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Hawaiian Volcano Observatory record book 1963, 1964, and 1965

The Hawaiian Volcano Observatory (HVO) record books are annual journals in which field observations of eruptive activity at Kīlauea and Mauna Loa volcanoes, on the Island of Hawaiʻi, were compiled by HVO staff for most years from 1912 through early 1966. In addition to descriptive observations, the record books also contain hundreds of annotated photographs and sketches, as well as temperature and transit measurements. When photographs are included, the camera settings and film types used are noted. The field notes, sketches, and photographs used to compile the record books provide an unparalleled record of eruptive activity and were the basis for published newspaper reports and periodic bulletins, such as the Hawaiian Volcano Observatory bulletins and The volcano letter . HVO staff also painstakingly prepared a second copy of each early record book, virtually identical to the original, complete with photographs and sketches. The original version of the record book was kept at HVO, while the duplicate went to the Hawaiian Volcano Research Association, which was created by a group of Honolulu, Hawaiʻi, businessmen to promote HVO’s work. The Hawaiian Volcano Research Association, which dissolved decades ago, held the duplicate books in Honolulu, where they were more readily accessible to the public. After 1923, HVO stopped duplicating the books—at least, no duplicate records for subsequent years have been found. During the decade of the 1940s, preparation of the record books stopped altogether. In 1952, compilation of the record books resumed and continued, with long gaps, until early 1966. Entries in these later volumes are sparse, and these books are little more than photo albums of the episodic eruptions of Kīlauea. The original record books were held at HVO and, later, at Hawai‘i Volcanoes National Park. In 1972, the original record books for the years 1912 through 1939 were transferred to the Bishop Museum, in Honolulu, for safekeeping. These record books are now stored in archival boxes at that institution (accession number 172.265). The duplicate set of record books held by the Hawaiian Volcano Research Association were bound into volumes at an unknown date and turned over to the U.S. Geological Survey. This set of record books, spanning 1912 through 1923, is now housed in the rare book room of the U.S. Geological Survey Library in Reston, Virginia (catalog number 220(950) H3d). The record books for 1952 through early 1966 remain in storage at HVO. After 1955, typed annotations were no longer placed in the record books, and only photographs, most with captions, were included. Although the volumes titled “Record Book” end with the 1955 volume, the photograph albums that followed are similar, and the record book designation is retained for simplicity. Note that the photographs in these albums were not all originally arranged in chronological order.

Hawaii

Hawaiian Volcano Observatory record book 1965

The Hawaiian Volcano Observatory (HVO) record books are annual journals in which field observations of eruptive activity at Kīlauea and Mauna Loa volcanoes, on the Island of Hawaiʻi, were compiled by HVO staff for most years from 1912 through early 1966. In addition to descriptive observations, the record books also contain hundreds of annotated photographs and sketches, as well as temperature and transit measurements. When photographs are included, the camera settings and film types used are noted. The field notes, sketches, and photographs used to compile the record books provide an unparalleled record of eruptive activity and were the basis for published newspaper reports and periodic bulletins, such as the Hawaiian Volcano Observatory bulletins and The volcano letter . HVO staff also painstakingly prepared a second copy of each early record book, virtually identical to the original, complete with photographs and sketches. The original version of the record book was kept at HVO, while the duplicate went to the Hawaiian Volcano Research Association, which was created by a group of Honolulu, Hawaiʻi, businessmen to promote HVO’s work. The Hawaiian Volcano Research Association, which dissolved decades ago, held the duplicate books in Honolulu, where they were more readily accessible to the public. After 1923, HVO stopped duplicating the books—at least, no duplicate records for subsequent years have been found. During the decade of the 1940s, preparation of the record books stopped altogether. In 1952, compilation of the record books resumed and continued, with long gaps, until early 1966. Entries in these later volumes are sparse, and these books are little more than photo albums of the episodic eruptions of Kīlauea. The original record books were held at HVO and, later, at Hawai‘i Volcanoes National Park. In 1972, the original record books for the years 1912 through 1939 were transferred to the Bishop Museum, in Honolulu, for safekeeping. These record books are now stored in archival boxes at that institution (accession number 172.265). The duplicate set of record books held by the Hawaiian Volcano Research Association were bound into volumes at an unknown date and turned over to the U.S. Geological Survey. This set of record books, spanning 1912 through 1923, is now housed in the rare book room of the U.S. Geological Survey Library in Reston, Virginia (catalog number 220(950) H3d). The record books for 1952 through early 1966 remain in storage at HVO. After 1955, typed annotations were no longer placed in the record books, and only photographs, most with captions, were included. Although the volumes titled “Record Book” end with the 1955 volume, the photograph albums that followed are similar, and the record book designation is retained for simplicity. Note that the photographs in these albums were not all originally arranged in chronological order.

