1
1 College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, 97331 [email protected], 2 Portland Water Bureau, 1120 Southwest 5th Avenue #600, Portland, OR 97204, 3 Department of Geophysics, Federal University of Rio Grande do Norte, Natal-Brazil, 4 Seahorse Geomatics, 2533 NE Clackamas St, Portland, OR. 97232 , 5 Environmental Science Resources, LLC, PO Box 726, Corvallis, OR., 97339 Marine and Lacustrine Turbidite Records: Testing Linkages and Estimating Ground Motions, Central Cascadia Margin, USA 1 Rachel Hausmann, 1 Bran Black, 2 Tim Collins, 1 Chris Romsos, 3 Leonardo Medeiros, 4 Mike Mutschler, 1 Steve Galer, Ann Morey 5 , Richard Raymond 5 Dr. Chris Goldfinger, 1 5 Preliminary Correlations PGA-PGV Sensitivity of Lake Sites 4 Seahorse Geomatics Abstract Setting & Signficance Adams J. 1990. Paleoseismicity of the Cascadia Subduction Zone: Evidence from Turbidites off the Oregon-Washington Margin. Tectonics, 9(4): 569-583 p. Atwater B.F, Satoko MR, Kenji S, Yoshinobu T, Kazue U, Yamaguchi D.K. 2005. The Orphan Tsunami of 1700: Japanese Clues to a Parent Earthquake in North America. Seattle (WA): University of Washington Press 133 p. Atwater B.F, et al. 1995. Summary of Coastal Geologic Evidence for Past Great Earthquakes at the Cascadia Subduction Zone. Earthquake Spectr, II(1): 1-18 p. DeVecchio D.E, Keller E.A. 2015. Natural Hazards: Earth’s Processes as Hazards, Disasters, and Catastrophes. Fourth edition. Upper Saddle River (NJ): 554 p. Goldfinger C. 2010. Submarine paleoseismology based on turbidite records. Annual Review of Marine Science [internet]. [cited 2011]: 3:35-66. Available from: marine.annualreviews.org Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gracia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2012, Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper 1661-F, Reston, VA, U.S. Geological Survey, p. 184 p, 64 Figures. http://pubs.usgs.gov/pp/pp1661f/ Melo, C., and Sharma, S., 2004, Seismic coefficients for pseudostatic slope analysis, 13th World Conference on Earthquake Engineering: August 1-6, 2004, Vancouver, B.C., Canada. Petersen, M.D., Moschetti, M.P., Powers, P.M., Mueller, C.S., Haller, K.M., Frankel, A.D., Zeng, Yuehua, Rezaeian, Sanaz, Harmsen, S.C., Boyd, O.S., Field, Ned, Chen, Rui, Rukstales, K.S., Luco, Nico, Wheeler, R.L., Williams, R.A., and Olsen, A.H., 2014, Documentation for the 2014 update of the United States national seismic hazard maps: U.S. Geological Survey Open-File Report 2014–1091, 243 p., http://dx.doi.org/10.3133/ofr20141091. Raymond R.B. 1983. The Paleolimnology of Bull Run Lake: Disruption and stability in a natural system [dissertation]. Portland State University. 1-127pp. Rong, Y., Jackson, D.D., Magistrale, H., and Goldfinger, C., 2014, Magnitude Limits of Subduction Zone Earthquakes, Bulletin of the Seismological Society of America, v. 104, no. 5, p. xx. doi: 10.1785/0120130287 Rowan, C. 2011. The slowly building threat of Cascadia- and the slow realization it was there (book review). All-geo.org. Image created using GeoMapApp. Morey A.E, Goldfinger C, Briles C.E, Gavin D.G, Colombaroli D, and Kusler J.E. 2013. Are Great Cascadia earthquakes recorded in the sedimentary records from small forearc lakes? Natural Hazards Earth System Sciences, 13, 2441-2463 p. Schnellmann M, Anselmetto F, Giardini D, McKenzie J, and Ward S. 2002. Prehistoric earthquake history revealed by lacustrine slump deposits. Geology, 30(12): 1131-1134 p. Snyder D.T, Brownell D.L. 1996. Hydrogeologic Setting and preliminary estimates of hydrologic components for Bull Run Lake and the Bull Run Lake drainage basin, Multnomah and Clackamas Counties, Oregon. U.S. Geological Survery: Water-resources investigations report 96-4064, 55p. Strasser M, Anselmetti F.S, Fah D, Giardini D, Schnellmann M. 2006. Magnitudes and source areas of large prehistoric northern Alpine earthquakes revealed by slope failures in lakes. Geology, 34 (12): 1005-1008 