Central Aleutian Islands Research Articles

Evaluating Network Analysis Indicators of Ecosystem Status in the Gulf of Alaska

This is the first study on the emergent properties for empirical ecosystem models that have been validated by time series information. Ecosystem models of the western and central Aleutian Islands and Southeast Alaska were used to examine indices of ecosystem status generated from network analysis and incorporated into Ecopath with Ecosim. Dynamic simulations of the two ecosystems over the past 40 years were employed to examine if these indices reflect the dissimilar changes that occurred in the ecosystems. The results showed that the total systems throughput (TST) and ascendancy (A) followed the climate change signature (Pacific decadal oscillation, PDO) in both ecosystems, whereas the redundancy (R) followed the inverse trend. The different trajectories for important species such as Steller sea lions (Eumetopias jubatus), Atka mackerel (Pleurogrammus monopterygius), pollock (Theragra chalcograma), herring (Clupea pallasii), Pacific cod (Gadus macrocephalus) and halibut (Hippoglossus stenolepis) were noticeable in the Finn cycling index (FCI), entropy (H) and average mutual information (AMI): not showing large change during the time that the Stellers sea lions, herring, Pacific cod, halibut and arrowtooth flounder (Atheresthes stomias) increased in Southeast Alaska, but showing large declines during the decline of Steller sea lions, sharks, Atka mackerel and arrowtooth flounder in the Aleutians. On the whole, there was a change in the emergent properties of the Aleutians around 1976 that was not seen in Southeast Alaska. Conversely, the emergent properties of both systems showed a change around 1988, which indicated that both systems were unstable after 1988.

At-sea and on-shore cycles of juvenile Steller sea lions (Eumetopias jubatus) derived from satellite dive recorders: A comparison between declining and increasing populations

Abstract We calculated the durations of time on-shore and at-sea for juvenile Steller sea lions (Eumetopias jubatus) using satellite dive recorders deployed between 2000 and 2002, and compared two genetically distinct populations; one increasing (eastern stock; n=42) and one that experienced an 80% decline in population since the mid-1970s (western stock; n=89). Data represented 24-h periods divided into 72 20-min increments indicating whether an animal was on-shore (dry) or at-sea (wet). Time apportioned between land and sea was described on a per-trip basis (rather than a 24-h cycle) and durations ranged from 20 min to several days. We tested differences in the durations of on-shore and at-sea events among sex, geographic region, year, and age at capture using mixed-effects models. Animal identifier was included as a random effect to account for repeated measures on the same individual. Sea lions from the eastern Aleutian Islands, central Aleutian Islands, and central Gulf of Alaska hauled out just after sunrise, and departure times coincided with dusk. For Prince William Sound and Southeast Alaska animals, arrivals and departures occurred throughout the day and were not related to crepuscular period. Mean duration on-shore did not differ among sex, region, year or age, and was unrelated to previous trip duration. This may suggest a minimum rest period for juvenile Steller sea lions or that dependant animals are maximizing their time on-shore suckling. Time spent at-sea varied among individuals from both populations and development of maternal independence, inferred from significant increases in time spent at sea, occurred approximately 10 months later in individuals from Prince William Sound and Southeast Alaska than in the other regions, suggesting environmental and developmental differences among regions.

Abundance, trends and distribution of baleen whales off Western Alaska and the central Aleutian Islands

Large whales were extensively hunted in coastal waters off Alaska, but current distribution, population sizes and trends are poorly known. Line transect surveys were conducted in coastal waters of the Aleutian Islands and the Alaska Peninsula in the summer of 2001–2003. Abundances of three species were estimated by conventional and multiple covariate distance sampling (MCDS) methods. Time series of abundance estimates were used to derive rates of increase for fin whales ( Balaenoptera physalus) and humpback whales ( Megaptera novaeangliae). Fin whales occurred primarily from the Kenai Peninsula to the Shumagin Islands, but were abundant only near the Semidi Islands and Kodiak. Humpback whales were found from the Kenai Peninsula to Umnak Island and were more abundant near Kodiak, the Shumagin Islands and north of Unimak Pass. Minke whales ( B. acutorostrata) occurred primarily in the Aleutian Islands, with a few sightings south of the Alaska Peninsula and near Kodiak Island. Humpback whales were observed in large numbers in their former whaling grounds. In contrast, high densities of fin whales were not observed around the eastern Aleutian Islands, where whaling occurred. Average abundance estimates (95% CI) for fin, humpback and minke whales were 1652 (1142–2389), 2644 (1899–3680), and 1233 (656–2315), respectively. Annual rates of increase were estimated at 4.8% (95% CI=4.1–5.4%) for fin and 6.6% (5.2–8.6%) for humpback whales. This study provides the first estimate of the rate of increase of fin whales in the North Pacific Ocean. The estimated trends are consistent with those of other recovering baleen whales. There were no sightings of blue or North Pacific right whales, indicating the continued depleted status of these species.