Hawaii

Effects of CRP field age and cover type on ring-necked pheasants in eastern South Dakota

Loss of native grasslands to tillage has increased the importance of Conservation Reserve Program (CRP) grasslands to maintain ring-necked pheasant (Phasianus colchicus) populations. Despite the importance of CRP to pheasants, little is known about the effects of CRP field age and cover type on pheasant abundance and productivity in the northern Great Plains. Therefore, we assessed effects of these characteristics on pheasant use of CRP fields. We stratified CRP grasslands (n=42) by CRP stand age (old [10-13 yrs] vs. new [1-3 yrs] grasslands) and cover type (CP1 [cool-season grasslands] vs. CP2 [warm-season grasslands]) in eastern South Dakota and used crowing counts and roadside brood counts to index ring-necked pheasant abundance and productivity. Field-age and cover-type effects on pheasant abundance and productivity were largely the result of differences in vegetation structure among fields. More crowing pheasants were recorded in old cool-season CRP fields than any other age or cover type, and more broods were recorded in cool- than warm-season CRP fields. Extending existing CRP contracts another 5-10 years would provide the time necessary for new fields to acquire the vegetative structure used most by pheasants without a gap in habitat availability. Cool-season grass-legume mixtures (CP1) that support higher pheasant productivity should be given equal or higher ratings than warm-season (CP2) grass stands. We also recommend that United States Department of Agriculture administrators and field staff provide broader and more flexible guidelines on what seed mixtures can be used in CRP grassland plantings in the northern Great Plains. This would allow landowners and natural resource professionals who manage pheasant habitat to plant a mosaic of cool- and warm-season CRP grassland habitats.

Wildlife Society Bulletin

Volcanic air pollution and human health: Recent advances and future directions

Volcanic air pollution from both explosive and effusive activity can affect large populations as far as thousands of kilometers away from the source, for days to decades or even centuries. Here, we summarize key advances and prospects in the assessment of health hazards, effects, risk, and management. Recent advances include standardized ash assessment methods to characterize the multiple physicochemical characteristics that might influence toxicity; the rise of community-based air quality monitoring networks using low-cost gas and particulate sensors; the development of forecasting methods for ground-level concentrations and associated public advisories; the development of risk and impact assessment methods to explore health consequences of future eruptions; and the development of evidence-based, locally specific measures for health protection. However, it remains problematic that the health effects of many major and sometimes long-duration eruptions near large populations have gone completely unmonitored. Similarly, effects of prolonged degassing on exposed populations have received very little attention relative to explosive eruptions. Furthermore, very few studies have longitudinally followed populations chronically exposed to volcanic emissions; thus, knowledge gaps remain about whether chronic exposures can trigger development of potentially fatal diseases. Instigating such studies will be facilitated by continued co-development of standardized protocols, supporting local study teams and procuring equipment, funding, and ethical permissions. Relationship building between visiting researchers and host country academic, observatory, and agency partners is vital and can, in turn, support the effective communication of health impacts of volcanic air pollution to populations, health practitioners, and emergency managers.