p. Yeats, R.S. 1998. Living with Earthquakes in the Pacific Northwest: A Survivor’s Guide. Second edition. Corvallis (OR): Oregon State University Press 390 p. 2 Questions We Address... How can we apply paleoseismic and geological engineering techniques to establish shaking levels for inland population centers? What can we learn about the Cascadia “Locked Zone” from inland paleoseismic records? We are investigating a potential paleoseismic record at Bull Run Lake, 165 km inland and 280 km landward of the tip of the plate boundary thrust, at the lati- tude of Portland, Oregon, central Cascadia margin. Bull Run is a landslide dammed lake in a cirque basin on the western flanks of Mt. Hood. Bull Run is poten- tially a good paleoseismic site, with no major stream inputs and a small catchment basin. The watershed and lake are faulted, and may contain ashes and evi- dence of crustal faulting. The lake was investigated by Raymond (1983), who cored the lake and found an orderly stratigraphic sequence with a number of minerogenic disturbance events (turbidites) and the Mazama Ash. The bulk ages of several of the disturbance events dated in Raymond’s cores overlap well- known Cascadia earthquakes, including the AD 1700 event and several prior earthquakes, suggesting potential for this site. We collected full coverage high- resolution multibeam and backscatter data, along with a high resolution grid of CHIRP sub-bottom profiles, and seven new sediment gravity cores. We find that the turbidite record in the lake is well imaged in the high-resolution chirp data, and is found throughout the lake, including at least one basin isolated from the main basin. The continuity of the turbidite record shows little or no relationship to the minor stream inlets, suggesting the disturbance beds are not likely to be storm related. Many faint laminae may contain a storm record. Subtle channels from north and south sides of the lake feed an axial channel that terminates at the eastern shore. Lake sidewall failures are evident on the north and south walls, and occur with and without imageable tabular blocky slide debris where sedimented slopes exceed ~ 22-25 deg. Smaller failures visible in backscatter data are found on slopes as low as 12 degrees. We conducted diver investigations of several of the landslide areas, collecting hand push core samples and in-situ vane shear torquemeter measurements. Initial slope stability models suggest that slopes less than ~ 25 degrees are statically stable. We are investigating the levels of ground motion required to destabilize surface sedi- ments around the lake, and radiocarbon dating the disturbance events for comparison to other paleoseismic records, including new offshore cores at a similar latitude. 1 3 Leland Lake Lake Sawyer Bull Run Lake Seattle Portland W FS (deterministic) = 0.980 FS (mean) = 1.009 PF = 56.000% RI (normal) = 0.049 RI (lognormal) = -0.039 Material Name Color Unit Weight (kN/m3) Sat. Unit Weight (kN/m3) Strength Type Cohesion (kPa) Phi (deg) Water Surface Hu Type B-Bar Mat. Weight Causes Excess Pore Pressure SOIL 1 16 20 Mohr-Coulomb 3.7 20 Water Surface Automa cally Calculated 1 No 0.15 ± 0.0933 ± Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+ 80 60 40 20 0 -20 -20 0 20 40 60 80 100 120 14 12degree LAKE slope.slim 12/8/2015 At Bull Run Lake, we measured the slope angle of a number of visible landslide scars. These ranged from 12deg. to 30deg. We u se the lowest slope with map- pable failures to assess the destabilization of the most stable slope. Using in-situ shear vane measurements acquired from the upper meter of nearby sedi - ments, we find an average cohesion value of 3.5-5 kPa at three sit es. Using an unsaturated mass 16 kN/m3, Saturated wt 20 kN/ m3 and Phi = 15 degrees, we estimate seismic stability coefficient for the 12 degree case to be 0.15-0.19g using pseudo-static stability methods (GLE Morgernstern-Price, Bishop simplified, Janbu