Long term monitoring for oil in the Exxon Valdez spill region

In the aftermath of the 1989 Exxon Valdez oil spill, a Long Term Environmental Monitoring Program (LTEMP) has been regularly sampling mussels (and some sediments) for polycyclic aromatic and saturated hydrocarbons (PAH and SHC) at sites in Port Valdez, Prince William Sound, and the nearby Gulf of Alaska region. After 1999, a decreasing trend appears in total PAH (TPAH) in tissues at all sites with current values below 100 ng/g dry weight (many below 50 ng/g). Currently, most samples reflect a predominantly dissolved-phase signal. This new low in TPAH likely represents ambient background levels. Synchrony in TPAH time-series and similarities in the hydrocarbon signatures portray regional-scale dynamics. The five inner Prince William Sound sites show similar composition and fluctuations that are different from the three Gulf of Alaska sites. The two Port Valdez sites represent a unique third region primarily influenced by the treated ballast water discharge from the Alyeska Marine Terminal. Prince William Sound has reverted to a stable environment of extremely low level contamination in which local perturbations are easily detected.     

Profiles of Fecal Porphyrins in River Otters Following the Exxon Valdez Oil Spill

Median levels of Coproporphyrin III (Copro III) in fecal samples of river otters (Lontra canadensis) collected from an oiled area in Prince William Sound, Alaska, USA, during 1990 were significantly higher than in samples collected from the same oiled area during 1996 (p=0.011, one way analysis of variance), a nonoiled reference area in Prince William Sound during 1996 (p=0.002) and a reference area in southeast Alaska during 1998 (p=0.004). An overall test of significance that combined probabilities from the statistical analysis of this porphyrin study with those from other biomarker studies revealed a significant difference in physiological response of river otters between oiled and nonoiled areas of the Sound for 1990 (p < 0.01). We demonstrated that changes in levels of fecal porphyrins may serve as a biomarker that may contribute to a health assessment of wild river otters.     

Seismicity of the Prince William Sound Region for over Thirty Years Following the 1964 Great Alaskan Earthquake

—In this paper we present results of body wave-form modeling of 19 earthquakes (generally m _b ≥5.7) occurring from 1964 to 1983 in the vicinity and down-dip of the large asperity within the Prince William Sound region that ruptured in 1964. These data are supplemented with source parameters from studies of more recent (post-1980) events. Our results suggest that moderate earthquakes which occurred in the region between 1964 and 1984 were predominantly located in the vicinity of the Prince William Sound asperity and could be assigned to two groups. The first group consists of events occurring above the plate interface within Prince William Sound along reverse faults or low angle thrusts. The second group occurs at 35 to 60 km depth in the region north of Prince William Sound, and represents normal to normal-oblique faulting within the subducted Pacific crust or upper mantle. These earthquakes occur below the northern edge of the 1964 asperity in a region where the subducting plate undergoes a rapid change in strike and dip. A third group of events occurs in Cook Inlet well down-dip of the 1964 asperity and below the plate interface. These events exhibit a variety of mechanisms and many at depths of 50 to 70 km may be associated with complexities in the shape of the downgoing slab. Most of the Cook Inlet events occurred after 1984, whereas a few events of similar magnitude have occurred in the vicinity of the Prince William Sound asperity since 1984.     

A hierarchical approach measures the aerial extent and concentration levels of PAH-contaminated shoreline sediments at historic industrial sites in Prince William Sound, Alaska