Bulletin of Volcanology

Paleomagnetic correlations and Potassium-Argon dating of Middle Tertiary ash-flow sheets in the eastern Great Basin, Nevada and Utah

Directions of natural remanent magnetization are used to identify and correlate individual cooling units in the middle Tertiary ash-flow province in central and eastern Nevada and western Utah. Potassium-argon dating indicates that the minimum time between eruptions of individual but genetically related ash-flow cooling units is on the order of 0.8 m.y. As this interval is long in comparison with the secular variation of the direction of the geomagnetic field, in a given volcanic province the direction of natural thermoremanent magnetization is a unique characteristic of each cooling unit. Ash-flow sheets investigated include the Stone Cabin Formation, the tuff of Pancake Summit, the Windous Butte Formation, the Needles Range Formation, the Bates Mountain Tuff, and the tuff of Clipper Gap, of Oliogcene to early Miocene age. The original areas of individual ash-flow cooling units are as great as 8,000 km 2 ; the volumes, 1,300 km 3 . The paleomagnetic correlations, made over distances up to 200 km, confirm most of the previously made lithologic correlations and allow more accurate delineation of single cooling units. These correlations are particularly useful in those parts of the Basin and Range province where the Tertiary stratigraphic section consists mainly of ash-flow sheets, and the outcrops are confined to mountain ranges that are separated by alluvium-filled valleys.

Nevada, Utah

Role of fluid pressure in mechanics of overthrust faulting: II. Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis

Pressures of interstitial fluids significantly greater than the normal hydrostatic pressure are known in many parts of the world. Many occurrences are in thick sections of relatively young sediments; some are in areas that have been intensely deformed. Abnormal fluid pressures in the Gulf Coast region are associated with thick bodies of shale or mudstone, and with high hydraulic gradients across bedding. The rocks there have been buried rather rapidly and are evidently not yet fully compacted. The mechanism by which clay consolidates under pressure affords a quantitative relationship among the variables—depth, strength of clay, and fluid pressure—and this relationship indicates that the Gulf Coast examples agree fairly well with observations on depth and porosity in Paleozoic shales of Oklahoma and Tertiary shales of Venezuela. Critical data are lacking, but permeability clearly decreases tremendously as clay rocks are compacted. This decrease in permeability provides a self-sealing mechanism that greatly retards the escape of pore water from deeply buried clay rocks. The relationship between rate of compaction and the development of abnormal fluid pressures probably applies not only to clay rocks but also to carbonates and possibly to micaceous and chloritic metamorphic rocks. Conditions of geosynclinal deposition are, in general, those most favorable to the development of abnormal fluid pressures. The hypothesis that large-scale overthrusting is facilitated by abnormal fluid pressures which, in turn, are associated with geosynclinal deposition is applied to the overthrust belt of western Wyoming and adjacent States. This is a long curving belt of several bedding-plane faults which have an aggregate horizontal displacement across the belt of 50 miles or more. The sedimentary rocks that make up the belt were evidently deposited in a major geosyncline bordered by uplands not far to the west. At any given locality, the rate of deposition of the sediments increased continuously until the beginning of intense deformation and overthrusting. The geosynclinal axis and the bordering uplands probably migrated slowly eastward across the belt. Several lines of indirect evidence suggest that abnormal fluid pressures developed in this region during final stages of rapid geosynclinal sinking and that thick plates of Paleozoic and Mesozoic sedimentary rocks sheared off from the underlying rocks and moved slowly eastward. Rate of movement probably was controlled by erosion of upfolds that arose at the front of each moving plate. The fundamental cause of the lateral stresses that propelled the overthrusts is not known, but it may be examined instructively in the light of the fluid-pressure hypothesis. The thrust sheets might, for example, have slid by simple gravitation down the western limb of the geosyncline on reasonable slopes and not improbable fluid pressure-overburden ratios. Such large-scale slumping of thrust sheets, however, seems to require gaps at the rear of the thrust sheets. The long intermontane valleys of Idaho and Utah may possibly have originated as such gaps or rifts, but no proof has yet been recognized that they were formed in this manner. An alternative possibility, regional compression, requires concentration of the lateral stresses within the upper few miles of the earth's crust; in this general region emplacement of the Idaho batholith seems the most likely source of such superficially concentrated stresses. However, this batholith is so far from the front edge of the overthrust belt that it would require extremely high fluid pressure-overburden ratios over a wide area. Perhaps some combination of the two forces—pushing wide thrust plates down a gentle slope—is the most likely explanation.

