Researchers PSU Professor Ashley Streig, with alumni Alison Horst, Maddy Blair, Erin Murphy, MS student Andrew Dunning, and Scott Bennett (USGS, research collaborator) studying the Twin Lakes fault on Mt. Hood in the summer of 2021. Image courtesy of Ashley Streig.
When we talk about earthquakes in the Pacific Northwest, the discussion is almost always about the inevitable "big one"--a magnitude nine or greater Cascadia subduction zone earthquake. The last time such an earthquake struck was 322 years ago, a long enough time for some geologists to put the odds of a big Cascadia quake occurring in the next fifty years at three to one.
Portland State University earthquake geologist, Ashely Streig, knows firsthand the kind of damage earthquakes can cause. Streig has served on several teams responding to major seismic events--the Chi-Chi earthquake, which struck Taiwan in 1999, and the 2011 Tohoku subduction zone earthquake and tsunami in Japan. Today, Streig's research focuses on the potential for significant seismicity after the rupture of the Cascadia subduction zone, not the shaking of aftershocks, but earthquakes originating from inland faults here in Oregon.
Streig recently received a five-year, $582,316 CAREER award from the National Science Foundation (NSF) to study some of these inland crustal faults in western Oregon, the northern Oregon Cascades, and the Strawberry Range in the eastern part of the state.
According to Streig, geologic forces are causing the landmass of the Pacific Northwest to rotate in a clockwise orientation relative to the rest of the North American continent. Over the past few decades, geologists have developed models that simulate this crustal deformation using GPS data. The models show landmasses moving between two and seven millimeters per year, depending on location in the state. Geologists presume fault systems bounding these blocks of landmasses are responsible for the movement. However, not all boundaries coincide with known faults, and faults can be difficult to identify using GPS data because of the relatively sparse instrumentation and short period of data collection. Furthermore, features such as dense vegetation can obscure what geologists can observe in the field.
Streig's study aims to improve seismic hazard models and maps used by agencies such as the U.S. Geological Survey and enhance our overall understanding of crustal deformation in Oregon. To do so, Streig and her research team will determine how these inland faults work together to accommodate the clockwise rotation.
"We don't have a lot of data on these faults and their relation to plate boundary motion," Streig said. "We only have about twenty years of GPS data, which isn't adequate if we want high-quality models. Additionally, we're in this period between subduction zone earthquakes, so the stress caused by the Juan de Fuca plate sinking beneath the North American plate is clamping down all these crustal faults. That could mean the faults are primed for activity if a subduction zone earthquake released that stress."
Such was the case after the Tohoku subduction zone earthquake when, one month later, similar crustal faults activated and unleashed destructive energy across the region. The potential for such seismic events here in Oregon is why understanding the Pacific Northwest's network of inland faults and their relationship to the rotation of landmasses is essential to preparing for future quakes and the risks associated with their seismicity.
Streig's approach to improving understanding of the network of inland crustal faults and landmass deformation in Oregon involves searching for unidentified fault structures along the boundaries of landmasses where deformation is occurring and conducting fieldwork to assess the timing of recent seismic activity along several faults.
To identify previously unknown fault structures along the boundaries of rotating landmasses--those that have generated surface rupturing earthquakes in recent geologic history, Streig and the research team will analyze Lidar (light detection and ranging) datasets. The advantage of Lidar, Streig noted, is the quality of the resolution it provides, allowing researchers to strip away vegetation and examine surface features in one- to two-meter units to identify characteristics associated with faults. Dating recent seismic activity along some of the faults will take the research team out into the field. There they'll collect samples of deposits of organic matter for carbon dating. That dating can provide clues as to when the site was last seismically active. In locations such as glacial moraines that lack organic material to date, the team will collect rock samples for a process of isotopic analysis. That analysis, along with the measurement of features of the fault structures, will allow the team to calculate when the last seismic activity occurred.
"You can think of these faults and the landmasses they constrain and move as kind of a patchwork quilt," Streig said. "The Lidar data will help us see pieces we didn't know were there and how everything fits together. Then we can combine that spatial data with the temporal data. Once we've done that, we can start to understand how the pieces move over time--how the crust is deforming and how the faults work together to accommodate the rotation of the land."
In addition to funding the science, the CAREER award supports graduate students and paid internships for undergraduate students that Streig plans to recruit from PSU's NSF-funded Louis Stokes Alliance for Minority Participation program. Streig notes that geology is one of the most underrepresented STEM fields. A project of this scope and duration provides an excellent opportunity for women and members of the BIPOC community to engage in earthquake geology. Streig also plans to partner with the Oregon Museum of Science and Industry (OMSI) in Portland via the OMSI Science Communications Fellowship program. The partnership will result in creating an interactive demonstration exhibit at the museum to educate the public about earthquake geology in the Pacific Northwest.