BLUE WHALE (BALAENOPTERA MUSCULUS) SIGHTINGS AND RECORDINGS SOUTH OF THE ALEUTIAN ISLANDS

Blue whales have been sighted throughout the North Pacific Ocean (Mizroch et al. 1984), and were harvested around the Pacific Rim from Mexico to Japan (Fig. 1). The International Whaling Commission (IWC) currently recognizes only one stock of blue whales in the North Pacific (Donovan 1991); however, there may be up to five (reviewed in Stafford 2003). While past shipboard and aerial surveys in the Aleutian Island chain found no blue whales (Rice and Wolman 1982, Stewart et al. 1987), their contemporary presence in this area has been well documented by acoustic monitoring (Watkins et al. 2000, Stafford et al. 2001). In this note we present the results of a shipboard survey during which blue whales were seen south of the Aleutian Islands and recordings of blue whale vocalizations were obtained. Blue whale calls vary geographically (Thompson et al. 1996), and these geographic differences may be useful in discriminating different populations of blue whales. Two geographically and spectrographically distinct blue whale call types have been recorded in the North Pacific. The northeastern Pacific blue whale call type is well known and has been recorded from locations in the northeast Pacific, ranging from the equator along the west coast of North America up to the Gulf of Alaska (Stafford 2003). The northwestern Pacific blue whale call type is less well known, but has been recorded from remote hydrophones near Kaneohe, Hawaii, Midway Island, and locations along the Aleutian Islands and northwest Pacific and Gulf of Alaska (Northrup et al. 1971, Thompson and Friedl 1982, Stafford et al. 2001, Stafford 2003). This call type is recorded annually from June through December by seafloor hydrophones just south of the central Aleutian Islands (Stafford et al. 2001). Prior to this report, the western calls had never been visually linked to a blue whale. The calls have nevertheless been attributed to blue whales based on their similarity to other blue whale calls worldwide: they are long (>10 s), low-frequency (fundamental <100 Hz), frequency-modulated (FM) tonal calls (Thompson et al. 1996, Stafford et al. 2001, Stafford 2003, Rankin et al. 2005). Although fin whale calls are also low

RISK OF EXTIRPATION OF STELLER SEA LIONS IN THE GULF OF ALASKA AND ALEUTIAN ISLANDS: A POPULATION VIABILITY ANALYSIS BASED ON ALTERNATIVE HYPOTHESES FOR WHY SEA LIONS DECLINED IN WESTERN ALASKA

AbstractWe estimated the risk that the Steller sea lion will be extirpated in western Alaska using a population viability analysis (PVA) that combined simulations with statistically fitted models of historical population dynamics. Our analysis considered the roles that density‐dependent and density‐independent factors may have played in the past, and how they might influence future population dynamics. It also established functional relationships between population size, population growth rate and the risk of extinction under alternative hypotheses about population regulation and environmental variability. These functional relationships can be used to develop recovery criteria and guide research and management decisions. Life table parameters (e.g., birth and survival rates) operating during the population decline (1978–2002) were estimated by fitting simple age‐structured models to time‐series of pup and non‐pup counts from 33 rookeries (subpopulations). The PVA was carried out by projecting all 33 subpopulations into the future using these estimated site‐specific life tables (with associated uncertainties) and different assumptions about carrying capacities and the presence or absence of density‐dependent population regulation. Results suggest that the overall predicted risk of extirpation of Steller sea lions as a species in western Alaska was low in the next 100 yr under all scenarios explored. However, most subpopulations of Steller sea lions had high probabilities of going extinct within the next 100 yr if trends observed during the 1990s were to continue. Two clusters of contiguous subpopulations occurring in the Unimak Pass area in the western Gulf of Alaska/eastern Aleutian Islands and the Seguam–Adak region in the central Aleutian Islands had relatively lower risks of extinction. Risks of extinction for a number of subpopulations in the Gulf of Alaska were reduced if the increases observed since the late 1990s continue into the future. The risks of subpopulations going extinct were small when density‐dependent compensation in birth and survival rates was assumed, even when random stochasticity in these vital rates was introduced.