corrected, Spencer; implemented in Slide 6.0). Various conversions from this coefficient to PGA are found in the literatu re. Some are very conservative, and intended for dam safety applications such as the 0.15 conversion factor to PGA used by the USACE. Melo and Sharma (2004) suggest a value of 40-45% of PGA for this coefficient, and several other modern studies use this value, and thus we adopt this value for this investigation. Applying this coefficient yields a range of 0.38- 0.33 PGA if the ground motions are allowed to add to fluid pressure, and 0.45-0.42 if not. We consider it likely that fluid pressure and stability are decreased under long duration seismic loading, and thus adopt the lower value of ~ 0.3-0.38g PGA. This value, given that it i s applies only to the lowest ob- served failure slope, represents the extreme value from mappable slides in Bull Run Lake. We cannot relate this slide to the H olocene deposits in the lake explic - itly, nor can we assume that it represents a Holocene event known in the paleoseismic record. If we assume however that this r esult applies to the largest of the recorded Holocene events, known as T11, its magnitude is estimated to be ~ 9.1 (Goldfinger et al., 2012; Rong et al., 2014). We can compare this event to the USGS 2014 national Seismic Hazard maps results. In Peterson et al. (2014), the extreme event is shown by the 2% probability of exceedance in 2500 years map shown to the left. The 0.3g PGA contour passes through the Mt Hood area, thus the USGS extreme event is similar to our 0.3g PG A slope stability result for maximum events in Bull Run Lake. 0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 -5 0 5 10 15 20 25 30 PGA vs. slope angle at failure Slope Angle PGA 3 3 30 Statically Unstable Using actual slope values for targeted lakes that range in sensitivity from high to low, we anticipate that it may be possible to bracket ground shaking near Seattle and Portland by calculating the mini- mum g values for a range of slope angles and estab- lishing minimums for the range. The most stable slopes that failed will set the minimum-maximum. 2% Probability of Exceedance in 2500 years (PGA) 12 degree slope Leland Lake Bull Run Lake Lake Sawyer Portland Transect Seattle Transect Cutoff angle for Bull Run Lake ~ 12 degrees for mappable slides along the lake margin. Leland Lake Lake Sawyer Bull Run Lake The Seattle Transect through Leland and Sawyer Lakes shows that both lakes contain near indentical stratigraphy of silty event beds. We interpret four facies in the lake, 1) silty “event beds, mostly with elevated lithic con- tent, sharp bases, common load structures, with fining upward single or multipulse structure. These are transported turbidites, most likely generated internally to the lake. 2) thin laminae that are just above background that number well above 100 in the Holocene, that we interpret as large storm events, or possible watershed events of some other type. 3) diffuse structureless units just above background that suggest post-seismic hillslope response and 4) background diatom rich productivity related sediment. A primary test of plate boundary earthquake origin of the sequence is correlation across Puget Sound, and whether the sequence weakens eastward. The sequence is both correlable, and weakens eastward in mean grain size and average and peak density per event, though not in bed thickness (not visible in these plots). Determing the Age of Portland’s Water Supply using Paleoseismology TWT (s) 0.118 0.115 0.117 0.116 0.114 0.113 0.112 0.111 0.110 Distance (m) 0 43 20 30 40 10 TWT (s) 0.005 0.001 0.004 0.003 0.000 -0.000 -0.001 86.800 87.575 88.350 89.125 89.900 90.675 91.450 Approximate Depth (m) Approximate Depth (m) 85.250 86.025 86.800 87.575 88.350 89.125 89.900 90.675 91.450 Distance (m) 0 43 20 30 40 10 TWT (s) 0.120 0.100 0.110 0.090 0.060 0.080 0.070 0.030 0.050 0.040 0.020 0.010 0.000 Approximate Depth (m) 93.00 77.50 