Robert C. Witter

Tsunami history of an Oregon coastal lake reveals a 4600 yr record of great earthquakes on the Cascadia subduction zone

Bradley Lake, on the southern Oregon coastal plain, records local tsunamis and seismic shaking on the Cascadia subduction zone over the last 7000 yr. Thirteen marine incursions delivered landward-thinning sheets of sand to the lake from nearshore, beach, and dune environments to the west. Following each incursion, a slug of marine water near the bottom of the freshwater lake instigated a few-year-to-several-decade period of a brackish (≤4‰ salinity) lake. Four additional disturbances without marine incursions destabilized sideslopes and bottom sediment, producing a suspension deposit that blanketed the lake bottom. Considering the magnitude and duration of the disturbances necessary to produce Bradley Lake’s marine incursions, a local tsunami generated by a great earthquake on the Cascadia subduction zone is the only accountable mechanism. Extreme ocean levels must have been at least 5–8 m above sea level, and the cumulative duration of each marine incursion must have been at least 10 min. Disturbances without marine incursions require seismic shaking as well. Over the 4600 yr period when Bradley Lake was an optimum tsunami recorder, tsunamis from Cascadia plate-boundary earthquakes came in clusters. Between 4600 and 2800 cal yr B.P., tsunamis occurred at the average frequency of ~3–4 every 1000 yr. Then, starting ~2800 cal yr B.P., there was a 930–1260 yr interval with no tsunamis. That gap was followed by a ~1000 yr period with 4 tsunamis. In the last millennium, a 670–750 yr gap preceded the A.D. 1700 earthquake and tsunami. The A.D. 1700 earthquake may be the fi rst of a new cluster of plate-boundary earthquakes and accompanying tsunamis. Local tsunamis entered Bradley Lake an average of every 390 yr, whereas the portion of the Cascadia plate boundary that underlies Bradley Lake ruptured in a great earthquake less frequently, about once every 500 yr. Therefore, the entire length of the subduction zone does not rupture in every earthquake, and Bradley Lake has recorded earthquakes caused by rupture along the entire length of the Cascadia plate boundary as well as earthquakes caused by rupture of shorter segments of the boundary. The tsunami record from Bradley Lake indicates that at times, most recently ~1700 yr B.P., overlapping or adjoining segments of the Cascadia plate boundary ruptured within decades of each other.

Great Cascadia earthquakes and tsunamis of the past 6700 years, Coquille River estuary, southern coastal Oregon

Cascadia subduction zone earthquakes dropped tidal marshes and low-lying forests to tidal flat elevations 12 times in the last 6700 cal yr B.P. at the Coquille River estuary in southwestern Oregon. The youngest buried soil, preserved in tidal marsh deposits near the estuary mouth, records the A.D. 1700 earthquake that ruptured the entire Cascadia margin. Eleven other buried marsh and upland soils found in tributary valleys of the estuary provide repeated evidence for rapid, lasting relative sea-level rise interpreted as coseismic subsidence. Additional stratigraphic criteria supporting a coseismic origin for soil burial include: lateral soil correlation over hundreds of meters, fossil diatom assemblages that indicate a maximum of 1.2‐3.0 m of submergence, and sand deposits overlying buried soils consistent with earthquakeinduced tsunamis that traveled 10 km up the estuary. Twelve earthquakes occurred in the last 6500‐6720 cal yr B.P., recurring on average every 570‐590 yr. Intervals between earthquakes varied from a few hundred years to over 1000. Comparisons of the Coquille record to earthquake histories from adjacent sites in Oregon, southwestern Washington, and northwestern California suggest that at least two earthquakes in the last 4000 yr did not rupture the entire length of the subduction zone. An earthquake 760‐1140 cal yr B.P. in southwestern Washington may have ruptured as far south as Coos Bay but probably stopped before it reached the Coquille estuary because no buried soil records the event, and tidal marsh conditions were set to record an earthquake. An earthquake limited to a southern segment of the Cascadia margin 1940‐2130 cal yr B.P. probably did not rupture north of the Coquille estuary. An analysis of relative sea-level histories from either side of the Coquille fault failed to find conclusive evidence for late Holocene vertical deformation. However, we cannot preclude recent upper-plate faulting. If the fault is active, as geomorphic features suggest, then constraints on the highest possible elevation of mean tide level allow a maximum vertical slip rate of 0.2‐ 0.4 mm/yr in the past 6200‐6310 cal yr B.P.

