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The 26 January 1700 CE Cascadia subduction zone earthquake ruptured much of the plate boundary and generated a tsunami that deposited sand in coastal marshes from northern California to Vancouver Island. Although the depositional record of tsunami inundation is extensive in some of these marshes, few sites have been investigated in enough detail to map the inland extent of sand deposition and depict variability in tsunami deposit thickness and grain size. We collected 129 cores in marshes of the Salmon River estuary in Oregon and reanalyzed 114 core logs from a 1987–88 study that mapped the inland extent of circa 1700 CE sandy tsunami deposits. The ca. 1700 CE tsunami deposit in the Salmon River estuary is easily recognized in cores ≤1 m deep in which a buried marsh peat is overlain by a well sorted sand bed with a sharp lower contact that thins and fines inland. We use tsunami deposit data and models of sandy tsunami sediment transport (using Delft3D‐FLOW) to test 15 rupture models that could represent a ca. 1700 CE earthquake. At least 12–16 m of slip offshore of the Salmon River, which results in 0.8–1.0 m of coastal coseismic subsidence, is required to match the ca. 1700 CE sand deposit's inland extent, which is consistent with models of heterogeneous megathrust slip in ca. 1700 CE. Our methods of detailed tsunami deposit mapping, combined with sediment transport modeling, can be used to test models of megathrust ruptures and their tsunamis to potentially improve earthquake and tsunami hazard assessments. Plain Language Summary: We rely on the geologic record of great earthquakes and tsunamis, often preserved in the stratigraphy of coastal estuaries, to determine the size, frequency, and location of prehistoric events. At the Cascadia subduction zone, the 1700 CE earthquake caused a meter or more of sudden coastal subsidence and a tsunami that not only inundated coastal areas in Cascadia but was also recorded in Japan. Computer modeling implies that the earthquake occurred around 9 p.m. on 26 January 1700. Although sandy tsunami deposits from the 1700 earthquake are widespread throughout Cascadia, few sites have been cored extensively enough to confidently identify the inland extent of sandy tsunami deposits. We used over 200 sediment cores from the Salmon River estuary in Oregon to map the extent of sandy tsunami deposits from 1700. We then ran numerical models of sediment transport driven by a variety of earthquake generation sources to determine that earthquakes causing at least 0.8 m of subsidence are needed to generate a modeled tsunami capable of recreating the 1700 tsunami deposits observed in cores. This study demonstrates that tsunami deposit mapping and modeling methods can be used to improve our understanding of the impacts of past great earthquakes and tsunamis. Key Points: Models of sediment transport test earthquake and tsunami sources by comparing the modeled and observed distribution of sandy depositsSandy deposits can define the minimum inundation of the last major tsunami, around 1700 CE, generated at the Cascadia subduction zoneTo match tsunami deposit data, models require an earthquake that caused ≥0.8 m of coastal coseismic subsidence at the Salmon River [ABSTRACT FROM AUTHOR]

Storms and associated surges are major coast-shaping processes. Nevertheless, no typical sequences for storm surge deposits in different coastal settings have been established. This study interprets a coarse-grained hurricane ridge deposit on the island of Bonaire, Netherlands Antilles. The sequence was deposited during Hurricane Lenny in November 1999. Insight is gained into the hydrodynamics of surge flow by interpreting textural trends, particle imbrication, and deposit geometry. Vertical textural variations, caused by time-dependent hydrodynamic changes, were used to subdivide the deposit into depositional units that correspond to different stages of the surge, such as setup, peak, and return flow. Particle size and imbrication trends and geometry of the units reflect landward bed-load transport of components during the setup, a nondirectional flow with sediment falling out of suspension during the peak, and a seaward bed-load transport during the return flow. Formation of a ridge during setup affected the texture of the return flow unit. Changing angles of imbrication reflect alternating flow velocities during each phase. Normal grading during setup and inverse grading during return flow are caused by decelerating and accelerating flow, respectively. Hence, the interpreted deposit seems to represent the first described complete hurricane surge sequence from a carbonate environment.

