Characterization of earthquake sources in the Gulf of Alaska

Abstract

The characterization of earthquake sources in the Gulf of Alaska and the relative significance of earthquake sources for establishing seismic design inputs at a typical site for engineering purposes are discussed. Earthquake sources in the complex tectonic environment can be divided into two groups: (a) a subduction zone that underlies the entire region (maximum magnitude M = 8.5); and (b) individual thrust and strike‐slip faults associated with the plate motions (maximum magnitude M = 6 to 7.5). The sources of either group and individual earthquake events can be represented as planar surfaces for consistency with the physical process and a mathematically tractable computational scheme.

Although the area is very active seismically, the degree of activity of individual sources varies significantly. Therefore, even for sources with the same maximum earthquakes, different magnitudes may apply for a selected design return period. The area is considered to be a “seismic gap.”; No great earthquakes have occurred in nearly 80 years. Estimates based on a temporally varying seismic function such as the semi‐Markov model indicate that the probability of occurrence of a great earthquake in the near future is significantly higher than the average probability inferred from a statistical analysis of historical seismicity data of the entire region.

Separate attenuation relationships should be used for calculating ground motions due to earthquakes on the dipping subduction zone in the northern portion of the gulf. The dominant earthquake source for almost the entire Gulf of Alaska region is the subduction zone that contributes over 80 percent of the seismic exposure at a typical site. The dominant magnitude range is M_s = 6.5 to 7.5. “Gap filling”; earthquakes (M_s = 7.5 to 8.25) contribute a little over a third of the seismic exposure at a typical site. Deterministic assessments of ground motion values using the maximum earthquake on the subduction zone at the closest distance yield values significantly higher than those calculated for even 500‐year return periods. Estimated 100‐year return period accelerations in the area range from 180 to 340 cm/sec².     

Computerized detection of seamounts from Seasat type geodetic data

Abstract

Geoid heights and vertical deflections derived from satellite radar altimetry contain characteristic signals that may be reproduced and explained by simple models for seamount gravitation acting on the sea surface. Computer algorithms capable of automatic operation and able to detect, approximately locate, and estimate parameters constraining the shape of actual sea‐mounts were written and tested. The computer program which utilized a digital high‐pass filter combined with a roughness sensor was effective in separating the seamount produced geoid undulation/vertical deflection pattern from the remaining data track features, simultaneously detecting and locating along the track such signals. Tests of the algorithm on several SEASAT passes over known bathymetry produced mixed results. Meaningful shape constraints were obtained by matching the geoid anomaly calculated from the seamount model to the actual mean sea level pattern for some seamounts. Results for other seamounts were poor and possible reasons for the failure are discussed. It is concluded that a computerized seamount detection program for radar altimetry data is feasible, but it will have to be more complex than the present one for fully successful operation.     

阿拉斯加湾逆温现象的时空变化特征

基于阿拉斯加湾(Gulf of Alaska, GOA)2002—2020年的Argo浮标数据,本研究分析了该海湾逆温现象的时空变化特征,并探讨其年际变化与水体垂向位温异常的联系。结果显示,阿拉斯加湾逆温现象存在明显的时空变化。空间上,逆温幅度(ΔT)在湾北部最大,湾东南部次之,湾西南部最弱。逆温厚度(ΔD)在湾西南部最厚,湾北部和东南部依次变薄。在季节尺度上,湾北部逆温现象的季节变化最明显,冬季ΔT最大和ΔD最厚,随季节性的加热和混合作用,ΔT持续减弱,ΔD不断变薄。在湾东南部,ΔT分别在3月和9月具有0.46、0.40℃的峰值;ΔD在整个海湾中最薄,冬季最大为44 m,而秋季最小仅为23 m。在湾西南部,ΔT介于0.24～0.33℃之间,分别于4月和10月具有0.31、0.33℃的峰值;ΔD冬季最大超过100 m,秋季最薄约为57 m。年际变化上,ΔT在2002、2007—2009、2012、2017年表现为正异常,但在2003—2005、2010、2013—2016、2018—2020年表现为负异常;ΔD在2002、2007—2008、2012、2017—2020年主要偏厚,在2...     

