Eileen Hemphill-Haley

Summary of Coastal Geologic Evidence for Past Great Earthquakes at the Cascadia Subduction Zone

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

Abrupt uplift within the past 1700 years at Southern Puget Sound, Washington

Shorelines rose as much as 7 meters along southern Puget Sound and Hood Canal between 500 and 1700 years ago. Evidence for this uplift consists of elevated wave-cut shore platforms near Seattle and emerged, peat-covered tidal flats as much as 60 kilometers to the southwest. The uplift was too rapid for waves to leave intermediate shorelines on even the best preserved platform. The tidal flats also emerged abruptly; they changed into freshwater swamps and meadows without first becoming tidal marshes. Where uplift was greatest, it adjoined an inferred fault that crosses Puget Sound at Seattle and it probably accompanied reverse slip on that fault 1000 to 1100 years ago. The uplift and probable fault slip show that the crust of the North America plate contains potential sources of damaging earthquakes in the Puget Sound region.

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.

Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern coastal Washington

Fossil diatoms from four stratigraphic sections along the tidal Niawiakum River, southwestern Washington, provide an independent paleoecological test of a relative sea-level rise that has been attributed to subsidence during an inferred earthquake in the Cascadia subduction zone about 300 yr ago. Diatom assemblages in a buried soil and overlying mud indicate a sudden and lasting shift from marshes and forests near or above highest tides to mud flats and incipient tidal marshes, with a progressive return to high-level tidal marshes by sediment aggradation and, perhaps, gradual tectonic uplift. The amount of coseismic submergence required to generate the paleoecological changes observed at these sites could have ranged from a minimum of 0.8–1.0 m to a maximum of ∼3.0 m. Fossil diatoms also provide an independent test of previous inferences that the subsidence was shortly followed by a tsunami. The inferred tsunami deposit is a distinct sandy interval that widely overlies the buried marsh and forest soil. Diatoms from this interval consist of species observed on modern sand flats of the open bay, identifying a bayward source for the sand. Occurrences of the same sand-flat species above the buried soil in the farthest up-valley outcrop where a sandy interval is not recognizable suggest that the tsunami extended farther landward than was previously inferred from the stratigraphy. These data rule out proposed alternatives to the coseismic subsidence model—that is, climatically induced sea-level rise, temporary submergence caused by storms—and support the hypothesis that a great earthquake struck southwestern Washington 300 yr ago.

Diatoms as an aid in identifying late-Holocene tsunami deposits

Diatoms (Bacillariophyta) help identify the onshore deposits of tsunamis from earthquakes on the Cascadia subduction zone along the Pacific coast of Oregon, Washington, and British Columbia, and on faults high in the North American plate in the Puget Sound area of Washington. At the Copalis River, Washington, diatom analyses suggest that a tsunami deposit about 300 calendric years old (300 yr BP) originated from sandy shoals of the lower estuary rather than nearby beaches or coastal dunes. At Cultus Bay and West Point, Washington, well-preserved benthic estuarine diatoms in sand sheets overlying tidal-marsh peat indicate that the deposits came from intertidal or nearshore areas of Puget Sound. On an abruptly uplifted mudflat at the landward end of Hood Canal at Lynch Cove, Washington, tidal-flat diatoms refute the possibility of a terrestrial source for the sand. Diatoms in 300-yr-BP tsunami deposits on the Niawiakum River, Washington, confirm that the sand in these deposits had a marine source, and help to identify the landward extent of tsunami inundation. Diatom assemblages in deposits of the 300 yr BP and AD 1964 tsunamis at Port Alberni, British Columbia, consist of different dominant taxa, but both indicate that the sand units originated from Alberni Inlet. Diatoms add to stratigraphic evidence that tsunamis flooded Bradley Lake, a freshwater lake on the south-central Oregon coast, three times during the past 1700 years. Planktonic marine diatoms only found above 1-70-cm-thick sand layers in otherwise clayey lacustrine sediment imply tsunami inundation.

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.

Earthquake recurrence inferred from paleoseismology

Publisher Summary This chapter describes three North American examples of earthquake history inferred from Quaternary geology and discusses earthquakes in the interior of the North America plate––in the New Madrid seismic zone of Missouri, Arkansas, and Tennessee. The study of prehistoric earthquakes––paleoseismology––provides long-term rates of earthquake occurrence to improve confidence in such forecasts. These earthquakes suggest the rates and patterns of recurrence that help define earthquake hazards. The eastern California shear zone, centered about 150 km northeast of Los Angeles, exhibits geologic evidence for prehistoric surface ruptures during episodes thousands of years apart. Typical intervals between the earthquakes span hundreds of years in the New Madrid and Cascadia examples and thousands of years in the eastern California example. Apart from enabling such estimates of recurrence intervals, paleoseismology can provide evidence for the regional clustering of earthquakes in seismic zones and for aperiodic rupture along the same part of a fault. Such findings have made paleoseismology an essential part of earthquake-hazard assessment in the United States.

Persistent late Pleistocene-Holocene seasonal upwelling and varves off the coast of California

Abstract Seasonal production of siliceous phytoplankton and a seasonal flux of terrigenous clastic material produced varved sediments along the continental slope off California during the late Pleistocene to mid-Holocene. Light-colored sediment within laminations and in sediment-trap samples was deposited during summer upwelling and contains an abundance of the diatoms Skeletonema costatum and Chaetoceros spp. resting spores. Dark-colored sediment deposited in the fall and winter contains abundant Thalassiosira pacifica, and has more terrigenous material. Distribution of diatoms in varves shows that seasonal upwelling has persisted along the California coast and has remained strongly seasonal since the late Pleistocene.

Evidence for a stronger oxygen-minimum zone off central California during late Pleistocene to early Holocene

Of 31 deep-sea cores collected along the central California continental slope, 18 have distinctly laminated sediment at depth, but none have laminations in the top few centimetres. The cores with laminated facies are restricted to water depths between 508 and 1508 m, but not all cores taken from this depth interval have laminated facies. 14 C dates yield an extrapolated age of 4700 B.P. for the top of the uppermost laminated unit. Comparisons of the diatom flora in the laminated couplets with diatom floras in a 13-month sediment-trap record suggest that the laminations are varvelike couplets of seasonal sedimentation. The laminated facies represent a period from the last global deglaciation to early Holocene when the oxygen-minimum zone along the northeastern Pacific Ocean was stronger than at present. A stronger oxygen-minimum zone during this time is inferred to be the result of intensified upwelling.