85.25 69.75 46.50 62.00 54.25 23.25 38.75 31.00 15.50 7.75 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Distance (m) 0 1 2.2 2 Distance (Km) Flattened Unflattened 10 cm 0 cm 20 cm 30 cm 40 cm 50 cm 60 cm 70 cm 80 cm BRL-09GC Mazama Ash airfall 7650 B.P. BRL-09 its n Depos tics - asalt ium rmatio ion Alluvium, Terrace Cinder & Pyroclas Columbia River B Glacial Till, Colluv Landslide Rhododendron Fo Troutdale Format Bull Run Lake 0 1 2 0.5 Kilometers Bull Run Lake, OR Dating Methodology: 1. We located the deepest horizon visible across the seismic profiles, Horizon H is most likely the surface of the valley floor covered by the landslide that formed the lake. 2. A 1650m/s seismic velocity was used to estimate the thick- ness of the sediment cover from lake floor to landslide Horizon H (see figure to right). 3. The sedimentation rate was determined by using the Mazama ash datum in the bottom of the core as an age control/constraint. The depth of the Mazama ash (80cm) in BRL-09 was divided by the age of Mazama’s eruptions (~7,650 B.p.) to equal a sedimentation rate of 10.46cm/1000 yrs (0.01046cm/yr). Horizon H Horizon H 4. We measured 375cm (3.75m) of sediment cover above Horizon H (Figure A, right). 5. The calculated age of Horizon H and the valley floor is ~28,000 years old. Bull Run Lake, OR Estimated age of Bull Run Lake ~28,000 years old. A Site Strategy What are desirable characteristics for conducting paleoseismology? Beaver Lake, WA Lake Sawyer, WA Moderate sedimentation rate Few incoming stream channels Small watershed Low relief, internal sediment traps Close proximity to active crustal faults Close proximity to steep local slopes Organic-rich sediments and gas production Lake Sawyer, WA Inland Lake Sites Offshore Sites Significance of Hydrate Ridge and Oceanus Basin sites L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L La a a a a a a a a a a a a a a a a a a a a a a a a a a ak k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k k ke e e e e e e e e e e e e e e e e e e S S S S S S S S S S S S S S S S S S S S Sa a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a aw w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w wy y y y y y y y y y y y y y y y y y y y y e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e er r r r r r r r r r r r r r r r r r r r r r r r r r, , , , , , , , , , , W W W W W W W W W W W W W W W W W W W W W W WA A A A A A A A A A A A A A A A A A A A A A Oceanus Basin and Hydrate Ridge are both important sites to investigate for paleo- seismic signals. Hydrate Ridge is an isolated basin that is only vuberable to local failures, compared to other sites that could contain turbidites sources from coastl rivers. The new cores at Oceanus Basin and an unnamed basin to the south appear to be similarly isolated. 48°0'0"N 46°0'0"N 44°0'0"N 42°0'0"N # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * # * E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Washington Oregon California Tofino Uclulet Coquille Coos Bay Nestucca Eel River Alsea Bay Long Beach Lake Sawyer Kanim Lake Sixes River Netarts Bay Ecola Creek Catala Lake Humboldt Bay Bradley Lake Siletz River Salmon River Stanley Lake Willapa Spit Kakawis Lake Port Alberni Rockaway Beach Columbia River Grayland Plain Swantown Marsh Leland Lake 122°0'0"W 124°0'0"W 126°0'0"W 48°0'0"N 46°0'0"N 44°0'0"N 42°0'0"N Upper Squaw Lake Sanger Lake ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( Legend E Coastal Paleoseismic Sites # * RR0207 Cores # * M9907 Cores ! Other Lakes Lakes sites discussed in similiar papers RR0207-56PC Ü 0 50 100 150 200 25 Kilometers Klamath Rogue Smith Hydrate Ridge Trinidad ! Bolan Lake ! ^ ` Triangle Lake ^ ` Bull Run Lake Little Lake ^ ` ^ ` ^ ` ^ ` ^ ` ^ ` Lakes sites discussed in this paper ^ ` ^ ` ^ ` 122°0'0"W 124°0'0"W 126°0'0"W 128°0'0"W ! Portland Seattle ! M9907-11TC OC0315-24GC Beaver Lake, WA Bull Run Lake, OR Figure 6 (above left) Slope map of Lake Sawyer. Black and white starts mark locations of shear vane tests. (Above right) Bathymetry map of Lake Sawyer. Figure 8 (above left) Slope map of Leland Lake. Black and white stars mark locations of shear vane tests. (Above right) Bathymetry map of Leland Lake. Figure 9 (above left) Slope map of Lake Sawyer. Black and white starts mark locations of shear vane tests. Lake Sawyer, WA Leland Lake, WA Mazama tephra layer from Lake Sawyer, WA. Lake sites that have the Mazama ash datum provide strong temporal constraints for the turbidite section. 