Population genetic structure and conservation of marbled murrelets (Brachyramphus marmoratus)

Marbled murrelets (Brachyramphus marmoratus) are coastal seabirds that nest from California to the Aleutian Islands. They are declining and considered threatened in several regions. We compared variation in the mitochondrial control region, four nuclear introns and three microsatellite loci among194 murrelets from throughout their range except Washington and Oregon. Significant population genetic structure was found: nine private control region haplotypes and three private intron alleles occurred at high frequency in the Aleutians and California; global estimates of FST or ΦST and most pairwise estimates involving the Aleutians and/or California were significant; and marked isolation-by-distance was found. Given the available samples, murrelets appear to comprise five genetic management units: (1) western Aleutian Islands, (2) central Aleutian Islands, (3) mainland Alaska and British Columbia, (4) northern California, and (5) central California.

Seabird distribution, abundance and diets in the eastern and central Aleutian Islands

AbstractWe examined the hypothesis that seabird distribution, abundance and diets differ among the eastern and central Aleutian Islands in response to distinct marine environments and energy pathways in each region. Research cruises were conducted in June 2001 and May–June 2002. We determined the distribution, abundance, diet and prey consumption of seabirds, and related these to zooplankton abundance and water masses that possess different physical properties. We found that distribution, abundance and diets of seabirds could be partitioned into two regions that correspond to marine environments determined by the extent of the Alaska Coastal Current along the eastern and central Aleutian Islands. Short‐tailed shearwaters (Puffinus tenuirostris) were the most abundant seabird in the coastal waters of the eastern Aleutian Islands, and northern fulmars (Fulmarus glacialis) were the most abundant seabird in the oceanic waters of the central Aleutian Islands. Seabird communities in the central and eastern Aleutian Islands were likely associated with different food webs. In the central Aleutian Islands, short‐tailed shearwaters and northern fulmars consumed shelf‐break species of euphausiids (Thyssanoesa longipes) and oceanic copepods (Neocalanus cristatus), respectively; in the eastern Aleutian Islands, both short‐tailed shearwaters and northern fulmars consumed shelf species of euphausiids (T. inermis). Carbon transport to seabirds was highest in Unimak and Akutan Passes where shearwaters removed large quantities of shelf euphausiids, followed by Samalga and Seguam Passes where northern fulmars removed large amounts of oceanic copepods.

Zooplankton distribution, abundance and biomass relative to water masses in eastern and central Aleutian Island passes

AbstractConductivity–temperature–depth (CTD), acoustic and net samples were taken through a series of passes in the eastern and central Aleutian Island archipelago to document regional differences in the physical and biological conditions. The water column in central passes had elevated salinity and depressed temperatures, indicating upwelling of deep water through the passes. The zooplankton community included the oceanic genera Neocalanus and Eucalanus, and the euphausiid Euphausia pacifica, all oceanic taxa. Eastern passes had significantly lower salinity and elevated temperature relative to the central passes, suggesting a more neritic environment. Calanus marshallae, Pseudocalanus, Acartia and Thysanoessa inermis, common neritic species, were observed in the eastern passes. Canonical correlation indicated that up to 50% of the observed variance in mean zooplankton abundance in the passes could be explained by salinity and temperature. Elevated sound scattering was observed in fronts and eddies in the passes, particularly at the northern ends of the passes, in association with elevated zooplankton abundance and biomass. The central passes were characterized as an oceanic environment, influenced primarily by Alaskan Stream water with its associated interzonal copepod species, while the eastern passes were influenced by the Alaska Coastal Current, which contains a mixture of oceanic and neritic zooplankton species.

Marine environment of the eastern and central Aleutian Islands

AbstractTo examine the marine habitat of the endangered western stock of the Steller's sea lion (Eumetopias jubatus), two interdisciplinary research cruises (June 2001 and May to June 2002) measured water properties in the eastern and central Aleutian Passes. Unimak, Akutan, Amukta, and Seguam Passes were sampled in both years, and three additional passes (Umnak, Samalga, and Tanaga) were sampled in 2002. In the North Pacific (and to a lesser extent in the Bering Sea), a strong front in water properties was observed near Samalga Pass in June of both years, with significantly warmer, fresher, and more nitrate‐poor water east of Samalga Pass than west of the pass. These water properties reflect differences in source waters (Alaska Coastal Current versus Alaskan Stream), mixing depth, and Bering Sea influence. Strong cross‐Aleutian gradients were also observed with warmer, fresher water on the North Pacific side of the archipelago. The nutrient content of the waters flowing through the passes, combined with the effects of mixing within the passes, influences the transport of nutrients into the Bering Sea. As water moves away from the strong mixing of the passes and becomes more stratified, phytoplankton can take advantage of the enhanced nutrient concentrations. Thus, the northern side of the Aleutian Islands (especially in the lee of the islands) appears to be more productive. Combined with evidence of coincident changes in many ecosystem parameters near Samalga Pass, it is hypothesized that Samalga Pass forms a physical and biogeographic boundary between the eastern and central Aleutian marine ecosystems.

