Alan Nelson

Alan Nelson
United States Geological Survey | USGS · Geologic Hazards Science Center

PhD

About

128
Publications
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Publications

Publications (128)
Article
The tsunami associated with the giant 9.5 Mw 1960 Chile earthquake deposited an extensive sand layer above organic‐rich soils near Queule (39.3°S, 73.2°W), south‐central Chile. Using the 1960 tsunami deposits, together with eye‐witness observations and numerical simulations of tsunami inundation, we tested the tsunami inundation sensitivity of the...
Article
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The amplification of coastal hazards such as distant-source tsunamis under future relative sea-level rise (RSLR) is poorly constrained. In southern California, the Alaska-Aleutian subduction zone has been identified as an earthquake source region of particular concern for a worst-case scenario distant-source tsunami. Here, we explore how RSLR over...
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A new history of great earthquakes (and their tsunamis) for the central and southern Cascadia subduction zone shows more frequent (17 in the past 6700 yr) megathrust ruptures than previous coastal chronologies. The history is based on along-strike correlations of Bayesian age models derived from evaluation of 554 radiocarbon ages that date earthqua...
Article
Lithology and microfossil biostratigraphy beneath the marshes of a central Oregon estuary limit geophysical models of Cascadia megathrust rupture during successive earthquakes by ruling out >0.5 m of coseismic coastal subsidence for the past 2000 yr. Although the stratigraphy in cores and outcrops includes as many as 12 peat-mud contacts, like thos...
Article
Holocene crustal faulting in the northern Olympic Peninsula of Washington State manifests in a zone of west-northwest-striking crustal faults herein named the North Olympic fault zone, which extends for ∼80 km along strike and includes the Lake Creek–Boundary Creek fault to the east and the Sadie Creek fault and newly discovered scarps to the west....
Article
We infer a history of three great megathrust earthquakes during the past 2000 years at the Nehalem River estuary based on the lateral extent of sharp (≤3 mm) peat-mud stratigraphic contacts in cores and outcrops, coseismic subsidence as interpreted from fossil diatom assemblages and reconstructed with foraminiferal assemblages using a Bayesian tran...
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Over the past 200 years of written records, the Hawaiian Islands have experienced tens of tsunamis generated by earthquakes in the subduction zones of the Pacific ‘Ring of Fire’ (for example, Alaska–Aleutian, Kuril–Kamchatka, Chile and Japan). Mapping and dating anomalous beds of sand and silt deposited by tsunamis in low‐lying areas along Pacific...
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Great megathrust earthquakes arise from the sudden release of energy accumulated during centuries of interseismic plate convergence. The moment deficit (energy available for future earthquakes) is commonly inferred by integrating the rate of interseismic plate locking over the time since the previous great earthquake. But accurate integration requi...
Article
The Arauco Peninsula (37°-38°S) in south-central Chile has been proposed as a possible barrier to the along-strike propagation of megathrust ruptures, separating historical earthquakes to the south (1960 AD 1837, 1737, and 1575) and north (2010 AD, 1835, 1751, 1657, and 1570) of the peninsula. However, the 2010 (Mw 8.8) earthquake propagated into t...
Article
The Lake Creek–Boundary Creek fault, previously mapped in Miocene bedrock as an oblique thrust on the north flank of the Olympic Mountains, poses a significant earthquake hazard. Mapping using 2015 light detection and ranging (lidar) confirms 2004 lidar mapping of postglacial (< 13 ka) and Holocene fault scarps along the 22-km-long eastern section...
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Comparisons of pre-earthquake and post-earthquake microfossils in tidal sequences are accurate means to measure coastal subsidence during past subduction earthquakes, but the amount of subsidence is uncertain, because the response times of fossil taxa to coseismic relative sea-level (RSL) rise are unknown. We measured the response of diatoms and fo...
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Light and electron microscope observations show that a brackish diatom taxon should be classified as a new species of a new genus; Pseudofrustulia lancea gen. et sp. nov. We propose separating Pseudofrustulia from other similar genera such as Frickea, Frustulia, Amphipleura, Muelleria, and Envekadea on the basis of its thickened axial ribs, raphe e...