GSA Bulletin

From anecdotes to quantification: Advances in characterizing volcanic eruption impacts on the built environment

Over the past 20 years, our understanding of volcanic eruption impacts on the built environment has transformed from being primarily observational with small datasets to one grounded in field investigations, laboratory experiments, and quantitative modeling, with an emphasis on stakeholder collaboration and co-creation. Here, we summarize key advances and knowledge gaps of impacts across volcanic hazards and built environment types from the past 20 + years. Studies have concentrated on impacts from tephra fall (ash) and to buildings, with less examination of other hazards’ impacts to critical infrastructure. As we look to the next decade, we speculate on likely research directions, including the increasing role of new technologies, higher resolution modeling, transdisciplinary collaborations, and evidence-based mitigative solutions.

Bulletin of Volcanology

Spatiotemporal earthquake clusters along the North Anatolian fault zone offshore Istanbul

We investigate earthquakes with similar waveforms in order to characterize spatiotemporal microseismicity clusters within the North Anatolian fault zone (NAFZ) in northwest Turkey along the transition between the 1999 İzmit rupture zone and the Marmara Sea seismic gap. Earthquakes within distinct activity clusters are relocated with cross-correlation derived relative travel times using the double-difference method. The spatiotemporal distribution of microearthquakes within individual clusters is resolved with relative location accuracy comparable to or better than the source size. High-precision relative hypocenters define the geometry of individual fault patches, permitting a better understanding of fault kinematics and their role in local-scale seismotectonics along the region of interest. Temporal seismic sequences observed in the eastern Sea of Marmara region suggest progressive failure of mostly nonoverlapping areas on adjacent fault patches and systematic migration of microearthquakes within clusters during the progressive failure of neighboring fault patches. The temporal distributions of magnitudes as well as the number of events follow swarmlike behavior rather than a mainshock/aftershock pattern.

Istanbul

A critical gap: In situ measurements of surface-atmosphere interactions from outside earth

This white paper demonstrates five points: (1) The lack of robust measurements of the vertical gradients of natural boundary layers and transport fluxes on other planetary bodies precludes adequate estimation of aeolian and other meteorological processes throughout our Solar System (§1). (2) Thus, there exist critical knowledge gaps within high-priority planetary science questions that motivate the need for in situ aeolian and other meteorological measurements on an extraterrestrial surface (§2). (3) Such measurements would be timely, because they will greatly enhance the utility of existing climate data/models, while also contributing to improved design, safety, and effectiveness of near-future missions (§3). (4) Acquisition of these next-generation measurements from Mars is technologically feasible within the coming decade (including via small spacecraft) (§4, 6) and (5) such measurements would also significantly contribute towards addressing key Mars science questions (§5). Our implicit recommendation is that this type of in situ science be considered in definition of high-priority planetary science questions and prioritization of mission concepts.

Bulletin of the AAS

Tsunami history of an Oregon coastal lake reveals a 4600 yr record of great earthquakes on the Cascadia subduction zone