A field study was conducted in 2003 to estimate the areal distribution and concentrations of polycyclic aromatic hydrocarbons (PAH) in intertidal sediments at sites of past human and industrial activity (HA sites) in Prince William Sound (PWS), Alaska, the site of the 1989 Exxon Valdez oil spill. More than 50 HA sites, primarily in western PWS, were identified through analysis of historic records and prior field studies, and nine sites were selected for detailed surveys. The areal assessment process consisted of seven steps: (1) identify site from historic records and field surveys; (2) locate visual evidence of surface oil/tar at a site; (3) prepare a site map and lay out a sampling grid over the entire site with 10-m grid spacing; (4) excavate pits to 50 cm depth on the grid; (5) perform a field colorimetric test to estimate total PAH (TPAH) in sediments from the wall of each pit and record the results in the ranges <1 ppm; 1-10 ppm; >10 ppm TPAH; (6) expand grid size if necessary if elevated PAH levels are detected colorimetrically; (7) select 20 samples from each site for same-day shipboard PAH analysis by immunoassay (SDI RaPID PAH) and, based on these results, select sediment samples from each site for full PAH analysis in the laboratory to identify PAH sources. A total of 416 pits were dug at the nine sites. Nine acres of sediments with TPAH >2500 ppb dry wt. were mapped at the nine sites. TPAH concentrations obtained by immunochemical analysis of 181 samples from the nine sites ranged from 20 to 1,320,000 ppb (wet wt.). The contaminants are mixtures of petroleum products (2-3 ring PAH) and combustion products (4-6 ring PAH) unrelated to the 1989 Exxon Valdez oil spill. Mussels and clams collected at these sites have elevated levels of PAH that are compositionally similar to the PAH in the sediments. These findings indicate that at least a portion of the sediment PAH is bioavailable. The PAH sources at these historic industrial sites are chronic. They include relict fuel oil tanks and works located above and within the intertidal zone, with contamination at some locations extending into nearshore sub-tidal sediments. This study shows how a hierarchical approach can be used to quickly and successfully map, quantify, and subsequently, identify sources of PAH in shoreline sediments.     

Relocations of Earthquakes (1899–1917) in South-Central Alaska

I have relocated 18 earthquakes occurring in the south-central Alaska region between 1899 and 1917 using a bootstrap relocation technique. Locations of events within the Yakutat region suggest that the 1899 sequence began on 4 September with a M_S = 7.9 event within the area of the Pamplona fault zone/western Transition fault zone, rupturing the western portion of the North American/Pacific plate interface. A M_S = 7.4 event on 10 September appears to have ruptured the offshore portion of the plate interface to the east of the 4 September event. This was followed by a M_S = 8.0 event that likely ruptured the onshore and down-dip portion of the plate interface. A M_S = 7.0 event in 1908 may have ruptured a small portion of the plate interface between the 4 September and 10 September events. Events occurring between 1911 and 1916 in the Prince William Sound region appear to be slab events occurring in similar locations to more recent seismicity. Within the Kodiak region the 1900 earthquake of M_S = 7.7 has a location consistent with the rupture of the Kodiak asperity which also ruptured during the 1964 great Alaska earthquake. Other large magnitude Kodiak events appear to be associated with regions of recent seismicity, including the Karluk Lake area of southwestern Kodiak Island and the Albatross Basin located offshore southeast of Kodiak Island. Space-time seismicity patterns since 1899 indicate that magnitude 6 to7 events have occurred with regularity in the Kodiak Island region; that there has been a lack of magnitude ≥ 6 events in the Prince William Sound region since 1964, and that the Yakutat region has remained notably quiescent at the magnitude ≥ 6 level.     

Performance of a nowcast/forecast system for Prince William Sound,Alaska

A coastal ocean extended Prince William Sound nowcast/forecast system (EPWS/NFS) has been running semi-automatically for an extended domain of Prince William Sound (PWS), Alaska for 2 years. To determine the performance of this modeling system, an assessment is conducted. EPWS/NFS and PWS/NFS (viz., its predecessor) nowcasts are compared with observed time series of sea surface temperature (SST) and coastal sea level (CSL) at a few stations, and to velocity profiles from a moored ADCP. With the extension of the model domain to include the continental shelf outside PWS and forced by an operational global ocean model (Global-Navy Coastal Ocean Model (Global-NCOM)) and a 2D tidal model at the open boundary, EPWS/NFS has achieved significant improvement over PWS/NFS, which covered only PWS per se, for most of the predicted variables in this study. In both magnitude and phase, EPWS/NFS accurately predicts the coastal tide fluctuations, as well as M2 tidal currents in Central Sound, although significant errors in coastal tides exist during some spring and neap tide cycles. Other than for the tidal motions, EPWS/NFS generally produces less energetic CSL and velocity variations than those observed. In comparison, although PWS/NFS well predicts the coastal tides, it suffers from the absence of low-frequency CSL variations, as well as misprediction of M2 tidal currents in Central Sound. For 40 h low-passed PWS/NFS and EPWS/NFS velocities, significant phase error occurs during the model–date comparison period, while EPWS/NFS nowcasts generally produce less root-mean-square-error (rmse) and smaller correlations with the observations than PWS/NFS does. Both observations and EPWS/NFS have similar vertical profiles of baroclinic velocity standard deviations, but some substantial discrepancies occur in the velocity direction. Also, in the Central Sound, EPWS/NFS predicts well the SST seasonal cycle and a major cooling event during the summer 2005. However, for periods shorter than 1 week, both PWS/NFS and EPWS/NFS SST underestimated the observed fluctuations by an order of magnitude.     