Northern Gulf of Alaska eddies and associated anomalies

In recent years, large anticyclonic eddies have been observed quasi-annually in the region seaward of Kodiak Island, Alaska. In situ sampling in 3 of these eddies was undertaken in 2002, 2003, and 2004. Satellite altimetry data showed that these 3 eddies had 3 different formation regions but their translation pathways were similar near Kodiak Island. Eddies in this region can persist for several years, moving southwestward along the Alaskan Peninsula to the Aleutian Archipelago. Water properties in the cores of the 2003 and 2004 eddies were significantly different from each other, probably because the 2003 eddy formed on the shelf near Yakutat while the 2004 eddy formed farther out in the basin in the northern Gulf of Alaska. Calculation of heat, salinity, and nutrient anomalies associated with the eddies showed that, in their subsurface core waters, the eddies carry excess heat, salt, nitrate and silicic acid seaward from the eddy formation regions.     

Modeling iron limitation of primary production in the coastal Gulf of Alaska

A lower trophic level NPZD ecosystem model with explicit iron limitation on nutrient uptake is coupled to a three-dimensional coastal ocean circulation model to investigate the regional ecosystem dynamics of the northwestern coastal Gulf of Alaska (CGOA). Iron limitation is included in the NPZD model by adding governing equations for two micro-nutrient compartments: dissolved iron and phytoplankton-associated iron. The model has separate budgets for nitrate (the limiting macro-nutrient in the standard NPZD model) and for iron, with iron limitation on nitrate uptake being imposed as a function of the local phytoplankton realized Fe:C ratio. While the ecosystem model represents a simple approximation of the complex lower trophic level ecosystem of the northwestern CGOA, simulated chlorophyll concentrations reproduce the main characteristics of the spring bloom, high shelf primary production, and “high-nutrient, low-chlorophyll” (HNLC) environment offshore. Over the 1998–2004 period, model-data correlations based on spatially averaged, monthly mean chlorophyll concentrations are on average 0.7, with values as high as 0.9 and as low as 0.5 for individual years. The model also provides insight on the importance of micro- and macro-nutrient limitation on the shelf and offshore, with the shelfbreak region acting as a transition zone where both nitrate and iron availability significantly impact phytoplankton growth. Overall, the relative simplicity of the ecosystem model provides a useful platform to perform long-term simulations to investigate the seasonal and interannual CGOA ecosystem variability, as well as to conduct sensitivity studies to evaluate the robustness of simulated fields to ecosystem model parameterization and forcing. The ability of the model to differentiate between nitrate-limited, and iron-limited growth conditions, and to identify their spatial and temporal occurrences, is also a first step towards understanding the role of environmental gradients in shaping the complex CGOA phytoplankton community structure.     

Intrinsic and forced interannual variability of the Gulf of Alaska mesoscale circulation

The response of the Gulf of Alaska (GOA) circulation to large-scale North Pacific climate variability is explored using three high resolution (15 km) regional ocean model ensembles over the period 1950-2004. On interannual and decadal timescales the mean circulation is strongly modulated by changes in the large scale climate forcing associated with PDO and ENSO. Intensification of the model gyre scale circulation occurs after the 1976-1977 climate shift, as well as during 1965-1970 and 1993-1995. From the model dynamical budgets we find that when the GOA experiences stronger southeasterly winds, typical during the positive phase of the PDO and ENSO, there is net large-scale Ekman convergence in the central and eastern coastal boundary. The geostrophic adjustment to higher sea surface height (SSH) and lower isopycnals lead to stronger cyclonic gyre scale circulation. The opposite situation occurs during stronger northwesterly winds (negative phase of the PDO).Along the eastern side of the GOA basin, interannual changes in the surface winds also modulate the seasonal development of high amplitude anticyclonic eddies (e.g. Haïda and Sitka eddies). Large interannual eddy events during winter-spring, are phase-locked with the seasonal cycle. The initial eddy dynamics are consistent with a quasi-linear Rossby wave response to positive SSH anomalies forced by stronger downwelling favorable winds (e.g. southwesterly during El Niño). However, because of the fast growth rate of baroclinic instability and the geographical focusing associated with the coastal geometry, most of the perturbation energy in the Rossby wave is locally trapped until converted into large scale nonlinear coherent eddies. Coastally trapped waves of tropical origin may also contribute to positive SSH anomalies that lead to higher amplitude eddies. However, their presence does not appear essential. The model ensembles, which do not include the effects of equatorial coastally trapped waves, capture the large Haïda and Sitka eddy events observed during 1982 and 1997 and explain between 40% and 70% of the tidal gauges variance along the GOA coast.In the western side of the GOA basin, interannual eddy variability located south of the Alaskan Stream is not correlated with large scale forcing and appears to be intrinsic. A comparison of the three model ensembles forced by NCEP winds and a multi-century-long integration forced only with the seasonal cycle, shows that the internal variability alone explains most of the eddy variance. The asymmetry between the eddy forced regime in the eastern basin, and the intrinsic regime in the western basin, has important implications for predicting the GOA response to climate change. If future climate change results in stronger wintertime winds and increased downwelling in the eastern basin, then increased mesoscale activity (perhaps more or larger eddies) might occur in this region. Conversely, the changes in the western basin are not predictable based on environmental forcing. Eastern eddies transport important biogeochemical quantities such as iron, oxygen and chlorophyll-a into the gyre interior, therefore having potential upscale effects on the GOA high-nutrient-low-chlorophyll region.     