122°52'30"W 122°52'30"W 122°53'0"W 122°53'0"W 47°54'0"N 47°54'0"N 47°53'30"N 47°53'30"N 0 0.5 1 0.25 Kilometers Updated Rupture Modes LLJ-07 Mazama Ash T14 T13 LLJ-7H 48.5 T12 T11b T11a LLJ-7G 48.5 T11 T10f T10c T10b T10 T9 LLJ-7E 58.5 T8 LLJ-1C 72 T7 LLJ-7D 20.5 T6 T5b LLJ-1b 71 T5 T4 Seattle LLJ-1B 27.5 T3 T2 T1 LLJ-7A 8.5 T0 2000 4000 6000 8000 Modelled date (BP) OxCal v4.2.4 Bronk Ramsey (2013); r:5 IntCal13 atmospheric curve (Reimer et al 2013) A B C T1 T2 T3 T4 T5 T5b? T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T17a T18 T5b? T10b? T10f? T3a T4a T5a T5c T8a or b T10a T10c T10d D T2a T8a or b T15a NB HB CB HB CB NB HB CB HB CB 434 years (110-1150) 350 years (110-1150) 220-240 years (40-600) 300-380 years (40-1150) ? ? ? ? <200 years T6a T6b T7a T9b T12a T14a T16a Additional unresolved events Barkley Astoria HR Rogue Barkley JDF Astoria HR Rogue Barkley JDF Astoria HR Rogue Barkley JDF Astoria HR Rogue Smith Klamath Trinidad Eel Smith Klamath Trinidad Eel Smith Klamath Trinidad Eel Smith Klamath Trinidad Eel OB OB OB OB F T15an T5b(alt) NB HB CB ? Barkley JDF Astoria HR Rogue Smith Klamath Trinidad Eel OB E T2b T2c T9c T10e NB HB CB ? Barkley Astoria HR Rogue Smith Klamath Trinidad Eel OB C’ T9a? T10d NB HB CB ? Barkley Astoria HR Rogue Smith Klamath Trinidad Eel OB Figure 13. (right) Model of Cascadia Holocene rupture lengths of Cascadia earthquakes from marine and onshore paleoseismology (Goldfingeret al., 2012 and references therein). The original four rupture modes inferred are now revised with new turbidite stratigraphic/14C corre- lation, supported by onshore radiocar- bon data at subsidence and tsunami sites. The northern limit of segments B and C were data limited, and are now revised northward with a new study just completed (Goldfinger et al. 2015 submitted). At least one, and perhaps three northern segments are now recognized, as well as an addtional southern segment (Goldfinger et al., 2013). The Washington lake data are compatible with this revision, showing evidence of the additional events known as T5b, T11a, T11b, T15an, and the northern extension of T10b and T10f. Leland Lake OxCal Age Model In Leland Lake, we have created an “event free” OxCal P-sequence depostional model to model the ages of event beds in the lake. Event bed stratigraphy, and the larger facies 2 and 3 units were removed from the stratigraphic sequence leaving background sedimentation, and remaining thin lamellae (storms?). This first order model will be refined with further attempts to discriminate these four facies. The model uses the radiocarbon ages to construct the ages of the remaining event beds, shown at right. The number of event beds in the lake closely matches the number of correlated earthquake generated beds offshore, and the radiocarbon ages and model ages are similarly a good fit for the earthquake sequence. Potential correlative beds couldalso include the Seattle Fault earthquake of ~ 1100 BP, and the 2001 Nisqually earthquake. Tentative correlation testing, Bull Run Lake and offshore cores from Oceanus Bsin and Hydrate Ridge. Although presently lacking radiocarbon ages, the well-log correlation between Oceanus Basin and Hydrate Ridge is very good, suggesting this core record is ~ 5500 years in length, and bottomed out above regional event T11. The record shows good correlation bed for bed with Hydrate Ridge, ~ 103 km to the south. Each