1. Paleoseismic Analysis of a Natural Fault Exposure Using Digital-Remote
Logging Techniques, Wasatch Fault Zone, Alpine, Utah.
Michael J. Arnoff, Nathan Toke
Utah Valley University Department of Earth Science
Abstract: The Wasatch Fault is an extensive, north-south trending normal fault stretching
throughout most of Utah and southern Idaho. The fault trace itself is discontinuous, and is
comprised of ten parallel segments previously thought to rupture independently of one another.
Recent paleoseismic studies have concluded that the Provo and Salt Lake City segments are the
most active, with radiocarbon samples indicating slip events as recent as 1kya. These two
segments border each other, but are separated by a ~10km, east-west trending stepover zone
where the fault trace is concealed. Trench data gathered within this stepover zone suggest that
segments within the WFZ may actually rupture simultaneously when triggered by a sufficiently
large earthquake event. The possibility of multisegment ruptures occurring along the WFZ has
great implications with respect to the safety and prosperity of Utah’s citizens living near the
fault. The goal of this research is to further investigate the rupture scheme along the WFZ by
observing quaternary sediment displacements and other surficial features that reflect
paleoseismic events. One such feature of interest is a small (~5x10m) channel located along the
northernmost portion of the Provo segment. The channel itself has experienced mass wasting
even within the last two to three years, exposing a ~2m long fault plane striking north-south
(perpendicular to the direction of mass wasting). Plant roots observed along this newly exposed
surface are aligned with the general slip direction of the fault as a whole, a phenomenon that
strongly indicates recent normal-oblique fault movement within the area. The short-lived nature
of this naturally-cut trench provided an opportunity to log sedimentary packages using digital-
remote logging techniques, primarily 3D modelling of the exposure using Agisoft Photoscan
Professional. The resulting 3D imagery was logged by hand, and no fewer than two seismic
events occurring roughly between 500-3000 years ago were identified. The amount of slip
resulting from these events was relatively minimal (between 0.5m &2.5m, respectively), but the
resulting destabilization of overlying soil is alarming to housing divisions immediately adjacent
to the fault. These events were roughly correlated with similar features observed within a nearby
USGS-dug trench. It is still unclear if these two events are the result of energy translating across
fault segments, and reliable radiocarbon samples are needed to properly date and correlate them
with slip events along the Salt Lake City fault segment.