Graduate Thesis Or Dissertation
 

Investigating the Paleoseismic Potential of Small, Southern Cascadia Lakes

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  • Marine and coastal sedimentary archives show that the Cascadia Subduction Zone can generate great earthquakes of ~M9, most recently in 1700 CE, yet we still know little of the impacts of these events inland of the Pacific coast. Inland lakes have recently been exploited for their use as paleoseismic records at Cascadia, but most of these studies have looked at large lakes. In this dissertation, I investigate small inland lakes from southern Oregon and northern California because they hold great potential as sensitive recorders of megathrust earthquakes. I address the following questions: 1) Do Cascadia earthquakes influence sedimentation in small lakes in Cascadia? 2) If Cascadia earthquakes influence lake sediments, are the resulting deposits different from other types of disturbance deposits? 3) How does the earthquake chronology compare to coastal and marine records of Cascadia earthquakes? 4) Do other types of disturbances (earthquakes produced by crustal faults and anomalous sedimentation resulting from floods) influence the sedimentary record in ways that can be uniquely identified? 5) Is it possible to correlate between marine and lake paleoseismic records? To answer the first four of the five questions, I investigated the sedimentary record from Squaw Lakes, Oregon, a pair of landslide dammed lakes at the California/Oregon border. These lakes were formed when a retrogressive landslide progressively dammed a drainage system forming the lower lake, then forming the upper lake. These lakes share some of the same characteristics because they are part of the same drainage system, and experience many of the same disturbances (such as earthquakes and floods), yet they are different in their volume, clastic supply, watershed characteristics and sediment provenance. This site is also ideal because Upper Squaw Lake has a published record of watershed-sourced disturbances (Colombaroli et al., 2018). To address the fifth question I looked at the timing and frequency of the relationship between Oregon and northern California lake records and compared them to other lake and marine records using radiocarbon ages and physical property data from lake cores as a proxy for disturbances in the sedimentary record. Finding evidence of megathrust earthquakes in small Cascadia lakes would provide the opportunity to better determine the source, and therefore hazard potential, of southern Cascadia earthquakes. An analysis of the sedimentary record from Lower Squaw Lake, a landslide dammed lake in southern Oregon, identified a disturbance deposit dated to 1680-1780 CE. This deposit has a sharp basal contact with evidence of loading, is composed of watershed-sourced silt (suspected to result from liquefaction of the lake’s delta), and has a long (5-20 cm) organic tail. To determine if this deposit was formed in response to the 1700 CE Cascadia megathrust earthquake, deposits with similar characteristics were identified downcore and the timing of these deposits was compared to the timing of published record of offshore and coastal paleoseismic records. Deposits with characteristics most similar to the deposit attributed to the 1700 CE Cascadia earthquake are temporal equivalents to offshore deposits T1-T6, where T2 and T4 equivalents in the lake record are thinner than the other events, similar to correlatives in the offshore record. A younger deposit (deposit H, dated to 1820-1880 CE; attributed to a slab earthquake which occurred in 1873 CE) also has similar characteristics. Deposit D, dated to 1870-1940 CE (possibly the result of the 1906 San Andreas earthquake), is also composed of watershed-sourced silt. A deposit, possibly equivalent to T2a (550-660 BP), is composed of schist, similar to deposit I (attributed to the failure of the landslide dam attributed to the 1873 CE earthquake), suggesting a different source. The record may also include equivalents to events T5b and T5c, prominent in the offshore record, but are of uncertain attribution sedimentologically. These results suggest that Lower Squaw Lake contains evidence of both Cascadia and other seismic (and possibly aseismic) events, and suggest that southern Cascadia lakes may provide useful information that can be used to determine the seismic sources influencing the offshore record.
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  • Pending Publication
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  • 2021-01-15 to 2021-05-24
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