Slope instability near the shelf break,Western Gulf of Alaska*

Abstract

The uppermost continental slope in the western Gulf of Alaska, from southern Albatross Bank to Portlock Bank, includes two broad areas where large submarine landslides occur and one intervening area where they are absent. In the areas containing large slides, seismic reflection records show evidence for active nearsurface folding and consequent slope steepening, which is apparently the ultimate control on this sliding. Evidence is lacking for similar active steepening in the area containing no large slides, where slope gradients are relatively gentle. Relatively small, shallow slides, fundamentally different from the larger ones, occur in all three areas on slopes that are not necessarily actively steepening. These slides are probably stratigraphically controlled, with failure occurring along weak subsurface strata. Strong earthquakes and the related accelerations are probably responsible for the actual triggering of many of the large and small slides. As long as the tectonic setting remains as it is today, future large‐scale sliding should remain confined to the two broad areas in which it now exists. Relatively small‐scale and shallow sliding might occur in any of the three areas.     

Coupling global and regional circulation models in the coastal Gulf of Alaska

As part of the US GLOBEC NE Pacific program, we are simulating currents in the Coastal Gulf of Alaska (CGOA) to explore sources of interannual and interdecadal variability. To do so, we have developed a coupled modeling system composed of linked regional and global circulation models. The regional model, configured with 13–22 km resolution in the CGOA, is forced at the surface by observed heat fluxes and wind stresses, at the continental boundaries by observed runoff, and at the open ocean boundaries by a combination of tracer climatologies and sub-tidal velocity and tidal elevation provided by a global finite element model. In this communication, we describe the coupled system, including its present method of intermodel coupling, describe a series of multi-year model hindcasts, compare hindcast results with Eulerian and Lagrangian field data obtained in the CGOA in fall 1996, and assess the impact of global information (barotropic sub-tidal velocities and tidal elevations) on the regional model under the present coupling strategy. We find that the regional model produces appropriate current systems (Alaskan Stream, Alaska Coastal Current) and scalar fields, but with mesoscale variability (of SSH and velocities) at somewhat reduced strength relative to data, and with temperature gradients somewhat larger than those observed. Barotropic sub-tidal information from the global model penetrates the regional model interior, supplying additional mesoscale variability, and modifying regional velocity and scalar fields in both shallow and deep areas. Tidal information exerts a significant influence on sub-tidal scalar and velocity structure only in specific shallow areas, where the tides (and tidal mixing) are strongest. Pending the exploration of alternate coupling schemes, we infer from these results that on a time scale of months, purely barotropic information from outside the CGOA will have a modest impact on its mean regional circulation, but a potentially stronger impact on the statistics and details of mesoscale eddies.     