Bradley Lake, on the southern Oregon coastal plain, records local tsunamis and seismic shaking on the Cascadia subduction zone over the last 7000 yr. Thirteen marine incursions delivered landward-thinning sheets of sand to the lake from nearshore, beach, and dune environments to the west. Following each incursion, a slug of marine water near the bottom of the freshwater lake instigated a few-year-to-several-decade period of a brackish (??? 4??? salinity) lake. Four additional disturbances without marine incursions destabilized sideslopes and bottom sediment, producing a suspension deposit that blanketed the lake bottom. Considering the magnitude and duration of the disturbances necessary to produce Bradley Lake's marine incursions, a local tsunami generated by a great earthquake on the Cascadia subduction zone is the only accountable mechanism. Extreme ocean levels must have been at least 5-8 m above sea level, and the cumulative duration of each marine incursion must have been at least 10 min. Disturbances without marine incursions require seismic shaking as well. Over the 4600 yr period when Bradley Lake was an optimum tsunami recorder, tsunamis from Cascadia plate-boundary earthquakes came in clusters. Between 4600 and 2800 cal yr B.P., tsunamis occurred at the average frequency of ??? 3-4 every 1000 yr. Then, starting ???2800 cal yr B.P., there was a 930-1260 yr interval with no tsunamis. That gap was followed by a ???1000 yr period with 4 tsunamis. In the last millennium, a 670-750 yr gap preceded the A.D. 1700 earthquake and tsunami. The A.D. 1700 earthquake may be the first of a new cluster of plate-boundary earthquakes and accompanying tsunamis. Local tsunamis entered Bradley Lake an average of every 390 yr, whereas the portion of the Cascadia plate boundary that underlies Bradley Lake ruptured in a great earthquake less frequently, about once every 500 yr. Therefore, the entire length of the subduction zone does not rupture in every earthquake, and Bradley Lake has recorded earthquakes caused by rupture along the entire length of the Cascadia plate boundary as well as earthquakes caused by rupture of shorter segments of the boundary. The tsunami record from Bradley Lake indicates that at times, most recently ???1700 yr B.P., overlapping or adjoining segments of the Cascadia plate boundary ruptured within decades of each other. ?? 2005 Geological Society of America.

Geological Society of America Bulletin

Gazetteer of planetary nomenclature 1994

Planetary nomenclature, like terrestrial nomenclature, is used to uniquely identify a feature on the surface of a planet or satellite so that the feature can be easily located, described, and discussed. This volume contains detailed information about all names of topographic and albedo features on planets and satellites (and some planetary ring and ring-gap systems) that the International Astronomical Union has named and approved from its founding in 1919 through its triennial meeting in 1994. This edition of the Gazetteer of Planetary Nomenclature supersedes an earlier informal volume distributed by the U.S. Geological Survey in 1986 as Open-File Report 84-692 (Masursky and others, 1986). Named features are depicted on maps of the Moon published first by the U.S. Defense Mapping Agency or the Aeronautical Chart and Information Center and more recently by the U.S. Geological Survey; on maps of Mercury, Venus, Mars, and the satellites of Jupiter, Saturn, and Uranus published by the U.S. Geological Survey; and on maps of the Moon, Venus, and Mars produced by the U.S.S.R. Although we have attempted to check the accuracy of all data in this volume, we realize that some errors will remain in a work of this size. Readers noting errors or omissions are urged to communicate them to the U.S. Geological Survey, Branch of Astrogeology, Rm. 409, 2255 N. Gemini Drive, Flagstaff, AZ 86001.

Bulletin

Stress transfer to the Denali and other regional faults from the M 9.2 Alaska earthquake of 1964

Stress transfer from the great 1964 Prince William Sound earthquake is modeled on the Denali fault, including the Denali-Totschunda fault segments that ruptured in 2002, and on other regional fault systems where M 7.5 and larger earthquakes have occurred since 1900. The results indicate that analysis of Coulomb stress transfer from the dominant earthquake in a region is a potentially powerful tool in assessing time-varying earthquake hazard. Modeled Coulomb stress increases on the northern Denali and Totschunda faults from the great 1964 earthquake coincide with zones that ruptured in the 2002 Denali fault earthquake, although stress on the Susitna Glacier thrust plane, where the 2002 event initiated, was decreased. A southeasterlytrending Coulomb stress transect along the right-lateral Totschunda-Fairweather-Queen Charlotte trend shows stress transfer from the 1964 event advancing slip on the Totschunda, Fairweather, and Queen Charlotte segments, including the southern Fairweather segment that ruptured in 1972. Stress transfer retarding right-lateral strike slip was observed from the southern part of the Totschunda fault to the northern end of the Fairweather fault (1958 rupture). This region encompasses a gap with shallow thrust faulting but with little evidence of strike-slip faulting connecting the segments to the northwest and southeast. Stress transfer toward failure was computed on the north-south trending right-lateral strike-slip faults in the Gulf of Alaska that ruptured in 1987 and 1988, with inhibitory stress changes at the northern end of the northernmost (1987) rupture. The northern Denali and Totschunda faults, including the zones that ruptured in the 2002 earthquakes, follow very closely (within 3%), for about 90° , an arc of a circle of radius 375 km. The center of this circle is within a few kilometers of the intersection at depth of the Patton Bay fault with the Alaskan megathrust. This inferred asperity edge may be the pole of counterclockwise rotation of the block south of the Denali fault. These observations suggest that the asperity and its recurrent rupture in great earthquakes as in 1964 may have influenced the tectonics of the region during the later stages of evolution of the Denali strike-slip fault system.