Comparison of mussels and semi-permeable membrane devices as intertidal monitors of polycyclic aromatic hydrocarbons at oil spill sites

Side-by-side comparisons of polycyclic aromatic hydrocarbon (PAH) concentrations in resident blue mussels (Mytilus trossulus) and in semi-permeable membrane devices (SPMDs) were made at four sites in Prince William Sound, Alaska. SPMDs were deployed for approximately 30 days on the surface of the beach sediment at three tidal elevations on each shore and in 0.5 m deep open pits in the middle intertidal zone. Total PAH (TPAH) concentrations in mussels and in SPMDs were correlated, but the PAH compositions were different. The lower molecular weight PAH were relatively more abundant in the SPMDs than in the mussels at oiled and HA sites. TPAH concentrations in SPMDs deployed in pits and mussels collected adjacent to those pits at oiled sites were higher than in SPMDs and mussels from non-pitted SPMD locations approximately 3-15 m from the pits. Pitting released buried oil making its PAH bioavailable. SPMDs deployed in the supratidal zone (+4.0 m tidal elevation) were exposed to atmospheric contaminants for a large fraction of the deployment time and accumulated primarily pyrogenic (combustion-sourced) PAH from the atmosphere. The SPMD strips supplied by the manufacturer contained significant amounts (approximately 125 ng/strip) of primarily alkylated 2-3 ring PAH. These blank levels make SPMDs unsuitable for shoreline assessments when environmental PAH concentrations are low. Consequently, where available, mussels are recommended for use in assessments of the bioavailability of buried oil residues sequestered in intertidal sediments following an oil spill. Mussels are the preferred monitoring tool when the assessments involve food-chain effects. At locations where the absence of mussels necessitates the use of SPMDs or other passive sampling devices, their limitations need to be carefully considered in the interpretation of results.     

Results from a sixteen year study on the effects of oiling from the Exxon Valdez on adult pink salmon returns

For sixteen years following the 1989 Exxon Valdez oil spill adult returns of pink salmon in Prince William Sound, Alaska were monitored to assess spill effects on survival. No evidence of spill effects was detected for either intertidal or whole-stream spawning fish. From 1989 through 2004 mean densities for oiled and reference streams tracked each other, illustrating similar responses of oiled and reference stream adult populations to naturally changing oceanographic and climactic conditions. Hatchery fish strayed into the study streams, but similar incursions occurred in oiled and reference streams, and their presence was compensated for to eliminate their influence on determining the success of the returning natural populations. These results, showing no detectable effects of oiling on pink salmon spawning populations, are supported by published field studies on pink salmon incubation success in oiled streams.     

Cytochrome P4501A biomarker indication of the timeline of chronic exposure of Barrow's goldeneyes to residual Exxon Valdez oil

We examined hepatic EROD activity, as an indicator of CYP1A induction, in Barrow’s goldeneyes captured in areas oiled during the 1989 Exxon Valdez spill and those from nearby unoiled areas. We found that average EROD activity differed between areas during 2005, although the magnitude of the difference was reduced relative to a previous study from 1996/1997, and we found that areas did not differ by 2009. Similarly, we found that the proportion of individuals captured from oiled areas with elevated EROD activity (?2 times unoiled average) declined from 41% in winter 1996/1997 to 10% in 2005 and 15% in 2009. This work adds to a body of literature describing the timelines over which vertebrates were exposed to residual Exxon Valdez oil and indicates that, for Barrow’s goldeneyes in Prince William Sound, exposure persisted for many years with evidence of substantially reduced exposure by 2 decades after the spill.     