Do patterns of Steller sea lion (Eumetopias jubatus) diet, population trend and cetacean occurrence reflect oceanographic domains from the Alaska Peninsula to the central Aleutian Islands?

AbstractShipboard surveys were conducted along the Aleutian Islands in 2001 and 2002 to assess the influence of a suite of biophysical parameters on regional patterns in the distribution of cetaceans and Steller sea lions (SSL; Eumetopias jubatus). Distributions of four large whale species: fin (Balaenoptera physalus), humpback (Megaptera novaeangliae), minke (B. acutorostrata) and sperm (Physeter macrocephalus) aligned with proposed metapopulation breaks in diet and population trend of SSLs. Dall's porpoise (Phocoenoides dalli) and killer whales (Orcinus orca) were widely distributed throughout the study area, and killer whales were particularly prevalent along the north Aleutian Island coastlines between Unimak Pass and Samalga Pass. Biopsies determined that most killer whales (92%) were of the piscivorous (resident) ecotype as opposed to the mammal‐eating (transient) ecotype observed in 2002 only. Generalized additive models (GAMs) were used to explore relationships between these multispecies patterns in distribution, oceanographic variables (salinity, temperature, fluorescence and depth) and proximity to six Aleutian passes. The GAMs indicated the best‐fit models and most significant correlations as determined by the Akaike function and Cp‐statistics were: depth and proximity to the nearest measured pass for SSLs and all cetaceans, respectively; frequencies of herring and salmon in SSL diet with population trend; fluorescence in the top 50 m with occurrence of humpback, minke, and killer whales; and surface temperature with occurrence of humpback, killer, and sperm whales. Results of the GAM analyses suggest foci for future investigation of relationships between physical variables and interspecific patterns of marine mammal distribution.

Composition and structure of the central Aleutian island arc from arc‐parallel wide‐angle seismic data

New results from wide‐angle seismic data collected parallel to the central Aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry. The 3‐D ray tracing/2.5‐D inversion of this sparse wide‐angle data set, which incorporates independent phase interpretations and new constraints on shallow velocity structure, produces a faster and smoother result than a previously published velocity model. Middle‐crustal velocities of 6.5–7.3 km/s over depths of ∼10–20 km indicate an andesitic to basaltic composition. High lower‐crustal velocities of 7.3–7.7 km/s over depths of ∼20–35 km are interpreted as ultramafic‐mafic cumulates and/or garnet granulites. The total crustal thickness is 35–37 km. This result indicates that the Aleutian island arc has higher velocities, and thus more mafic compositions, than average continental crust, implying that significant modifications would be required for this arc to be a suitable building block for continental crust. Lateral variations in average crustal velocity (below 10 km) roughly correspond to trends in major element geochemistry of primitive (Mg # &gt; 0.6) lavas. The highest lower‐crustal velocities (and presumably most mafic material) are detected in the center of an arc segment, between Unmak and Unalaska Islands, implying that arc segmentation exerts control over crustal composition.

Lopholiparis flerxi: A New Genus and Species of Snailfish (Scorpaeniformes: Liparidae) from the Aleutian Islands, Alaska

A new genus and species of the scorpaeniform family Liparidae, Lopholiparis flerxi, is described from the central Aleutian Islands, Alaska. The new genus differs from all known liparid genera in having supraorbital ridges, produced by enlarged and heavily ossified frontals and sphenotics, and enlarged opercles. It also has seven spinous dorsal-fin rays, the anteriormost six forming a distinct lobe; an elliptical pupil; and low dorsal- and anal-fin ray counts. It is most similar to Temnocora candida, which also possesses an anterior spinous dorsal-fin lobe and an elliptical pupil.