Reactive iron delivery to the Gulf of Alaska via a Kenai eddy

Mesoscale anticyclonic eddies in the Gulf of Alaska are an important mechanism for cross-shelf exchange of high iron, low nitrate coastal waters and low iron, high nitrate offshore waters. A Kenai eddy was sampled in September 2007, 8 months after formation. The subsurface eddy core layer contained reactive iron concentrations more than eight times greater than waters at the same depths outside the eddy. The subsurface core of the Kenai eddy (25.4≤σ_θ≤25.8) is suggested to be seasonally important as these waters can be brought to the surface with storm events and deep winter mixing. The deeper core layer (25.8≤σ_θ≤27.0) is suggested to be a source of iron to HNLC waters on a longer timescale, due to isopycnal mixing and eventual eddy relaxation. The subsurface and deeper core layers are important reservoirs of iron that can promote and sustain primary productivity over the lifetime of the Kenai eddy. In addition, dissolved and leachable particulate manganese are shown to be excellent tracers of eddy surface and subsurface waters, respectively.     

Depth-stratified community zonation patterns on Gulf of Alaska rocky shores

Vertical zonation patterns have been considered ubiquitous in intertidal ecosystems but questions remain about their generality for individual taxonomic groups and over broad spatial scales, and whether they continue into adjacent shallow subtidal habitats. Taxon richness, invertebrate abundance, and macroalgal biomass were examined in the summer of 2003 along a vertical gradient in the rocky intertidal and shallow subtidal habitats around Kodiak Island, Kachemak Bay, and Prince William Sound, all within the Gulf of Alaska. Replicate samples of benthic organisms were taken in the high (～ 7 m), mid (～ 4 m) and low (～ 0 m) intertidal (relative to MLLW), and at 1, 5, 10 and 15 m water depths at three sites in each region, and identified to the lowest possible taxonomic level. Our primary goals were to assess (1) how estimates of taxon richness, invertebrate abundance, and macroalgal biomass vary among intertidal heights and subtidal depths and (2) how general these patterns are when considered across the Gulf of Alaska. Our results show that when all invertebrates were considered together, most of the variation in taxon richness was accounted for by differences among depths (i.e. intertidal heights and subtidal depths) (～ 51%), and among replicate samples within each depth (～ 26%). Little to none of the variation was accounted for by differences among sites within each region (～ 1%) or among regions themselves (～ 0%). When considered across the Gulf of Alaska, total taxon richness and organism abundance were greatest in the low intertidal/shallow subtidal and decreased with increasing height/depth. When separated by phylum and examined together with macroalgae, variation in abundance and/or biomass among depths was significant and accounted for most of the variability. Differences among regions and sites within each region were not significant and accounted for little to none of the variance. Because the pattern of zonation varied among sites within each region, it reduced the generality of a single zonation pattern for the Gulf of Alaska. Likewise, when community composition was compared among depths, geographic regions and sites within each region using multivariate analyses, vertical zonation patterns were evident at a regional scale, but high variability in these patterns among sites within each region reduced the generality of these patterns.     

On flow in Northwestern Gulf of Alaska,May 1972

Southwestward volume transport (referred to 1,500 db) out of the Gulf of Alaska seaward of the continental shelf in May 1972 was 12.5 Sv, and nearly 3/4 of this flow occurred within 50 km of the shelf edge. Mean geostrophic velocities of about 50 cm s^–1 occurred in a band 20 km wide, which extended 500 km along the shelf edge; a maximum velocity of 98 cm s^–1 (nearly 2 knots) was obtained. Bottom flow along the inshore part of the shelf as determined by seabed drifters was generally onshore at 0.5 cm s^–1. Evidence is presented of a large cyclonic gyre on the shelf encompassing the Portlock and Albatross Banks, perturbations in surface flow along the shelf edge, and relations between coastal tidal heights and fluctuations in geopotential topography at the shelf edge.     