Plate-boundary earthquakes and tsunamis of the past 5500 yr, Sixes River estuary, southern Oregon

Eleven plate-boundary earthquakes over the past 5500 yr have left a stratigraphic signature in coastal wetland sediments at the lower Sixes River valley in south coastal Oregon. Within a 1.8 km2 abandoned meander valley, 10 buried wetland soils record gradual and abrupt relative sea-level changes back in time to ;6000 yr ago. An additional, youngest buried soil at the mouth of the Sixes River subsided during the A.D. 1700 Cascadia earthquake. Multiple lines of evidence indicate that tectonic subsidence caused soil burial, including permanent relative sea-level rise following burial, lateral continuity of buried soil horizons over hundreds of meters, diatom assemblages showing that sea level rose abruptly at least 0.5 m, and sand deposits on top of buried soils demonstrating coincidence of coseismic subsidence and tsunami inundation. For at least two of the buried soils, liquefaction of sediment accompanied subsidence. The 11 soil-burial events took place between 300 and ;5400 yr ago, yielding an average recurrence interval of plateboundary earthquakes of ;510 yr. Comparing paleoseismic sites in southern Washington and south coastal Oregon with the Sixes River site for the past 3500 yr indicates that the number and timing of recorded plate-boundary earthquakes are not the same at all sites. In particular, a Sixes earthquake at ;2000 yr ago lacks a likely correlative in southern Washington. Therefore, unlike the A.D. 1700 Cascadia earthquake, some Cascadia plate-boundary earthquakes do not rupture the entire subduction zone from southern Oregon to southern Washington. In the lower Sixes River valley, the upperplate Cape Blanco anticline deforms sediment of late Pleistocene and Holocene age directly above the subduction zone. Differential tectonic subsidence occurred during two of the plate-boundary earthquakes when a blind, upper-plate reverse fault, for which the Cape Blanco anticline is the surface fold, slipped coseismically with rupture of the plate boundary. During these two earthquakes, sites ;2 km from the anticline axis subsided ;0.5 m more than sites ;1 km from the axis.

Response of a small Oregon estuary to coseismic subsidence and postseismic uplift in the past 300 years

The Sixes River estuary, south coastal Oregon, sits above the locked portion of the Cascadia subduction zone, which intermittently releases in subduction-zone earthquakes. One such Cascadia earthquake ∼300 years ago caused subsidence and a tsunami at the Sixes estuary. The subsidence raised the river9s base level, resulting in an ∼3 km upstream shift of the head of tide of the estuary. At the upper end of the expanded estuary, more than 4 m of overbank sediment was deposited in the first decades or century after subsidence. Subsequent incision through the overbank deposits accompanied the gradual emergence of the estuary, and attendant downstream shift of the head of tide, as relative sea level fell in response to interseismic uplift.

Evidence for Late Holocene Earthquakes on the Utsalady Point Fault, Northern Puget Lowland, Washington