of teh Bull Run cores bottomed in the Mazama Ash as described in Raymond (1983). The overlying stratigraphy includes an ash (Mt Hood) of ~ 900-1200 years BP in age. The remaining event beds show a potential correlation to the offshore Oceanus and Hy- drate Ridge cores, pending further analysis. Primary Result (Preliminary) We suggest that only two possible sources are likely to be common across the region, a climate signal, and earthquakes. To test the commonality of the signal, we compare the stratigraphy and geophysical signatures between lakes, and to the seismoturbidite record in Juan de Fuca Channel (JDF), Hydrate Ridge, and Oceanus. In the case of Leland and Sawyer lakes, fire is excluded for beds that are correla- tive between the two as such a fire would have to span Puget Sound and a distance of 90 km. We observe a surprising correspondence between the well-log signatures of event beds for the three lakes shown, and to the offshore records (see below). What constitutes an “event” is somewhat uncer- tain. For the Washington lakes, the event beds are clearly turbidtes, with load structures and fining upward sequences. Bull Run Lake is less clear, with no load features and more diffuse structures. The more mature Washington Lake study has good 14C age control, and an “event Free” OxCal age model that is an excellent match to the offshore and coastal great earthquake records, constrained by teh Mazama ash. The Washington lakes may also record the Nisqually and ~ 1100 Seattle Fault earth- quakes. We suggest that several poor 14C fits are likely reworked material (Leland Lake), based on comparison to the ages at Sawyer for equivalent stratigraphy. Bull Run Lake does not yet have 14C AMS age control, but has several older bulk dates. The larger event beds in Leland and Sawyer lakes appear to have an overlying diffuse ”tail” that may be a post seismic hillslope effect. Numerous thin laminae appear in both lakes just above backgound, which we interpret as most likley storm events. Plate boundary earthquakes remain the best candidate for the Washington lake event beds, matching the offshore frequency, timing, and with good well-log correlation. We tentatively conclude that the disturbance events in these lakes are a permissive fit to the earthquake record, though climate events are likely included as mostly thin laminae We show tentative correlation ties of several events to illus- trate the possible linkage at Bull Run Lake, where radiocarbon ages are pending. Transport Mechanisms and preliminary Interpretation delta landslide underwater failures channel to shelf canyon system B. Marine A. Lacustrine canyon wall failures liquefaction and transport downslope In lacustrine systems, a post-seismic hill- slope input is expected, unlike tmarine sys- tems isolated from coastal river systems as in Cascadia. Th T The Se S at ttl tl t e Tr T ansect t t thr h ough h L Lel land d and d Sa S wyer L Lak kes sho h ws t tha h t t bo b th th l lak kes conta t in i near in i de d nt tic i al l str t at tig i raph hy of f si ilt lty event t be b ds d . We interpret four facies in the lake, 1) silty “event beds, mostly with elevated lithic con - tent, sharp bases, common load structures, with fining upward single or multipulse structure. These are transported turbidites, most likely generated internally to the lake. 2) thin laminae that are just above background that number well above 100 in the Holocene, that we interpret as large storm events, or possible watershed events of some other type. 3) diffuse structureless units just above background that suggest post-seismic hillslope response and 4) background diatom rich productivity related sediment. A primary test of plate boundary earthquake origin of the sequence is correlation across Puget Sound, and whether the sequence weakens eastward. The sequence is both correlable, and weakens eastward in mean grain size and average and peak density per event, though