Alaska

Advanced quantitative precipitation information: Improving monitoring and forecasts of precipitation, streamflow, and coastal flooding in the San Francisco Bay area

Advanced Quantitative Precipitation Information (AQPI) is a synergistic project that combines observations and models to improve monitoring and forecasts of precipitation, streamflow, and coastal flooding in the San Francisco Bay Area. As an experimental system, AQPI leverages more than a decade of research, innovation, and implementation of a statewide, state-of-the-art network of observations, and development of the next generation of weather and coastal forecast models. AQPI was developed as a prototype in response to requests from the water management community for improved information on precipitation, riverine, and coastal conditions to inform their decision-making processes. Observation of precipitation in the complex Bay Area landscape of California’s coastal mountain ranges is known to be a challenging problem. But, with new advanced radar network techniques, AQPI is helping fill an important observational gap for this highly populated and vulnerable metropolitan area. The prototype AQPI system consists of improved weather radar data for precipitation estimation; additional surface measurements of precipitation, streamflow, and soil moisture; and a suite of integrated forecast modeling systems to improve situational awareness about current and future water conditions from sky to sea. Together these tools will help improve emergency preparedness and public response to prevent loss of life and destruction of property during extreme storms accompanied by heavy precipitation and high coastal water levels—especially high-moisture laden atmospheric rivers. The Bay Area AQPI system could potentially be replicated in other urban regions in California, the United States, and worldwide.

California

Steps toward a satellite-based global volcano monitoring and early warning system: From pilot to demonstrator to GVEWERS

The consequences of volcanic eruptions span a broad range. Large explosive eruptions that recur every several hundred to thousands of years can impact global climate, but even small eruptions of a magnitude that takes place multiple times each year someplace on Earth can have devastating effects given the large number of people who live near volcanoes. Ground-based monitoring is inadequate or lacking at many of the world’s active volcanoes, but satellite observations can fill important gaps and provide operational awareness of volcanic unrest and eruptions. The need for satellite monitoring of volcanoes has been articulated in numerous international agreements, and for some datasets, like thermal and ultraviolet imagery, daily or sub-daily monitoring has been incorporated into operational volcano surveillance. Utilization of high-resolution synthetic aperture radar (SAR) and optical datasets, however, has lagged. Since 2014, the Committee on Earth Observing Satellites (CEOS) has sponsored a series of projects designed to enhance access to, and utilization of, high-resolution SAR and optical satellite imagery for volcano monitoring. The efforts have culminated in the development of the Global Volcano Early Warning and Eruption Response from Space (GVEWERS) initiative—a global and sustainable program intended to support volcano hazards assessment and mitigation. GVEWERS builds on lessons learned from a decade of CEOS volcano projects while also providing a foundation for continuing to improve the accessibility and exploitation of satellite data for better anticipating and responding to hazardous volcanic eruptions.