Mesoscale eddies dominate surface phytoplankton in northern Gulf of Alaska

The HNLC waters of the Gulf of Alaska normally receive too little iron for primary productivity to draw down silicate and nitrate in surface waters, even in spring and summer. Our observations of chlorophyll sensed by SeaWiFS north of 54°N in pelagic waters (>500 m depth) of the gulf found that, on average, more than half of all surface chlorophyll was inside the 4 cm contours of anticyclonic mesoscale eddies (the ratio approaches 80% in spring months), yet these contours enclosed only 10% of the total surface area of pelagic waters in the gulf. Therefore, eddies dominate the chlorophyll and phytoplankton distribution in surface pelagic waters. We outline several eddy processes that enhance primary productivity. Eddies near the continental margin entrain nutrient - (and Fe) - rich and chlorophyll-rich coastal waters into their outer rings, advecting these waters into the basin interior to directly increase phytoplankton populations there. In addition, eddies carry excess nutrients and iron in their core waters into pelagic regions as they propagate away from the continental margin. As these anticyclonic eddies decay, their depressed isopycnals relax upward, injecting nutrients up toward the surface layer. We propose that this transport brings iron and macro-nutrients toward the surface mixed layer, where they are available for wind-forced mixing to bring them to surface. These mesoscale eddies decay slowly, but steadily, perhaps providing a relatively regular upward supply of macro-nutrients and iron toward euphotic layers. They might behave as isolated oases of enhanced marine productivity in an otherwise iron-poor basin. We note that much of this productivity might be near or just below the base of the surface mixed layer, and therefore poorly sampled by colour-sensing satellites. It is possible, then, that eddies enrich phytoplankton populations to a greater extent than noted from satellite surface observations only.     

A synoptic survey of young mesoscale eddies in the Eastern Gulf of Alaska

Eddies in the Gulf of Alaska are important sources of coastal water and associated nutrients, iron, and biota to the high-nutrient, low-chlorophyll central Gulf of Alaska. Three primary eddy formation regions along the eastern boundary of the gulf have been identified, (from south to north, Haida, Sitka, and Yakutat). In the spring of 2005, three eddies (one of each type) were sampled soon after their formation. The subsurface eddy core water in all three eddies was defined by high iron concentrations and low dissolved oxygen compared with surrounding basin water. The Sitka and Yakutat core waters also exhibited a subsurface temperature maximum (mesothermal water) coincident in depth with the iron maximum, suggesting that eddies may play a role in the formation of temperature inversions observed throughout the Gulf of Alaska. The data suggest different formation regions, with the Yakutat eddy forming in shallow shelf water with riverine input, while the Sitka and Haida eddies appear to form in deeper water.     

Modeled transport of freshwater from a line-source in the coastal Gulf of Alaska

A set of multiply nested atmospheric (The Penn State/NCAR Mesoscale Modeling system—MM5) and oceanic (Regional Ocean Modeling System—ROMS) models has been developed to investigate ecosystem forcing as part of the US. GLOBEC program. This study focuses on the most finely nested oceanic model in the hierarchy, that of the coastal Gulf of Alaska (CGOA) during 2001–2002, and compares the model's results to data collected by GLOBEC investigators. The 3-km resolution model realistically generates two physical features needed to reproduce the CGOA ecosystem: the cross-shelf water mass structure on the Seward Shelf, and the seasonal cycle of vertical structure. In addition, the temporal variability of currents and tracer fields generated by the model is greatly improved compared to previous work, as is the resolution of the Alaska Coastal Current (ACC). However, the treatment of the line-source freshwater source along the coast of Alaska still presents difficulties, because the model cannot resolve the many inlets and fjords where mixing takes place initially. This issue is investigated by testing the model's sensitivity to various forcing mechanisms which could compensate for this weakness, such as the addition of tidal mixing, the use of finely resolved winds, and the use of brackish runoff rather than purely freshwater for the line-source.     

Multiple late Quaternary episodes of exceptional diatom production in the Gulf of Alaska

Hole 887B of the Ocean Drilling Program (ODP) comprises a 44 m (750 kyr) long continuous section recovered from the Patton–Murray Rise, an elevated plateau that is largely isolated from turbidite deposition. The Patton–Murray area is centered under the Alaska Gyre, a region characterized by the domal upwelling of nutrient-rich waters. Marked increases in productivity and rapid settling of biogenic matter are suggested throughout the section by the episodic accumulation of diatomaceous oozes up to ∼1 m thick that are accompanied by barium enrichments. Significant δ¹³C_org maxima in the major diatomaceous bands suggest that mixed-layer [CO_2(aq)] may have been drawn down significantly during some of the productivity events. The episodes of enhanced productivity at Site 887 occur synchronously with short-lived minima in planktonic foram δ¹⁸O, suggesting a direct link to low salinity, or less likely, warming, events in the Gulf of Alaska. There is no obvious explanation for the events, but they may be related to seasonal incursions of meltwater from Alaska. We speculate that episodic input of meltwater- or dust-borne iron from Asian or Alaskan sources may have promoted the extraordinary diatom production events recorded in the sedimentary record.     