Trenches across the Utsalady Point fault in the northern Puget Lowland of Washington reveal evidence of at least one and probably two late Holocene earthquakes. The “Teeka” and “Duffers” trenches were located along a 1.4-km-long, 1- to 4-m-high, northwest-trending, southwest-facing, topographic scarp recognized from Airborne Laser Swath Mapping. Glaciomarine drift exposed in the trenches reveals evidence of about 95 to 150 cm of vertical and 200 to 220 cm of left-lateral slip in the Teeka trench. Radiocarbon ages from a buried soil A horizon and overlying slope colluvium along with the historical record of earthquakes suggest that this faulting occurred 100 to 400 calendar years b.p. (a.d. 1550 to 1850). In the Duffers trench, 370 to 450 cm of vertical separation is accommodated by faulting (∼210 cm) and folding (∼160 to 240 cm), with probable but undetermined amounts of lateral slip. Stratigraphic relations and radiocarbon ages from buried soil, colluvium, and fissure fill in the hanging wall suggest the deformation at Duffers is most likely from two earthquakes that occurred between 100 to 500 and 1100 to 2200 calendar years b.p., but deformation during a single earthquake is also possible. For the two-earthquake hypothesis, deformation at Teeka trench in the first event involved folding but not faulting. Regional relations suggest that the earthquake(s) were M ≥ ∼6.7 and that offshore rupture may have produced tsunamis. Based on this investigation and related recent studies, the maximum recurrence interval for large ground-rupturing crustal-fault earthquakes in the Puget Lowland is about 400 to 600 years or less.

Testing the use of microfossils to reconstruct great earthquakes at Cascadia

Coastal stratigraphy from the Pacific Northwest of the United States contains evidence of sudden subsidence during ruptures of the Cascadia subduction zone. Transfer functions (empirical relationships between assemblages and elevation) can convert microfossil data into coastal subsidence estimates. Coseismic deformation models use the subsidence values to constrain earthquake magnitudes. To test the response of foraminifera, the accuracy of the transfer function method, and the presence of a pre-seismic signal, we simulated a great earthquake near Coos Bay, Oregon, by transplanting a bed of modern high salt-marsh sediment into the tidal flat, an elevation change that mimics a coseismic subsidence of 0.64 m. The transplanted bed was quickly buried by mud; after 12 mo and 5 yr, we sampled it for foraminifera. Reconstruction of the simulated coseismic subsidence using our transfer function was 0.61 m, nearly identical to the actual elevation change. Our transplant experiment, and additional analyses spanning the A.D. 1700 earthquake contact at the nearby Coquille River 15 km to the south, show that sediment mixing may explain assemblage changes previously interpreted as evidence of pre-seismic land-level change in Cascadia and elsewhere.

Unusually large tsunamis frequent a currently creeping part of the Aleutian megathrust

Current models used to assess earthquake and tsunami hazards are inadequate where creep dominates a subduction megathrust. Here we report geological evidence for large tsunamis, occurring on average every 300–340 years, near the source areas of the 1946 and 1957 Aleutian tsunamis. These areas bookend a postulated seismic gap over 200 km long where modern geodetic measurements indicate that the megathrust is currently creeping. At Sedanka Island, evidence for large tsunamis includes six sand sheets that blanket a lowland facing the Pacific Ocean, rise to 15 m above mean sea level, contain marine diatoms, cap terraces, adjoin evidence for scour, and date from the past 1700 years. The youngest sheet and modern drift logs found as far as 800 m inland and >18 m elevation likely record the 1957 tsunami. Previously unrecognized tsunami sources coexist with a presently creeping megathrust along this part of the Aleutian Subduction Zone.

Simulated tsunami inundation for a range of Cascadia megathrust earthquake scenarios at Bandon, Oregon, USA

Characterizations of tsunami hazards along the Cascadia subduction zone hinge on uncertainties in megathrust rupture models used for simulating tsunami inundation. To explore these uncertainties, we constructed 15 megathrust earthquake scenarios using rupture models that supply the initial conditions for tsunami simulations at Bandon, Oregon. Tsunami inundation varies with the amount and distribution of fault slip assigned to rupture models, including models where slip is partitioned to a splay fault in the accretionary wedge and models that vary the updip limit of slip on a buried fault. Constraints on fault slip come from onshore and offshore paleoseismological evidence. We rank each rupture model using a logic tree that evaluates a model’s consistency with geological and geophysical data. The scenarios provide inputs to a hydrodynamic model, SELFE, used to simulate tsunami generation, propagation, and inundation on unstructured grids with w 8.7–9.2. Simulated tsunami inundation agrees with sparse deposits left by the A.D. 1700 and older tsunamis. Tsunami simulations for large (22–30 m slip) and medium (14–19 m slip) splay fault scenarios encompass 80%–95% of all inundation scenarios and provide reasonable guidelines for land-use planning and coastal development. The maximum tsunami inundation simulated for the greatest splay fault scenario (36–44 m slip) can help to guide development of local tsunami evacuation zones.

