Documenting the Earthquake History of the Thousand Springs Fault in Summer Lake Basin, Oregon, USA

<p>Defining seismic hazards in low-strain-rate regions such as the northwestern Basin and Range can be difficult due to the infrequency of earthquakes. Revealing the earthquake records of low-strain-rate regions can refine our understanding of the variability of earthquake sizes and recurrence intervals, however, which can ultimately improve hazard analysis. Four active normal faults form the Summer Lake basin, in the northwestern Basin and Range: The Thousand Springs (TSF), Ana River (ARF), Slide Mountain, and Winter Ridge faults. Other than the TSF, the faults in the Summer Lake basin have documented histories that include surface-rupturing (>M6) earthquakes. Scarps along the TSF were only recently mapped and its earthquake history has not been previously documented. The TSF cuts through an area with relatively low sedimentation rates and numerous tephras from the past ~250,000 years and thus earthquakes on this fault are preserved and dateable through trenching. We dug two trenches across the TSF in 2019, exposing multiple episodes of offset bracketed by deep to shallow-water lake sediments, a sand dune, and tephras which were identified based on correlations of their physical characteristics, stratigraphic sequence, glass chemistry, and radiocarbon dates from the lake sediments: Tephra 2 (Ice Quarry tephra), Pumice Castle tephra, Mount St Helens Cy tephra, Wono tephra, Trego Hot Springs tephra, Mount St Helens Mp, and a black tephra. These tephras, and a sand dune most likely containing reworked Mazama ash and lacustrine sediments, are offset by a fault zone that spans a minimum of four meters with at least five fault strands. The only unit that was able to be correlated across the fault zone, MSH Cy tephra, had a total offset amount of 2.4 m. The next youngest tephra, Wono, is offset by 2.0 m and THS is offset by 1.8 m, which were both determined by extrapolating missing sections. Based on offset of individual tephras and the comparison between the two trenches, we have identified at least five surface-rupturing earthquakes. The events in chronological order are as follows: The oldest event (event 5) occurred 54.1 – 71 ka, event 4 occurred 30.5 – 45.6 ka, event 3 occurred 24.8 – 29.1 ka, event 2, which was a folding event at our sites, occurred 7.6 – 12.7 ka, and the most recent event (event 1) occurred after 7.6 ka. These results suggest that the TSF is just as active as the nearby Ana River Fault, which has had at least 8 earthquakes in the past ~80 ka compared to the TSF’s 5 earthquakes. Comparing the TSF’s activity to lake level changes in the basin during the Quaternary suggests that the crust could be responding to the changes of the lake level, causing variability in the earthquake recurrence intervals. These insights are applicable to the forecasting of earthquakes in the northwestern Basin and Range and other low-strainrate regions.</p>

Name thesis:degree_name

Master of Science (MS)

Discipline thesis:degree_discipline

Geological Sciences

Year dc:date.available

2020

Author dc:creator

• Curtiss, Elizabeth

Contributors dc:contributor

• Anne E. Egger
• Ray J. Weldon
• Walter Szeliga

• Paleoseismology
• Northwestern Basin and Range
• Trenching
• Geology
• Stratigraphy
• Tectonics and Structure

Repository record dc:identifier

https://digitalcommons.cwu.edu/etd/1363

OAI identifier oai:identifier

oai:digitalcommons.cwu.edu:etd-2387