Main meiofauna taxa as an indicator for assessing the spatial and seasonal impact of fish farming

The present study aimed to establish potential indicators of fish farming pollution in muddy substrate, by means of meiofauna, and to test whether the effect of the fish farm is more important to determine the meiofauna community than the seasonal environmental conditions. Sampling was performed in spring, after several months of light feeding, and in summer, at high food supply. Samples were collected in three directions at various distances from the floating cages. Harpacticoid copepods and kinorhynchs, whose abundance decreased under the cages, were put forward as indicator taxa. However, harpacticoid copepods were sensitive to fish farm only, while kinorhynchs showed responsiveness to fish farm and to seasonal environmental conditions. Total meiofauna density was dependent on season sensu stricto. The nMDS clearly showed a ‘cage community’ and ‘control community’ in both sampling occasions; therefore it is a good tool for impact assessment.     

Note on the 1964 Alaska Tsunami Generation by Horizontal Displacements of Ocean Bottom. Numerical Modeling of the Runup in Chenega Cove, Alaska

A numerical model of the wave dynamics in Chenega Cove, Alaska during the historic M _w 9.2 megathrust earthquake is presented. During the earthquake, locally generated waves of unknown origin were identified at the village of Chenega, located in the western part of Prince William Sound. The waves appeared shortly after the shaking began and swept away most of the buildings while the shaking continued. We model the tectonic tsunami in Chenega Cove assuming different tsunami generation processes. Modeled results are compared with eyewitness reports and an observed runup. Results of the numerical experiments let us claim the importance of including both vertical and horizontal displacement into the 1964 tsunami generation process. We also present an explanation for the fact that arrivals of later waves in Chenega were unnoticed.     

A model study of the Copper River plume and its effects on the northern Gulf of Alaska

A three-level nested Regional Ocean Modeling System was used to examine the seasonal evolution of the Copper River (CR) plume and how it influences the along- and across-shore transport in the northern Gulf of Alaska (NGoA). A passive tracer was introduced in the model to delineate the growth and decay of the plume and to diagnose the spread of the CR discharge in the shelf, into Prince William Sound (PWS) and offshore. Furthermore, a model experiment with doubled discharge was conducted to investigate potential impacts of accelerated glacier melt in future climate scenarios. The 2010 and 2011 simulation revealed that the upstream (eastward) transport in the NGoA is negligible. About 60 % of the passive tracer released in the CR discharge is transported southwestward on the shelf, while another one third goes into PWS with close to 60 % of which exiting PWS to the shelf from Montague Strait. The rest few percent is transported across the shelf break and exported to the GoA basin. The downstream transport and the transport into PWS are strongly regulated by the downwelling-favorable wind, while the offshore transport is related to the accumulation of plume water in the shelf, frontal instability, and the Alaskan Stream. It takes weeks in spring for the buoyancy to accumulate so that a bulge forms outside of the CR estuary. The absence of strong storms as in the summer of 2010 allows the bulge continue growing to trigger frontal instability. These frontal features can interact with the Alaskan Stream to induce transport pulses across the shelf break. Alternatively as in 2011, a downwelling-favorable wind event in early August (near the peak discharge) accelerates the southwestward coastal current and produces an intense downstream transport event. Both processes result in fast drains of the buoyancy and the plume content, thereby rapid disintegration of the plume in the shelf. The plume in the doubled discharge case can be two to three times in size, which affects not only the magnitude but also the timing of certain transport events. In particular, the offshore transport increases by several folds because the plume appears to be more easily entrained by the seaward flow along the side of Hinchinbrook Canyon.     

Hawaiian oral tradition describes 400 years of volcanic activity at Kīlauea

Culturally significant oral tradition involving Pele, the Hawaiian volcano deity, and her youngest sister Hi'iaka may involve the two largest volcanic events to have taken place in Hawai'i since human settlement: the roughly 60-year-long ‘Ailā’au eruption during the 15th century and the following development of Kīlauea's caldera. In 1823, Rev. William Ellis and three others became the first Europeans to visit Kīlauea's summit and were told stories about Kīlauea's activity that are consistent with the Pele–Hi'iaka account and extend the oral tradition through the 18th century. Recent geologic studies confirm the essence of the oral traditions and illustrate the potential value of examining other Hawaiian chants and stories for more information about past volcanic activity in Hawai‘i.     