Past Earthquake-Induced Rapid Subsidence along the Northern San Andreas Fault: A Paleoseismological Method for Investigating Strike-Slip Faults

Evidence of rapid relative sea level changes preserved in sediment of coastal estuaries along the north coast segment of the San Andreas fault provides information on the dates of past earthquakes. This approach, although successfully used to document large subduction zone earthquakes, has not been used previously to constrain the dates of San Andreas fault or other strike-slip fault earthquakes. Data on prehistoric San Andreas fault earthquakes is needed for development of robust probabilistic hazard assessments in northern California and the San Francisco Bay area. Evidence of earthquake-induced subsidence is preserved in marshes at the northern margin of Bolinas Lagoon and the southeastern margin of Bodega Harbor. These sites occupy structural basins or troughs along the northern San Andreas fault. Radiocarbon dating and identification of the first occurrence of nonnative pollen near abrupt sedimentological changes in cores are used to constrain the dates of San Andreas fault earthquakes over about the last 800 years. Estuarine sediment from northern Bolinas Lagoon preserves evidence of an earthquake that occurred about 750 years ago. Evidence for this earthquake includes a marsh soil that was abruptly buried by tidal flat mud or coarse, poorly sorted deltaic deposits containing a mixture of terrestrial sediment and marine mud. Evaluation of diatom assemblages from above and below the buried soil horizon provides evidence of at least several decimeters of sustained relative sea level rise. Three buried soils in a marsh at the southern end of Bodega Harbor are likely a result of coseismic subsidence. All three buried marsh soils have abrupt upper contacts, although diatom evidence for decimeters of sustained subsidence is not as strong as it is for the buried soil in Bolinas Lagoon. Existing paleoseismic data on the northern San Andreas fault is compared with results of this study in order to evaluate the dates of prehistoric large earthquakes and length of past ruptures. These data indicate that an earthquake occurred about 400 years ago and another earthquake occurred about 700 years ago. The data allow for 1906-type ruptures of the fault from the Santa Cruz Mountains to at least Point Arena. However, a sequence of closely timed, smaller earthquakes would produce a similar set of paleoseismic data.

Stratigraphic and microfossil evidence for a 4500-year history of Cascadia subduction zone earthquakes and tsunamis at Yaquina River estuary, Oregon, USA

We infer that each buried soil represents a Cascadia subduction zone earthquake because the soils are laterally extensive and abruptly overlain by sandy deposits and mud. Preservation of coseismically buried soils occurred from 4500 yr ago until ~500–600 yr ago, after which preservation was compromised by cessation of gradual relative sea-level rise, which in turn precluded drowning of marsh soils during instances of coseismic subsidence. Based on grain-size and microfossil data, sandy deposits overlying buried soils accumulated immediately after a subduction zone earthquake, during tsunami incursion into Sallys Bend. The possibility that the sandy deposits were sourced directly from landslides triggered upstream in the Yaquina River basin by seismic shaking was discounted based on sedimentologic, microfossil, and depositional site characteristics of the sandy deposits, which were inconsistent with a fluvial origin. Biostratigraphic analyses of sediment above two buried soils—in the case of two earthquakes, one occurring shortly before 1541–1708 cal. yr B.P. and the other occurring shortly before 3227–3444 cal. yr B.P.—provide estimates that coseismic subsidence was a minimum of 0.4 m. The average recurrence interval of subduction zone earthquakes is 420–580 yr, based on an ~3750–4050-yr-long record and seven to nine interearthquake intervals.