Bulletin of Volcanology

Structure of the continental margin of Liberia, West Africa

Geophysical surveys made by R/V Unitedgeo I (USGS–IDOE Cruise Leg 5), combined with earlier surveys and available geologic information, provide the basis for interpreting the structure of the continental margin of Liberia. This area lies at the junction of the Americas and Africa in published reconstructions of Gondwanaland prior to the opening of the North and South Atlantic in Jurassic and Cretaceous time, respectively. Three fracture zones (St. Paul, Cape Palmas, and Grand Cess) are inferred in the area southeast of 9°30′ W. on the basis of magnetic and gravity data, which is supported by bathymetric and seismic reflection data. The three fracture zones appear to exist as separate lineaments near the African coast. Farther seaward, they may be part of the same transform fault crossing the Atlantic (St. Paul fracture zone). The magnetic anomalies associated with these fracture zones, which may have originated in Cretaceous time at the opening of the South Atlantic, are continuous with magnetic anomalies over crust of Eburnean age (∼2,000 m.y.) in southeast Liberia and its continental shelf. This suggests that Eburnean age structures may have been zones of weakness that were reactivated in Cretaceous time. A positive gravity anomaly (∼50 mgal) along the coast and continental shelf of Liberia is attributed to deep crustal rocks that were uplifted and exposed in Pan-African time (∼550 m.y.). The land boundary of this anomaly coincides with a shear zone that marks the boundary between the Pan-African and the Liberian age province (∼2,700 m.y.); the shearing (in a thrust-fault sense) may be the result of compressive stress associated with the closing of a proto-Atlantic ocean. Liberian age magnetic anomalies in the area northwest of 9°30′ W. cross the Pan-African province (and the positive coastal gravity anomaly) and continue over the continental shelf and slope to about the 3,000-m bathymetric contour; the seaward limit of the anomalies is interpreted as representing the seaward limit of the old continental crust. This westward extension of the continental crust does not completely fill the gap in fit in various published reconstructions of Gondwanaland, and we suggest that the northern Florida block may have been located near the Liberian margin at one time. Magnetic data indicate a thick section of sedimentary rock, possibly as great as 8 km, on the continental slope. Comparison of gravity data over magnetically inferred basins in the shelf, slope, and rise suggests that low-density sedimentary rocks constitute a greater proportion of the section in basins beneath the shelf and beneath the slope and rise northwest of 9°30' W. than beneath the slope and rise in the area of the fracture zones. The gravitational attraction that corresponds to a crust-mantle boundary dipping 45° to 60° can be computed to fit observed data – as might be expected at a rifted continental margin. A shallow high-density block beneath the coast and continental shelf is required to fit the coastal positive anomaly; this block is represented by exposures on land of granulite-grade metamorphic rock of the Pan-African province.

Atlantic Ocean

Infrequent triggering of tremor along the San Jacinto Fault near Anza, California

We examine the conditions necessary to trigger tremor along the San Jacinto fault (SJF) near Anza, California, where previous studies suggest triggered tremor occurs, but observations are sparse. We investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near Anza, California. We examine 44 M w ≥7.4 teleseismic events between 2001 and 2011; these events occur at a wide range of back azimuths and hypocentral distances. In addition, we included one smaller‐magnitude, regional event, the 2009 M w 6.5 Gulf of California earthquake, because it induced extremely high strains at Anza. We find the only episode of triggered tremor occurred during the 3 November 2002 M w 7.8 Denali earthquake. The tremor episode lasted 300 s, was composed of 12 tremor bursts, and was located along SJF at the northwestern edge of the Anza gap at approximately 13 km depth. The tremor episode started at the Love‐wave arrival, when surface‐wave particle motions are primarily in the transverse direction. We find that the Denali earthquake induced the second highest stress (~35 kPa) among the 44 teleseismic events and 1 regional event. The dominant period of the Denali surface wave was 22.8 s, at the lower end of the range observed for all events (20–40 s), similar to periods shown to trigger tremor in other locations. The surface waves from the 2009 M w 6.5 Gulf of California earthquake had the highest observed strain, yet a much shorter dominant period of 10 s and did not trigger tremor. This result suggests that not only the amplitude of the induced strain, but also the period of the incoming surface wave, may control triggering of tremors near Anza. In addition, we find that the transient‐shear stress (17–35 kPa) required to trigger tremor along the SJF at Anza is distinctly higher than what has been reported for the well‐studied San Andreas fault.