Modern microfossil distributions reflect site‐specific habitats and provide an opportunity to assess sediment transport pathways in the nearshore environment. When applied to overwash deposits in the geological record, they provide insight into sediment provenance and transport, factors important for understanding patterns of frequency and intensity of past storms and tsunamis. Modern distribution studies are rare and often the first established ones occur immediately after an overwash event as part of a post‐event field survey. This is problematic because it is unclear what effect overwash events have on nearshore microfossil assemblages and what time interval is necessary for them to return to pre‐event conditions. This study documents the impacts of Hurricane Irma on nearshore sediments off the coast of Anegada, British Virgin Islands, using distributions of Homotrema rubrum, an encrusting foraminifer with a defined provenance in coral reefs. At four sampling intervals spanning two years, from six months pre‐Hurricane Irma to eighteen months after, surface sediment was collected from three transects on the northern and southern shores of the island. Partitioning Around Medoids cluster analysis revealed that Hurricane Irma introduced an influx of well‐preserved fragments into the reef flat and made the sediments more uniform, limiting the foraminifer's utility as a known sediment transport indicator. The mixing of sediments along the two northern transects (reef proximal) persisted for seven to eighteen months before returning to near pre‐hurricane conditions. However, the southern transect (absence of reef), where Homotrema rubrum concentrations are significantly less, failed to recover within the time period assessed by this study, indicating a variable recovery period between Atlantic Ocean and Caribbean Sea facing shorelines. Results from this study suggest that a waiting period of at least eighteen months after a major storm is recommended before collecting surface sediment from the nearshore environments of reef‐dominated coastlines. [ABSTRACT FROM AUTHOR]

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 coasts, even those distant from subduction zones, is critical for assessing tsunami hazard throughout the Pacific basin. This study searched for evidence of tsunami inundation using stratigraphic and sedimentological analyses of potential tsunami deposits beneath present and former Hawaiian wetlands, coastal lagoons, and river floodplains. Coastal wetland sites on the islands of Hawai΄i, Maui, O΄ahu and Kaua΄i were selected based on historical tsunami runup, numerical inundation modelling, proximity to sandy source sediments, degree of historical wetland disturbance, and breadth of prior geological and archaeological investigations. Sand beds containing marine calcareous sediment within peaty and/or muddy wetland deposits on the north and north‐eastern shores of Kaua΄i, O΄ahu and Hawai΄i were interpreted as tsunami deposits. At some sites, deposits of the 1946 and 1957 Aleutian tsunamis are analogues for deeper, older probable tsunami deposits. Radiocarbon‐based age models date sand beds from three sites to ca 700 to 500 cal yr bp, which overlaps ages for tsunami deposits in the eastern Aleutian Islands that record a local subduction zone earthquake. The overlapping modelled ages for tsunami deposits at the study sites support a plausible correlation with an eastern Aleutian earthquake source for a large prehistoric tsunami in the Hawaiian Islands. [ABSTRACT FROM AUTHOR]

Far‐field tsunami deposits observed in the Kahana Valley, O'ahu, Hawai'i (USA), were investigated for their organic‐geochemical content. During short high‐energy events, (tsunamis and storms) organic and chemical components are transported with sediment from marine to terrestrial areas. This study investigates the use of anthropogenic based organic geochemical compounds (such as polycyclic aromatic hydrocarbons, pesticides and organochlorides) as a means to identify tsunami deposits. Samples were processed by solid–liquid extraction and analyzed using gas chromatography–mass spectrometry. A total of 21 anthropogenic marker compounds were identified, of which 11 compounds were selected for detailed analysis. Although the tsunami deposits pre‐date industrial activity in Hawai'i by several hundred years, distinct changes were found in the concentrations of anthropogenic marker compounds between sandy tsunami deposits and the surrounding mud/peat layers, which may help in identifying tsunami deposits within cores. As expected, low overall concentrations of anthropogenic markers and pollutants were observed due to the lack of industrial input‐sources and little anthropogenic environmental impact at the study site. This geochemical characterization of tsunami deposits shows that anthropogenic markers have significant potential as another high‐resolution, multi‐proxy method for identifying tsunamis in the sedimentary record. [ABSTRACT FROM AUTHOR]

Abstract: Morphological trends of three distinct intertidal environments in South San Francisco Bay were investigated using a combination of measurement and modeling tools. Because of the inherent relationship between the physical environment and the sediment properties, the sediment properties provide a good indicator of morphologic trends. A significant finding of this study is that surface sediment erodibility increases as the energy level in the environment increases. Conversely subsurface sediment erodibility shows a strong relationship to the long-term history of the site. The combination of the measured sediment properties, the history of deposition and erosion, and simple modeling of the physical environment illustrate the interaction of these properties such that an understanding of intertidal flat behavior is developed. [Copyright &y& Elsevier]