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 great subduction earthquakes worldwide. The assumption of uniform distribution in previous rupture models was due partly to the large uncertainties of then available paleoseismic data used to constrain the models. In this work, we use more precise estimates of subsidence in 1700 from detailed tidal microfossil studies. We develop a 3‐D elastic dislocation model that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip slip extensions, the more precise subsidence estimates are best explained by a model with along‐strike slip heterogeneity, with multiple patches of high‐moment release separated by areas of low‐moment release. For example, in A.D. 1700, there was very little slip near Alsea Bay, Oregon (~44.4°N), an area that coincides with a segment boundary previously suggested on the basis of gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for more precise, high‐quality estimates of subsidence or uplift during prehistoric earthquakes from the coasts of southern British Columbia, northern Washington (north of 47°N), southernmost Oregon, and northern California (south of 43°N), where slip distributions of prehistoric earthquakes are poorly constrained.

A 4500-year record of hurricane-induced storm surges is developed from sediment cores collected from a coastal sinkhole near Apalachee Bay, Florida. Recent deposition of sand layers in the upper sediments of the pond was found to be contemporaneous with significant, historic storm surges at the site modeled using SLOSH and the Best Track, post-1851 A.D. dataset. Using the historic portion of the record for calibration, paleohurricane deposits were identified by sand content and dated using radiocarbon-based age models. Marine-indicative foraminifera, some originating at least 5 km offshore, were present in several modern and ancient storm deposits. The presence and long-term preservation of offshore foraminifera suggest that this site and others like it may yield promising microfossil-based paleohurricane reconstructions in the future. Due to the sub-decadal (~ 7 year) resolution of the record and the site's high susceptibility to hurricane-generated storm surges, the average, local frequency of recorded events, approximately 3.9 storms per century, is greater than that of previously published paleohurricane records from the region. The high incidence of recorded events permitted a time series of local hurricane frequency during the last five millennia to be constructed. Variability in the frequency of the largest storm layers was found to be greater than what would likely occur by chance alone, with intervals of both anomalously high and low storm frequency identified. However, the rate at which smaller layers were deposited was relatively constant over the last five millennia. This may suggest that significant variability in hurricane frequency has occurred only in the highest magnitude events. The frequency of high magnitude events peaked near 6 storms per century between 2800 and 2300 years ago. High magnitude events were relatively rare with about 0-3 storms per century occurring between 1900 to 1600 years ago and between 400 to 150 years ago. A marked decline in the number of large storm deposits, which began around 600 years ago, has persisted through present with below average frequency over the last 150 years when compared to the preceding five millennia.

How climate controls hurricane variability has critical implications for society is not well understood. In part, our understanding is hampered by the short and incomplete observational hurricane record.

Coseismic subsidence along the Cascadia subduction zone causes abrupt relative sea-level (RSL) rise that is recorded in coastal stratigraphy and foraminiferal assemblages. RSL reconstructions therefore provide insight into the magnitude, nature, and frequency of great earthquakes that can constrain deformation models and quantify the seismic risk faced by coastal populations. These reconstructions are commonly generated using transfer functions that are calibrated from counts of modern (surface) foraminifera and corresponding elevation measurements. We developed four transfer functions of increasing complexity to explore how and why the composition of the modern dataset and the choice of transfer-function type affects subsidence reconstructions. Application of these four models to stratigraphic contacts (mud abruptly overlying peat or soil) representing the A.D. 1700 Cascadia earthquake and a field experiment that simulated subsidence show that a Bayesian transfer function (BTF) calibrated using a large modern dataset (19 sites from California to Vancouver Island) and incorporating prior information from stratigraphic context produces systematically larger subsidence estimates than a weighted-averaging transfer function calibrated using a smaller modern dataset (8 sites in Oregon) that does not leverage stratigraphic context. This difference arises from ( 1) training set composition, (2) taxa-elevation relationships in the BTF that are not assumed to be unimodal, and(3) stratigraphic prior information that compensates for postdepositional, downward mixing of postearthquake foraminifera into pre-earthquake sediment, which biases reconstructions at some sites toward smaller subsidence. Our reconstructions support a heterogeneous rupture model for the A.D. 1700 earthquake, but indicate that slip estimates in patches from Alsea Bay to Netarts Bay (Oregon) and from Netarts Bay to Vancouver Island should be increased. Electronic Supplement:Table listing counts of foraminifera and sample elevations used to construct the West Coast modern training set.

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