Mixing and biological production at eddy margins in the eastern Gulf of Alaska

We performed a multi-day shipboard experiment in June 2001 to test whether combining water from within an anticyclonic mesoscale eddy in the eastern Gulf of Alaska with water from outside could result in enhanced phytoplankton growth and to determine how mixing might influence planktonic assemblages. Initially, the eddy had lower standing stocks of algal pigments (chlorophyll a [chl a] and accessory pigments), nutrients, phytoplankton, and particulate organic carbon/nitrogen compared to waters outside of the eddy. The eddy possessed a greater diversity and abundance of coastal diatoms while the outside waters had a greater proportion of oceanic species, including the endemic pennate diatom, Nitzschia cylindroformis. After one week of incubation, rates of primary production were significantly higher in the mixed water compared to both the eddy and outside treatments. Pigment concentrations (except chl c₃, alloxanthin, and zeaxanthin) and the proportion of large diatoms (mainly Pseudo-nitzschia spp.) and heterotrophic dinoflagellates were greater in the mixed water than would be expected from the simple combination of inside and outside waters. Nutrient limitation (most likely by trace metals) appeared to be less severe in the mixed water. Chl a was enhanced in the mixed water, particularly when compared to the eddy water. The mixing of eddy and outside water masses stimulated primary production by ∼20%, but more importantly, the mixing resulted in a distinct planktonic assemblage. The biomass enrichment was short-lived, indicating that the maintenance of elevated chl a would require further mixing events in a physical setting that also permits an accumulation of biomass. We note that submesoscale processes, including the intensification of ageostrophic circulation that elicits strong vertical mixing in the presence of strain, might explain observed patterns of high phytoplankton standing stocks at the inner edges of Haida eddies in the field.     

Submarine faults and slides on the continental shelf,northern Gulf of Alaska

Abstract

Submarine faults and slides or slumps of Quaternary age are potential environmental hazards on the outer continental shelf (OCS) of the northern Gulf of Alaska. Most faults that approach or reach the seafloor cut strata that may be equivalent in age to the upper Yakataga Formation (Pliocene‐Pleistocene). Along several faults, the seafloor is vertically offset from 5 to 20 m. A few faults appear to cut Holocene sediments, but none of these shows displacement at the seafloor. Submarine slides or slumps have been found in two places in the OCS region: (1) seaward of the Malaspina Glacier and Icy Bay, an area of 1200 km² with a slope of less than 0.5°, and (2) across the entire span of the Copper river prodelta, an area of 1730 km², having a slope of about 0.5°. Seismic profiles across these areas show disrupted reflectors and irregular topography commonly associated with submarine slides or slumps. Potential slide or slump areas have been delineated in areas of thick sediment accumulation and relatively steep slopes. These areas include (1) Kayak Trough, (2) parts of Hinchinbrook Entrance and Sea Valley, (3) parts of the outer shelf and upper slope between Kayak Island and Yakutat Bay, and (4) Bering Trough.     

Simulated eddy induced vertical velocities in a Gulf of Alaska model

Mesoscale eddies can distribute nutrients, heat and fresh water into the Gulf of Alaska (GOA) from the coastal margins. While many studies have investigated the physical characteristics of GOA eddies, their effects on passive-dispersive particles have not been previously simulated to investigate eddy induced upwelling. A climatologically forced Parallel Ocean Program simulation of the north Pacific Ocean with an online particle tracking scheme was used to simulate passive-dispersive particles in the Gulf of Alaska. In-eddy vertical Lagrangian velocities of the particles were calculated both inside and outside the eddies and showed upwelling rates are generally greater inside the eddies where the vertical velocities of the particles ranged from 0.2 to 0.7 m/day.     

Scientific inference and experiment in Ecosystem Based Fishery Management,with application to Steller sea lions in the Bering Sea and Western Gulf of Alaska

Learning about ecosystem processes and patterns is an essential component of Ecosystem Based Fishery Management and the sustainable use of natural resources.