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The stratigraphy within coastal river valleys in south-central Chile clarifies and extends the region’s history of large, earthquakes and accompanying tsunamis. Our site at Quidico (38.1°S, 73.3°W) is located in an overlap zone between ruptures of magnitude 8–9 earthquakes in 1960 and 2010, and, therefore, records tsunamis originating from subducti...
Article
Stratigraphic, sedimentologic (including CT 3D X-ray tomography scans), foraminiferal, and radiocarbon analyses show that at least six of seven abrupt peat-to-mud contacts in cores from a tidal marsh at Talbot Creek (South Slough, Coos Bay), record sudden subsidence (relative sea-level rise) during great megathrust earthquakes at the Cascadia subdu...
Conference Paper
Full-text available
Stratigraphic sequences beneath salt marshes along the U.S. Pacific Northwest coast preserve 7000 years of plate-boundary earthquakes at the Cascadia subduction zone. The sequences record rapid rises in relative sea level during regional coseismic subsidence caused by great earthquakes and gradual falls in relative sea level during interseismic upl...
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A new regional dataset comprising 425 intertidal diatom taxa from 175 samples from 11 ecologically diverse Oregon and Washington estuaries illustrates the importance of compiling a large modern dataset from a range of sites. Cluster analyses and detrended correspondence analysis of the diatom assemblages identify distinct vertical zones within supr...
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Despite the Alaska-Aleutian megathrust's role as the source of some of the largest earthquakes and tsunamis, the history of its pre-20th-century tsunamis is largely unknown west of the rupture zone of the great (M9.2) 1964 earthquake. Stratigraphy in core transects at two boggy lowland sites on Chirikof Island’s southwest coast preserves tsunami de...
Article
We studied 18 sampling stations along a transect to investigate the similarity between live (rose Bengal stained) foraminiferal populations and dead assemblages, their small-scale spatial variations and the distribution of infaunal foraminifera in a salt marsh (Toms Creek marsh) at the upper end of the South Slough arm of the Coos Bay estuary, Oreg...
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We investigated the influence of inter-annual and seasonal differences on the distribution of live and dead foraminifera, and the inter-annual variability of stable carbon isotopes (δ13C), total organic carbon (TOC) values and carbon to nitrogen (C/N) ratios in bulk sediments from intertidal environments of Bandon Marsh (Oregon, USA). Living and de...
Chapter
Sea-level histories are reconstructed from sequences of coastal sediment using standard methods of sedimentology and stratigraphy. Sediment is sampled with hand-operated gouge corers, chamber corers, and piston corers and less commonly with engine-driven vibracorers and geoslicers. But coastal sediment is most accurately mapped and interpreted in e...
Conference Paper
Instrumental and historical records have proved too short to estimate the potential magnitudes and recurrence intervals of rare events such as the 2004 Indian Ocean and 2011 Tohoku-Oki great earthquakes and tsunamis. Paleoseismology improves our understanding of subduction zone hazards by extending earthquake histories thousands of years into the p...
Article
Sources of seismic hazard in the Puget Sound region of northwestern Washington include deep earthquakes associated with the Cascadia subduction zone, and shallow earthquakes associated with some of the numerous crustal (upper-plate) faults that crisscross the region. Our paleoseismic investigations on one of the more prominent crustal faults, the D...
Article
Earthquake prehistory of the southern Puget Lowland, in the north-south compressive regime of the migrating Cascadia forearc, reflects diverse earthquake rupture modes with variable recurrence. Stratigraphy and Bayesian analyses of previously reported and new C-14 ages in trenches and cores along backthrust scarps in the Seattle fault zone restrict...
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We report stratigraphic evidence of land-level change and tsunami inundation along the Alaska-Aleutian megathrust during prehistoric and historical earthquakes west of Kodiak Island. On Sitkinak Island, cores and tidal outcrops fringing a lagoon reveal five sharp lithologic contacts that record coseismic land-level change. Radiocarbon dates, 137Cs...
