Although most of the focus of earthquake resilience efforts and research efforts in the Pacific Northwest is on the Cascadia Subduction Zone and its potential for M 9 earthquakes, local earthquakes on crustal faults remain a potential, but poorly understood, threat to the region.  Few active crustal faults have been identified in western Oregon, in part because of the thick forest that covers mountainous regions of the Coast Range and regions west of the crest of the Cascade Range. Using high resolution lidar topography collected by Oregon Department of Geology and Mineral Industries (DOGAMI) I have identified several geomorphic lineaments in the northern and southern Coast Range, and along the western margin of the Willamette Valley, Oregon. I have mapped these fault structures at a level of detail that was not possible prior to the acquisition of high resolution topographic data.

Using high resolution topographic data from lidar, we mapped surface expression of the Gales Creek Fault, located 45 minutes west of Portland Oregon, in the Coast Range. This fault is classified as Quaternary active in the USGS fault and fold database, magnetic data show 10-15 km of dextral offset in the magnetic Eocene basement, but little is known about its Holocene activity. Additionally, models for crustal deformation in the PNW suggest NW trending faults accommodate crustal strain from clockwise rotation at this latitude, we can test this by evaluating the surface morphology and paleoseismic record along mapped NW striking structures. Relatively new lidar data reveal that the Gales Creek fault has youthful surface expression, tectonic geomorphic features include right laterally deflected drainages, broad side-hill benches, and older landslide deposits cut by linear scarps that are on trend with the fault.  Quaternary activity of this fault was first investigated by the US Bureau of Reclamation (Redwine et al., 2017), our efforts on the fault were focused on a northeastern strand of the fault located 10 km north of their 2017 studies. A student intern, and I mapped the northern section of the Gales Creek fault using lidar derivatives and conducted field reconnaissance along the mapped traces to locate a paleoseismic site. At our Clear Creek paleoseismic site, we?ve identified evidence of at least two surface deforming events. Evidence for deformation includes a buried soil that is tilted and dips into the hillslope, and drops >50 cm over a 2 m distance and is overlain by a coarse-grained wedge shaped deposit. The buried soil and colluvial wedge units are truncated in a shear zone that coincides with an inflection in the ground surface that divides steep upslope topography and the flat side-hill bench. Preliminary age results reveal that the most recent surface rupturing Gales Creek fault earthquake occurred sometime between 800 ? 1000 cal. years before present, and overlaps in time with a moderately constrained Cascadia subduction zone earthquake. The Gales Creek fault is ~ 45 miles from downtown Portland, and based on empirical scaling relationships between rupture length and magnitude the fault could generate a M 6.8 to 7 earthquake and is clearly a seismic hazard for the region. Ultimately, with denser paleoseismic observations from neighboring crustal faults the timing of rupture on these forearc faults can be compared to the long paleoseismically determined record of ruptures along the Cascadia subduction zone.

This project evaluates whether recurrence intervals for crustal forearc faults in the Pacific Northwest are intrinsically linked with the long recurrence of Cascadia subduction zone events. Triggered crustal fault rupture might be a common feature of subduction zones, but too few megathrust events have occurred in the historic, well instrumented period to observe this relationship. The M_w 9.0 - 2011 Tohoku-oki, Japan earthquake caused significant eastward movement of the coast of up to 5.3 meters (Ozawa et al., 2011), and changed the primary stress state of the onshore region from interseismic E-W compression to roughly E-W extension (Toda and Tsutsumi, 2013; Yoshida et al., 2012). In Cascadia, we find large on-shore stress changes following subduction zone earthquakes. We have also identified crustal faults that have ruptured in the last 7,000 years, and the timing of the most recent large magnitude earthquake on one of these faults appears to overlap in time with a geologically constrained Cascadia subduction zone earthquake. These preliminary results suggest that there may be a relationship between subduction zone earthquake cycle and the interval in which crustal faults in the Cascadia forearc fail.

Last Modified: 12/03/2018
Modified by: Ashley R Streig