not in bed thickness (not visible in these plots). LLJ-07 Mazama Ash T14 T13 LLJ-7H 48.5 T12 T11b T11a LLJ-7G 48.5 T11 T10f T10c T10b T10 T9 LLJ-7E 58.5 T8 LLJ-1C 72 T7 LLJ-7D 20.5 T6 T5b LLJ-1b 71 T5 T4 Seattle LLJ-1B 27.5 T3 T2 T1 LLJ-7A 8.5 T0 2000 4000 6000 8000 Modelled date (BP) OxCal v4.2.4 Bronk Ramsey (2013); r:5 IntCal13 atmospheric curve (Reimer et al 2013) A B C T1 T2 T3 T4 T5 T5b? T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T17a T18 T5b? T10b? T10f? T3a T4a T5a T5c T8a or b T10a T10c T10d D T2a T8a or b T15a NB HB CB HB CB C NB HB CB HB CB 434 years (110-1150) 350 years (110-1150) 220-240 years (40-600) 300-380 years (40-1150) ? ? ? ? ? <200 years < T6a T6b T7a T9b T12a T14a T16a Additional unresolved events Barkley Astoria HR Rogue Barkley JDF Astoria HR Rogue Barkley JDF Astoria HR Rogue Barkley JDF Astoria HR Rogue Smith Klamath Trinidad Eel Smith Klamath Trinidad Eel Smith Klamath Trinidad Eel Smith Klamath Trinidad Eel OB OB OB OB F T15an T5b(alt) NB HB CB ? Barkley JDF Astoria HR Rogue Smith Klamath Trinidad Eel OB E T2b T2c T9c T10e NB HB CB ? Barkley Astoria HR Rogue Smith Klamath Trinidad Eel OB ? C’ T9a? T10d NB HB CB ? Barkley Astoria HR Rogue Smith Klamath Trinidad Eel OB Figure 13. (right) Model of Cascadia Holocene rupture lengths of Cascadia earthquakes from marine and onshore paleoseismology (Goldfingeret al., 2012 and references therein). The original four rupture modes inferred are now revised with new turbidite stra ti graphic/14C corre - la ti on, supported by onshore radiocar - bon data at subsidence and tsunami sites. The northern limit of segments B and C were data limited, and are now revised northward with a new study just completed (Goldfinger et al. 2015 submitted). At least one, and perhaps three northern segments are now recognized, as well as an addtional southern segment (Goldfinger et al., 2013). The Washington lake data are compatible with this revision, showing evidence of the additional events known as T5b, T11a, T11b, T15an, and the northern extension of T10b and T10f. Leland Lake OxCal Age Model In Leland Lake, we have created an “event free” OxCal P-sequence depostional model to model the ages of event beds in the lake. Event bed stratigraphy, and the larger facies 2 and 3 units were removed from the stratigraphic sequence leaving background sedimentation, and remaining thin lamellae (storms?). This first order model will be refined with further attempts to discriminate these four facies. The model uses the radiocarbon ages to construct the ages of the remaining event beds, shown at right. The number of event beds in the lake closely matches the number of correlated earthquake generated beds offshore, and the radiocarbon ages and model ages are similarly a good fit for the earthquake sequence. Potential correlative beds couldalso include the Seattle Fault earthquake of ~ 1100 BP, and the 2001 Nisqually earthquake. Tentative correlation testing, Bull Run Lake and offshore cores from Oceanus Bsin and Hydrate Ridge. Although presently lacking radiocarbon ages, the well-log correlation between Oceanus Basin and Hydrate Ridge is very good, suggesting this core record is ~ 5500 years in length, and bottomed out above regional event T11. The record shows good correlation bed for bed with Hydrate Ridge, ~ 103 km to the south. Each of teh Bull Run cores bottomed in the Mazama Ash as described in Raymond (1983). The overlying stratigraphy includes an ash (Mt Hood) of ~ 900-1200 years BP in age. The remaining event beds show a potential correlation to the offshore Oceanus and Hy - drate Ridge cores, pending further analysis. 120°0'0"W 120°0'0"W 125°0'0"W 125°0'0"W 50°0'0"N 50°0'0"N 49°0'0"N 49°0'0"N 48°0'0"N 48°0'0"N 47°0'0"N 47°0'0"N 46°0'0"N 46°0'0"N 45°0'0"N 45°0'0"N 44°0'0"N 44°0'0"N 43°0'0"N 43°0'0"N 42°0'0"N 42°0'0"N TT053 TT048 Lake Sawyer Hydrate Ridge Leland Lake Bull Run Lake Oceanus Basin 3560 (3370-3727) 1270 (930-1570)
