake in the region of the 1886 earthquake. One of the unique features of the Charleston-Summerville area is the high potential for widespread liquefaction and damage to the many bridges in the area. Therefore, Day 2 will focus on the potential for damage from a major earthquake on bridges and highly liquefiable sites by visiting a bridgeport area and then a barrier island. The visit to the barrier island highlights one of the main problems in Charleston in the event of an earthquake, the isolation of communities, with over 720 bridges and many more culverts in the area it is expected that people will be isolated in small communities for long periods of time.

The San Jacinto right-lateral strike-slip fault zone is crucial for understanding plate-boundary dynamics, regional slip partitioning, and seismic hazards within the San Andreas fault system of southern California, yet its age of initiation and long-term average slip rate are controversial. This synthesis of prior and new detailed studies in the western Salton Trough documents initiation of structural segments of the San Jacinto fault zone at or slightly before the 1.07-Ma base of the Jaramillo subchron. The dextral faults changed again after ca. 0.5–0.6 Ma with creation of new fault segments and folds. There were major and widespread basinal changes in the early Pleistocene when these new faults cut across the older West Salton detachment fault. We mapped and analyzed the complex fault mesh, identified structural segment boundaries along the Clark, Coyote Creek, and San Felipe fault zones, documented linkages between the major dextral faults, identified previously unknown active strands of the Coyote Creek fault 5 and 8 km NE and SW of its central strands, and showed that prior analyses of these fault zones oversimplify their complexity. The Clark fault is a zone of widely distributed faulting and folding SE of the Santa Rosa Mountains and unequivocally continues 20–25 km SE of its previously inferred termination point to the San Felipe Hills. There the Clark fault zone has been deforming basinal deposits at an average dextral slip rate of ≥10.2 +6.9/−3.3 mm/yr for ~0.5–0.6 m.y. Five new estimates of displacement are developed here using offset successions of crystalline rocks, distinctive marker beds in the late Cenozoic basin fill, analysis of strike-slip–related fault-bend folds, quantification of strain in folds at the tips of dextral faults, and gravity, magnetic, and geomorphic data sets. Together these show far greater right slip across the Clark fault than across either the San Felipe or Coyote Creek faults, despite the Clark fault becoming “hidden” in basinal deposits at its SE end as strain disperses onto a myriad of smaller faults, strike-slip ramps and flats, transrotational systems of cross faults with strongly domain patterns, and a variety of fault-fold sets. Together the Clark and Buck Ridge–Santa Rosa faults accumulated ~16.8 +3.7/−6.0 km of right separation in their lifetime near Clark Lake. The Coyote Ridge segment of the Coyote Creek fault accumulated ~3.5 ± 1.3 km since roughly 0.8–0.9 Ma. The San Felipe fault accumulated between 4 and 12.4 km (~6.5 km preferred) of right slip on its central strands in the past 1.1–1.3 Ma at Yaqui and Pinyon ridges. Combining the estimates of displacement with ages of fault initiation indicates a lifetime geologic slip rate of 20.1 +6.4/−9.8 mm/yr across the San Jacinto fault zone (sum of Clark, Buck Ridge, and Coyote Creek faults) and about ~5.4 +5.9/−1.4 mm/yr across the San Felipe fault zone at Yaqui and Pinyon ridges. The NW Coyote Creek fault has a lifetime slip rate of ~4.1 +1.9/−2.1 mm/yr, which is a quarter of that across the Clark fault (16.0 +4.5/−9.8 mm/yr) nearby. The San Felipe fault zone is not generally regarded as an active fault in the region, yet its lifetime slip rate exceeds those of the central and southern Elsinore and the Coyote Creek fault zones. The apparent lower slip rates across the San Felipe fault in the Holocene may reflect the transfer of strain to adjacent faults in order to bypass a contractional bend and step at Yaqui Ridge. The San Felipe, Coyote Creek, and Clark faults all show evidence of major structural adjustments after ca. 0.6–0.5 Ma, and redistribution of strain onto new right- and left-lateral faults and folds far removed from the older central fault strands. Active faults shifted their locus and main central strands by as much as 13 km in the middle Pleistocene. These changes modify the entire upper crust and were not localized in the thin sedimentary basin fill, which is only a few kilometers thick in most of the western Salton Trough. Steep microseismic alignments are well developed beneath most of the larger active faults and penetrate basement to the base of the seismogenic crust at 10–14 km. We hypothesize that the major structural and kinematic adjustments at ca. 0.5–0.6 Ma resulted in major changes in slip rate within the San Jacinto and San Felipe fault zones that are likely to explain the inconsistent slip rates determined from geologic (1–0.5 m.y.; this study), paleoseismic, and geodetic studies over different time intervals. The natural evolution of complex fault zones, cross faults, block rotation, and interactions within their broad damage zones might explain all the documented and implied temporal and spatial variation in slip rates. Co-variation of slip rates among the San Jacinto, San Felipe, and San Andreas faults, while possible, is not required by the available data. Together the San Jacinto and San Felipe fault zones have accommodated ~25.5 mm/yr since their inception in early Pleistocene time, and were therefore slightly faster than the southern San Andreas fault during the same time interval. If the westward transfer of plate motion continues in southern California, the southern San Andreas fault in the Salton Trough may change from being the main plate boundary fault to defining the eastern margin of the growing Sierra Nevada microplate, as implied by other workers.