PCB exposure in sea otters and harlequin ducks in relation to history of contamination by the Exxon Valdez oil spill

Exposure to contaminants other than petroleum hydrocarbons could confound interpretation of Exxon Valdez oil spill effects on biota at Prince William Sound, Alaska. Hence, we investigated polychlorinated biphenyls (PCBs) in blood of sea otters and harlequin ducks sampled during 1998. PCB concentrations characterized by lower chlorinated congeners were highest in sea otters from the unoiled area, whereas concentrations were similar among harlequin ducks from the oiled and unoiled area. Blood enzymes often elevated by xenobiotics were not related to PCB concentrations in sea otters. Only sea otters from the unoiled area had estimated risk from PCBs, and PCB composition or concentrations did not correspond to reported lower measures of population performance in sea otters or harlequin ducks from the oiled area. PCBs probably did not influence limited sea otter or harlequin duck recovery in the oiled area a decade after the spill.     

Hydrological and catchment controls on event-scale dissolved organic carbon dynamics in boreal headwater streams

Hydrological events transport large proportions of annual or seasonal dissolved organic carbon (DOC) loads from catchments to streams. The timing, magnitude and intensity of these events are very sensitive to changes in temperature and precipitation patterns, particularly across the boreal region where snowpacks are declining and summer droughts are increasing. It is important to understand how landscape characteristics modulate event-scale DOC dynamics in order to scale up predictions from sites across regions, and to understand how climatic changes will influence DOC dynamics across the boreal forest. The goal of this study was to assess variability in DOC concentrations in boreal headwater streams across catchments with varying physiographic characteristics (e.g., size, proportion of wetland) during a range of hydrological events (e.g., spring snowmelt, summer/fall storm events). From 2016 to 2017, continuous discharge and sub-daily chemistry grab samples were collected from three adjacent study catchments located at the International Institute for Sustainable Development-Experimental Lakes Area in northwestern Ontario, Canada. Catchment differences were more apparent in summer and fall events and less apparent during early spring melt events. Hysteresis analysis suggested that DOC sources were proximal to the stream for all events at a catchment dominated by a large wetland near the outlet, but distal from the stream at the catchments that lacked significant wetland coverage during the summer and fall. Wetland coverage also influenced responses of DOC export to antecedent moisture; at the wetland-dominated catchment, there were consistent negative relationships between DOC concentrations and antecedent moisture, while at the catchments without large wetlands, the relationships were positive or not significant. These results emphasize the utility of sub-daily sampling for inferring catchment DOC transport processes, and the importance of considering catchment-specific factors when predicting event-scale DOC behaviour.     

Extreme precipitation drives groundwater recharge: the Northern High Plains Aquifer,central United States, 1950–2010

Future extreme precipitation (EP, daily rainfall amount over certain thresholds) is projected to increase with global climate change; however, its effect on groundwater recharge has not been fully explored. This study specifically investigates the spatiotemporal dynamics of groundwater recharge and the effects of extreme precipitation (daily rainfall amount over the 95th percentile, which is tagged by ranking the percentiles in each season for a base period) on groundwater recharge from 1950 to 2010 over the Northern High Plains (NHP) Aquifer using the Soil Water Balance Model. The results show that groundwater recharge significantly (p < 0.05) increased in the eastern NHP from 1950 to 2010, where the highest annual average groundwater recharge occurs compared to the central and the western NHP. In the eastern NHP, 45.1% of the annual precipitation fell as EP, which contributed 56.8% of the annual total groundwater recharge. In the western NHP, 30.9% of the annual precipitation fell as extreme precipitation, which contributed 62.5% of the annual total groundwater recharge. In addition, recharge by extreme precipitation mainly occurred in late spring and early summer, before the maximum evapotranspiration rate, which usually occurs in mid‐summer until late fall. A dry site in the western NHP and a wet site in the eastern NHP were analysed to indicate how recharge responds to EP with different precipitation regimes. The maximum daily recharge at the dry site exceeded the wet site when there was EP. When precipitation fell as non‐extreme rainfall, most recharge was less than 5 mm at both the dry and wet sites, and the maximum recharge at the dry site became lower than the wet site. This study shows that extreme precipitation plays a significant role in determining groundwater recharge. © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd.     