Conference Paper
Estuarine marshes along the US-Pacific coast host unique stratigraphic sequences that record coseismic land-level changes and tsunamis during the most recent magnitude 8-9 earthquakes at the Cascadia subduction zone. Earlier studies have shown the great potential of microfossil reconstructions of land-level changes during past earthquakes, but thes...
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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 va...
Article
[1] Past earthquake rupture models used to explain paleoseismic estimates of coastal subsidence during the great A.D. 1700 Cascadia earthquake have assumed a uniform slip distribution along the megathrust. Here we infer heterogeneous slip for the Cascadia margin in A.D. 1700 that is analogous to slip distributions during instrumentally recorded gre...
Article
Marsh sediments fringing estuaries provide a unique record of plate-boundary earthquakes at the Cascadia subduction zone during the Holocene. Stratigraphic sequences preserved in these estuaries provide geologic evidence of coseismic subsidence, recognized as a rapid rise in relative sea-level, and interseismic uplift characterized by a gradual fal...
Chapter
Tidal-marsh sediment is an archive of Holocene environmental changes, including movements of sea and land levels, and extreme events such as hurricanes, earthquakes, and tsunamis. Accurate and precise radiocarbon dating of environmental changes is necessary to estimate rates of change and the recurrence interval (frequency) of events. Plant macrofo...
Article
We describe the modern distribution of salt-marsh and tidal-flat foraminifera from Sitkinak Island (Trinity Islands) and Simeonof Island (Shumagin Islands), Alaska, to begin development of a dataset for later use in reconstructing relative sea-level changes caused by great earthquakes along the Alaska-Aleutian subduction zone. Dead foraminifera wer...
Chapter
Records of relative sea-level change archive the response of the Earth's surface to tectonic processes. Much of our understanding of tectonic processes during the Quaternary comes from study of strandlines (shoreline landforms and deposits) along tectonically active coasts. Regional patterns of strandline uplift or subsidence reflect primary tecton...
Conference Paper
Abrupt coastal subsidence induced by the great AD 1700 Cascadia earthquake has been estimated from paleoseismic evidence of buried soils and overlying mud and associated tsunamis deposits. These records have been modeled using a rather uniform rupture model, a mirror image of the uniform interseismic fault locking based on modern GPS observations....
Article
Recent magnitude-9 (M9) earthquakes at subduction zones in Sumatra and Japan are focusing research efforts to learn the history of these infrequent earthquakes at other subduction zones worldwide. We estimate the magnitude of prehistoric earthquakes at the Cascadia subduction zone by comparing elevation-sensitive ecologic communities of modern inte...
Article
A complex sequence of cyclical marine and glaciomarine lithofacies exposed along the coastal lowlands of the Qivitu Peninsula was deposited during the marine transgressions and regressions that affected this area during repeated glaciation of the region. Multivariate analysis of amino acid ratios measured on shell fragments of the pelecypods Hiatel...
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Foraminiferal zones, radiocarbon ages on shells, and corrected ages on pretreated organic sediment from four cores from the eastern Baffin Island continental shelf suggest a three-stage deglacial to postglacial history (Late Wisconsin to Holocene). The earliest sediments in the cores contain foraminiferal species (Elphidium excavatum, Cassidulina r...
Article
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Quantitative estimates of land-level change during the giant AD 1700 Cascadia earthquake along the Oregon coast are inferred from relative sea-level changes reconstructed from fossil foraminiferal assemblages preserved within the stratigraphic record. A transfer function, based upon a regional training set of modern sediment samples from Oregon est...
Article
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In late 1994, sand dykes, large sand blows, and deformed strata were exposed in the walls of an excavation at Annacis Island on the Fraser River delta near Vancouver, British Columbia. The features record liquefaction during a large earthquake about 1700 years ago; this was perhaps the largest earthquake to affect the Vancouver area in the last 350...
Article
Deciphering the earthquake histories of faults over the past few thousands of years in tectonically complex forearc regions relies on detailed site-specific as well as regional geologic maps. Here we present examples of site-specific USGS maps used to reconstruct earthquake histories for faults in the Puget Lowland. Near-surface faults and folds in...