Identification of parasitoses in a child burial from Adak Island (Central Aleutian Islands, Alaska)

Bothriocephalid ( Diphyllobothrium pacificum) and Ascarid ( Ascaris lumbricoides) eggs have been identified in a sample taken in the abdominal cavity of a child skeleton found in Zeto Point (ADK-011), an archaeological site on Adak Island in the Central Aleutian Islands (Alaska).

STABLE ISOTOPE ANALYSES OF TOOTH ANNULI REVEAL TEMPORAL DIETARY RECORDS: AN EXAMPLE USING STELLER SEA LIONS

AbstractStable isotope analysis of teeth of marine mammals can provide valuable information on trophic level and source of feeding. However, the isotopic analysis of whole teeth presents only an average dietary estimate for individuals across the period of growth of that tooth. While such analyses can be valuable, particularly in the case of fossil material, in contrast, isotopic analysis of individual annuli of teeth can provide dietary information for each year of tooth growth, in some cases representing the whole of the animal's life. We measured stable‐carbon isotope ratios (13C/12C) in the inorganic (hydroxyapatite) and stable‐nitrogen isotope ratios (15N/14N) in the organic (primarily collagenous) components of individual tooth annuli of 18 male Steller sea lions (Eumetopias jubatus) obtained from archived collections from the Bering Sea and Gulf of Alaska and from single northern fur seals (Callorbinus ursinus) and northern elephant seals (Mirounga angustirostris) from the central Aleutian Islands and eastern Gulf of Alaska, respectively. In several individuals, we detected considerable variation in stable isotope values among annuli, up to 6.1%O for δ15N and 5.1%O for δ13C values. Enrichment in δ15N and depletion of δ13C values in the first annulus may correspond to dietary inputs from mother's milk during the period of suckling. Other variations among years may be caused by dietary changes or movements of individuals between regions differing in isotopic signatures of foodweb primary production. Our study indicates that the isotopic signatures of foodweb primary production. Our study indicates that the isotopic analysis of individual tooth annuli represents a fine‐seale tool for dietary reconstructions involving marine mammals, and cautions against the use of whole‐tooth material averaged over several annuli.

Lichens of Adak Island, Central Aleutian Islands, Alaska

Two hundred nineteen taxa of lichens including one lichen parasite are reported from Adak Island in the central Aleutian Islands, Alaska. Stereocaulon leprocephalum is new to North America; Caloplaca ferrugineofusca and Verrucaria ditmarsica are new to Alaska. The lichen flora is dominated by wide-ranging arctic-alpine and boreal species; a coastal element is moderately represented, while amphi-Beringian species form a minor element. This phytogeographic pattern is very similar to that of Attu Island, the westernmost Aleutian Island. The Aleutian Islands are a chain of mountainous volcanic islands extending in an arc 1,900 km west from the tip of the Alaska Peninsula approaching the Commander Islands of northeast Asia. Adak Island is in the Andreanof Islands group within the central Aleutian Islands. Adak, the largest (748 km2) of this island group, is dominated by rugged mountainous terrain and broad rolling lowland areas; modified volcanic cones, Mt. Moffett and Adagdak, occur in the northern part and narrow beaches are usually found adjacent to sea cliffs. As one of the central islands in the Aleutian chain, Adak Island is of considerable phytogeographic interest. Hitherto the lichen flora of Adak Island has received little attention. The only existing literature records are found in three reports that collectively report 32 species. Degelius (1937) reported lichen specimens collected by the Swedish botanist Eric Hult6n in 1932 that included 14 lichens from Adak Island. Later, Stair (1948) added five species to the flora based on a collection of W. J. Christian. Hein (1980) increased the list by 13 species. Of the species reported on Adak Island by previous authors only five, Alectoria sarmentosa (Ach.) Ach., Cetraria ciliaris Ach. (= Tuckermannopsis ciliaris), Cladonia pseudoevansii Asah. (= Cladina pseudoevansii), Pertusaria glomerata (Ach.) Schaerer, and Umbilicaria proboscidea (L.) Schrader, were not collected by the present authors. Comparative data from the nearest Aleutian Islands for which the lichen flora is relatively well known are Amchitka Island, 250 km to the west of Adak Island, and Attu Island, 650 km to the west. Thomson and Sowl (1989) reported the known total of Amchitka Island as 149 species and Talbot et al. (1991) reported 160 taxa for Attu Island. In the Tuxedni Wilderness Area of western lower Cook Inlet, Alaska, 1,800 km to the east of Adak Island, Talbot et al. (1992) reported 218 taxa. Caribou were introduced to Adak Island during the 1940's and lichens are known to be important in their diet (Sjenneberg & Slagsvold 1979; Sveinbjornsson 1990). Our qualitative observations of Adak Island lichens suggest that lichen cover may have been reduced due to grazing pressure from caribou. As a pioneering effort, the present study provides a checklist and voucher collection if future range assessment becomes necessary.