Transport through Prince William Sound: numerical study in a nowcast/forecast system

Using 1-year simulated data from extended Prince William Sound (PWS) nowcast/forecast system, both barotropic and baroclinic transports through two-strait, semi-enclosed PWS are examined. With major tidal constituents removed, hourly time series of volume transports through two straits are significantly correlated with net transport well balanced by the time rate of change of the PWS spatial-mean sea level. A transition frequency band occurs within the coherence function of hourly volume transports, which is characterized by a nearly 180° phase shift between low-frequency (>30 h) and high-frequency (<6 h) bands. The transition band is implicitly related to the horizontally divergent and horizontally non-divergent flows inside the Sound. Further investigation of monthly and annual mean volume transports indicates strong seasonal variability of flows through two straits. On the other hand, baroclinic transport through PWS demonstrates the transition between a two-layered flow structure during the wintertime and a well-defined three-layered structure, i.e., inflow in both the surface and bottom layer with outflow in the intermediate layer, in the remainder of the year. This three-layer exchange flow is determined to be mainly buoyancy-driven, geostrophic flow, and thus largely affected by seasonal variability of buoyancy over the shelf and PWS.     

Estimating background and threshold nitrate concentrations using probability graphs

Because of the ubiquitous nature of anthropogenic nitrate (NO3(-)) in many parts of the world, determining background concentrations of NO3(-) in shallow ground water from natural sources is probably impossible in most environments. Present-day background must now include diffuse sources of NO3(-) such as disruption of soils and oxidation of organic matter, and atmospheric inputs from products of combustion and evaporation of ammonia from fertilizer and livestock waste. Anomalies can be defined as NO3(-) derived from nitrogen (N) inputs to the environment from anthropogenic activities, including synthetic fertilizers, livestock waste, and septic effluent. Cumulative probability graphs were used to identify threshold concentrations separating background and anomalous NO(3)-N concentrations and to assist in the determination of sources of N contamination for 232 spring water samples and 200 well water samples from karst aquifers. Thresholds were 0.4, 2.5, and 6.7 mg/L for spring water samples, and 0.1, 2.1, and 17 mg/L for well water samples. The 0.4 and 0.1 mg/L values are assumed to represent thresholds for present-day precipitation. Thresholds at 2.5 and 2.1 mg/L are interpreted to represent present-day background concentrations of NO(3)-N. The population of spring water samples with concentrations between 2.5 and 6.7 mg/L represents an amalgam of all sources of NO3(-) in the ground water basins that feed each spring; concentrations > 6.7 mg/L were typically samples collected soon after springtime application of synthetic fertilizer. The 17 mg/L threshold (adjusted to 15 mg/L) for well water samples is interpreted as the level above which livestock wastes dominate the N sources.     

The rupture process and tectonic implications of the great 1964 Prince William Sound earthquake

We have determined the rupture history of the March 28, 1964, Prince Williams Sound earthquake (M _w=9.2) from long-period WWSSNP-wave seismograms. Source time functions determined from the long-periodP waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. Because of the large size of this event and thus a deficiency of on-scale, digitizableP-wave seismograms, it is impossible to uniquely invert for the location of moment release. However, if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spread over the fault surface in a semi-circular fashion until the full width of the fault was activated. The rupture then extended toward the southwest approximately 300 km (Ruff andKanamori, 1983). The second moment pulse was located in the vicinity of Kodiak Island, starting at 500 km southwest of the epicenter and extending to about 600 km. Although the aftershock area extends southwest past the second moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major, but smaller, asperity in the Kodiak Island region.The zone that ruptured in the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behaved independently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 1854, 1844, and 1792. In contrast, the Prince William Sound region has much longer recurrence intervals on the order of 400–1000 years.