Article
Changes in species assemblages of intertidal foraminifera can be used to estimate the amount of earthquake-related subsidence during plate-boundary earthquakes at the Cascadia subduction zone. The accuracy and precision of foraminiferal methods in paleoenvironmental reconstruction is underpinned by the relations between contemporary taxa and their...
Article
Paleoseismology is the study of prehistoric earthquakes, especially their location, timing, and size. Paleoseismology differs from more general geologic studies of slow to rapid crustal movements during the late Cenozoic (for example, neotectonics) in its focus on the almost instantaneous deformation of landforms and sediments during earthquakes. T...
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A reconnaissance of Holocene stratigraphy beneath fringing marshes of the Valdivia estuary, where an M 9:5 earthquake caused 1–2 m of regional coseismic subsidence in 1960, shows only fragmentary evidence of prehistoric coseismic sub-sidence. In most of the 150 hand-driven cores that were examined, a distinct uncon-formity separates 0.5–1.5 m of la...
Article
Assessments of the seismic hazard from the Cascadia subduction zone in central western North America use estimates of the amount of coseimic subsidence, geodetic strain, and thermal data to constrain coseismic slip and rupture area, thus deducing earthquake magnitude. Until recently, the amount of coseismic subsidence was approximated using qualita...
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Full-text available
A key question in earthquake hazard analysis is whether individual faults within fault zones represent independent seismic sources. For the Seattle fault zone, an upper plate structure within the Cascadia convergent margin, evaluating seismic hazard requires understanding how north-side-up, bedding-plane reverse faults, which generate late Holocene...
Article
The width of plate-boundary fault rupture at the Cascadia subduction zone, a dimension related to earthquake magnitude, remains uncertain because of the lack of quantitative information about land-level movements during past great-earthquake deformation cycles. Beneath a marsh at Alsea Bay, on the central Oregon coast, four sheets of tsunami-deposi...
Article
Continuing subduction of the Juan de Fuca plate beneath the North America plate in central western North America constitutes a major seismic hazard but the history of great earthquakes in the region remains unclear. The tsunami accompanying the last great earthquake along the Cascadia subduction zone was widely recorded in Japanese records as an "o...
Article
Comparison of histories of great earthquakes and tsunamis at eight coastal sites above the Cascadia subduction zone suggests both long (>500 km) and short (
Article
Comparison of histories of great earthquakes and accompanying tsunamis at eight coastal sites suggests plate-boundary ruptures of varying length, implying great earthquakes of variable magnitude at the Cascadia subduction zone. Inference of rupture length relies on degree of overlap on radiocarbon age ranges for earthquakes and tsunamis, and relati...
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The Indian Ocean tsunami of 26 December 2004 shows poignantly that catastrophic tsunamis are too infrequent for their hazard to be characterized by historical records alone. Long-term geologic records provide opportunities to assess water depth and velocity of past inundations, estimate source locations, and aid in understanding how tsunamis affect...
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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 bo...
Article
Nine muddy sand beds interrupt a 2500-yr-old sequence of peat deposits beneath a tidal marsh at the head of Discovery Bay on the south shore of the Strait of Juan de Fuca, Washington. An inferred tsunami origin for the sand beds is assessed by means of six criteria. Although all the sand beds contain marine diatoms and almost all the beds display i...
Article
Comparisons of regional Cascadia earthquake records to earthquake chronologies at the Coquille River and Sixes River estuaries in southern Oregon suggest that, in contrast to the M 9 A.D. 1700 earthquake, some events ruptured shorter segments of the subduction zone and failed to trigger offshore turbidites. Between 2,000 and 4,700 years ago, earthq...
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Four buried tidal marsh soils at a protected inlet near the mouth of the Salmon River yield definitive to equivocal evidence for coseismic subsidence and burial by tsunami-deposited sand during great earthquakes at the Cascadia subduction zone. An extensive, landward-tapering sheet of sand overlies a peaty tidal-marsh soil over much of the lower es...