Lichens of Adak Island, Central Aleutian Islands, Alaska

Lichens of Adak Island, Central Aleutian Islands, Alaska

The influence of an Alaskan Stream eddy on flow through Amchitka Pass

Flow into the Bering Sea through Amchitka Pass in the central Aleutian Islands is shown to respond to the presence of a large meander/eddy in the Alaskan Stream south of the pass. Maximum flow into the Aleutian Basin of the Bering Sea occurs when the eddy center lies nearest to the axis of Amchitka Pass. A conceptual model of low‐frequency circulation variability within the Bering Sea, based upon results of empirical orthogonal function analyses of sea surface height anomalies measured by the Geosat radar altimeter, is presented.

The shedding of an anticyclonic eddy from the Alaskan stream as observed by the Geosat altimeter

The evolution of a meander into an anticyclonic eddy which separates from the Alaskan Stream in the vicinity of the central Aleutian Islands is observed in two years of sea surface height anomalies computed from GEOSAT Exact Repeat Mission altimeter data. The trajectory of the center of mass is estimated using a modified perpendicular bisector method. Eddy separation is inferred to occur where the trajectory makes a distinct turn to the southwest. The observed region of meander growth, and eddy separation agrees with that predicted by theory. The evolution of meander/eddy height and radius is estimated from least squares fits of the data to an axially symmetric Gaussian profile. The heights and radii are used to calculate the strength (vorticity) and energetics of the meander/eddy. Ekman dynamics are assumed to govern the eddy decay. Spin down times based on vorticity are much longer than those based on energetics. This is consistent with an eddy's ability to maintain its integrity as it decays. Representative height, diameter, and velocity scales of the eddy are characteristic of rings spawned by other current systems. Assuming a ring structure for this eddy the estimated ring volume represents a 21% transport anomaly referenced to the mean annual transport into the Bering Sea.

Seismotectonics of intermediate-depth earthquakes from properties of aftershock sequences

Aftershock sequences following two intermediate-depth earthquakes have been analyzed to determine their spatial and temporal distributions. The Kanaga Pass earthquake ( M b = 6.0) occurred at 105 km depth in the central Aleutian Islands. Data were provided by the Central Aleutians Seismic Network. The Iwaizumi Town earthquake ( m b = 6.4) occurred at 75 km depth, under northern Honshu, Japan. Data were provided by the network of Tohoku University. For both events the hypocenter distribution of the aftershocks agreed well with focal mechanism solutions derived by others, with down-dip compression for both. Both events were followed by numerous aftershocks that decayed with time initially according to the modified Omori relation, with p-value close to 1. In both cases, activity at a higher rate than that expected from the Omori relation began after several months, without a strong aftershock to trigger it. The Kanaga Pass activity is modeled by two Omori sequences, the second beginning 64 days after the mainshock. The p-values are 0.92 and 1.04, respectively. The corresponding b-values are 1.01 and 1.03. The Iwaizumi Town sequence is modeled by a single Omori sequence, p = 0.96, plus a constant rate of activity, 0.25 events/day, for 63 days, beginning 120.5 days after the mainshock. The b-value of the main sequence was 0.91 ± 0.14, up to the start of the linear surge, for which the b-value was 1.23 ± 0.47. No strong aftershock or event in the vicinity of the aftershock zone occurred to trigger the secondary activity, in either case. The cause of the secondary activity is not known. The two sequences occurred in subduction zones with similar properties, and in both cases previous seismicity shows a deficiency of strong earthquakes in the main thrust zone above the aftershock source volumes. A strong creep event, or silent earthquake, in a weakly-coupled interplate contact zone is a plausible explanation for the secondary sequences. The unusual setting of the Iwaizumi event, at the bottom of the upper plane of the double-planed Wadati-Benioff zone under Honshu, may play a role in that case.

Andesites from northeastern Kanaga Island, Aleutians

Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO2 contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO2 content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO2 contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO2 contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO2 content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO2) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with ≤63 wt % SiO2 that increase the thickness and lowers the overall SiO2 content of the upper silicic zone. It is suggested that the maximum SiO2 content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall.