Active Tectonics Group, Ocean Admin Bldg 104, Corvallis OR 97333 Recording and Preservation Sensitivities for Paleoseismic Data on the Cascadia Margin* Chris Goldfinger College of Earth, Ocean and Atmospheric Sciences, Oregon State University Active Tectonics Group, Ocean Admin Bldg 104, Corvallis OR 97333 gold@coas.oregonstate.edu With many thanks to: C. Hans Nelson†, Joel E. Johnson*, Steve Galer, Jeffrey Beeson, Bran Black, Ann E. Morey*, Julia Gutiérrez-Pastor†, Eugene Karabanov**, Andrew T. Eriksson*°, Rob Witter and George Priest s, Eulàlia Gràcia****, Kelin Wang***, Joseph Zhang S, Gita Dunhill††, Jason Patton*, Randy Enkin***, Audrey Dallimore*** , Tracy Vallier§, and the Shipboard Scientific Parties (52 students, colleagues, technicians)   *and elsewhere perhaps

How do we compare sites, evaluate correlations and integrate multiple data streams? In paleoseismology, we commonly start with radiocarbon data. This is the primary tool for this in terrestrial paleoseismology. There are few other ways to do it.

How do we compare sites, evaluate correlations and integrate multiple data streams? But, a lot happens before we get the age. Interpretation of stratigraphy Interpretation of event horizons and terminations Sampling of detrital material in the vicinity of the event horizons Interpretation of how the site fits into the local tectonics Relationship of the samples to the events of interest. Local site sensitivity to ground motion Sensitivity of the site for recording and preservation.

How do we compare sites, evaluate correlations and integrate multiple data streams? But, a lot happens before we get the age. Interpretation of stratigraphy Interpretation of event horizons and terminations Sampling of detrital material in the vicinity of the event horizons Interpretation of how the site fits into the local tectonics Relationship of the samples to the events of interest. Local site sensitivity to ground motion Sensitivity of the site for recording and preservation.

So then, with that long list of interpretive steps, we then hope to compare two sites with similar lists (the uncertainty budget) using mainly the radiocarbon ages. Maybe the other sites were done by other people, using different methods, and different interpretive frameworks. Uh oh.

Such comparisons commonly lead to confusion, and sometimes lead to segmentation models! And once that happens, it’s in the literature forever, right or wrong, leaving a trail of ambiguity that’s hard to ever reconcile.

So lets take a look at one example from Cascadia Cascadia is today one of the best known faults in the world, based almost solely on paleoseismology. Remarkable, but the paleoseismic work spans decades of technological improvement, and multiple methodologies from marsh subsidence records, to tsunami records, to turbidites offshore, and now turbidites in lakes. The multiplicity of records leads to inevitable ambiguities based on radiocarbon, and presence-absence at various sites. But how much of the remaining ambiguity is attributable to the recording and preservation sensitivities of the various sites?

Coastal Subsidence Paleoseismology What is the recording threshold for subsidence? It can be as good as +/- 0.2 m, certainly better than 0.6 cm vertical using the present understanding of transfer functions (Engelhart et al. 2013. If this were measured near the subsidence maximum, recent simulations show that ruptures of Mw ~ 8.20-8.25 generate subsidence values of 0.5-0.75m locally in southern Cascadia (Priest et al., 2014). This should be well within modern methods detection capability. However, this improved precision exists at just a few sites, and is not applicable to most of the region.

Ground deformation for “Segment D” ruptures, southern Cascadia margin. Typical subsidence is < 1 m, 0.5-0.75 m at coastal sites

Coastal Subsidence Paleoseismology What is the recording threshold for subsidence? It can be as good as +/- 0.2 m, certainly better than 0.6 cm vertical using the present understanding of transfer functions (Engelhart et al. 2013. If this were measured near the subsidence maximum, recent simulations show that ruptures of Mw ~ 8.20-8.25 generate subsidence values of 0.5-0.75m locally in southern Cascadia (Priest et al., 2014). This should be well within modern methods detection capability. However, this improved precision exists at just a few sites, and is not applicable to most of the region. The coastal subsidence sensitivity at most sites is closer to ~ > 1m +/- 0.5 m (Atwater, 1992). This could correspond to a minimum Mw~ > 8.5.

Coastal Tsunami Paleoseismology What is the recording threshold for coastal tsunami? One of the best documented sites is Bradley Lake (Kelsey et al. 2005) At mid tide, a tsunami of ~ 5.5 m above MTL is required to reach the lake at all, and larger is required to leave a significant deposit (and more so in recent times) The corresponds to ~ Mw = 8.5-8.6 from empirical averages (e.g. Blaser et al. 2010).

The 5.5 m berm guarding Bradley Lake restricts tsunami input from earthquakes of Mw < 8.5 (+/-tide) It also effectively blocks tsunami coming from ruptures the terminate south of Cape Blanco, a proposed structural segment boundary (Priest et al., 2014; 2017).

Offshore Turbidite Paleoseismology What are the triggering and recording thresholds for turbidity currents/turbidites? Combined triggering and recording threshold was estimated to be ~ Mw=7.1 for Cascadia, but is likely much lower. Requirement is available sediment (abundant!), modest slopes, a depocenter and PGA > tenths of a G. These conditions are met virtually everywhere on the continental slope with slopes > ~ 10 degrees. What magnitude does this correspond to? Likely the value is < Mw=6 as demonstrated in numerous lake records globally. This may be near the detection limit in geophysical data from the cores.

Comparing sensitivities Most land records appear capable of recording and preserving ~ Mw=8.5 events. Better at a few sites, but more high precision work is needed. Numerous marine sites capable of recording > Mw=7.1, and perhaps much lower. Lake records have similar sensitivities to offshore sites. The difference is ~ Mw 1.4 units, or 125 times more energy required to preserve a terrestrial record that a marine record (as published) in Cascadia. > > Sensitivities

So should a few discrepancies be surprising? With such large differences in site and process sensitivity, some differences should be expected. In northern Cascadia, there are virtually no differences, attests probably to the dominance of very large events, and lack of smaller ones. In southern Cascadia, the marine record contains about twice the number of events as the marsh subsidence records. The larger events match well. The smaller events do not appear in the marsh record (possible exception at Humboldt Bay).

Bradley Lake however, is intermediate between the two, suggesting as proposed by Kelsey et al. 2005, and Goldfinger et al. 2012 that smaller ruptures are represented there. If “Segment D” ruptures are limited in northern extent to Cape Blanco, much of the difference between Bradley Lake and the offshore record vanishes. This possibility is not constrained by offshore paleoseismic data due to lack of site density. It is constrained by tsunami modeling.

Despite these differences in sensitivity, there are options 94BR-E 94BR-F Several of he Bradley Lake cores have now been CT scanned, adding a new data stream to the excellent existing core descriptions. Bradley Lake cores contain very well described tsunami sand sheets. They also appear to include pre-tsunami turbidites.

Despite these differences in sensitivity, there are options 94BR-E 94BR-F Bradley Lake cores contain very well described tsunami sand sheets. They also appear to include pre-tsunami turbidites. Possible turbidite below DE7 Bradley Lake DE7 tsunami deposit Pre DE7 turbidite?

Patton and Goldfinger (2016 SSA) have examined two cores and suggest a possible presence of lake turbidites for many of the “missing” events. This should be expected with the higher sensitivity expected for lake turbidites. Marine event Dist. event

Even if these southern events terminate south of the Lake, the lake may still be sensitive enough to record the events as turbidites, where they would likely not overtop the ~ 5.5 berm/barrier to the lake. Marine event Dist. event

Proposed correlation between four southern Cascadia lakes and the marine turbidite record (Morey et al., 2013). These records also resolve much of difference between land and marine paleoseismic sequences in southern Cascadia.

Conclusions Each deposit type has it’s own sensitivities for generation, transport and preservation of a geologic record. Each paleoseismic site type and method also has it’s own sensitivities. We should expect these to be different. Reconciliation of these records cannot depend on radiocarbon alone. Onshore lakes are the missing link between land and marine records in Cascadia

Thanks for your attention!

Questions?