California

Number and size of last-glacial Missoula floods in the Columbia River valley between the Pasco Basin, Washington, and Portland, Oregon

Field evidence and radiocarbon age dating, combined with hydraulic flow modeling, provide new information on the magnitude, frequency, and chronology of late Pleistocene Missoula floods in the Columbia River valley between the Pasco Basin, Washington, and Portland, Oregon. More than 25 floods had discharges of > 1.0 x 10 6 m 3 /s. At least 15 floods had discharges of >3.0 x 10 6 m 3 /s. At least six or seven had peak discharges of >6.5 x 106 m 3 /s, and at least one flood had a peak discharge of >10 x 106 m 3 /s, a value consistent with earlier results from near Wallula Gap, but better defined because of the strong hydraulic controls imposed by critical flow at constrictions near Crown and Mitchell Points in the Columbia River Gorge. Stratigraphy and geomorphic position, combined with 25 radiocarbon ages and the widespread occurrence of the ca. 13 ka (radiocarbon years) Mount St. Helens set-S tephra, show that most if not all the Missoula flood deposits exposed in the study area were emplaced after 19 ka (radiocarbon years), and many were emplaced after 15 ka. More than 13 floods perhaps postdate ca. 13 ka, including at least two with discharges of >6 x 10 6 m 3 /s. From discharge and stratigraphic relationships upstream, we hypothesize that the largest flood in the study reach resulted from a Missoula flood that predated blockage of the Columbia River valley by the Cordilleran ice sheet. Multiple later floods, probably including the majority of floods recorded by fine- and coarse-grained deposits in the study area, resulted from multiple releases of glacial Lake Missoula that spilled into a blocked and inundated Columbia River valley upstream of the Okanogan lobe and were shunted south across the Channeled Scabland.

Geological Society of America Bulletin

Alaska-Aleutian range batholith: Geochronology, chemistry, and relation to circum-Pacific plutonism

Potassium-argon mineral ages and reconnaissance mapping of approximately 65,000 sq km in south-central Alaska indicate that the Mesozoic and Cenozoic plutonic rocks in the region were emplaced during three discrete intrusive epochs. Most of the plutonic rocks are part of the Alaska-Aleutian Range batholith; the remainder appear as outcrops in isolated plutons northeast of the main batholith. Maximum and minimum concordant mineral ages on coexisting biotite and hornblende are used to mark the beginning and ending, respectively, of each intrusive epoch. The oldest intrusive epoch, Early and Middle Jurassic, contains plutonic rocks emplaced between about 176 and 154 m.y. ago. Jurassic plutonism occurred along a magmatic arc at least 1,300 km long, extending from about 480 km southwest of Becharof Lake northeast to the Talkeetna Mountains. Aeromagnetic data suggest that the magmatic arc, which represents the roots of the arc portion of an early Mesozoic arc-trench system, also continues southwest into the Bering Shelf. The associated trench is thought to be represented by an imbricated mélange of ophiolite and submarine lava with associated chert and argillite that occupies a belt 140 km southeast of the magmatic arc. Clastic sediments more than 4.5 km thick occupy the 140-km-wide arc-trench gap. Late Cretaceous and early Tertiary plutonic rocks, emplaced between about 83 and 58 m.y. ago, are found mainly in the northern part of the batholith and in isolated plutons to the northeast toward Mount McKinley. Middle Tertiary plutonic rocks ranging in age from 38 to 26 m.y. occur in two areas within the batholith and also in the Mount McKinley area. Although some of the Late Cretaceous and Tertiary plutons are associated with extrusive rocks, these plutons are largely post-tectonic, and magma was generated beneath both stable platform areas as well as beneath former eugeosynclinal regions in which deformation had essentially ceased. Magma for the Jurassic plutonic rocks appears to have been generated along or above an early Mesozoic subduction zone. The region southeast of the zone is considered to represent a classic example of continental accretion of eugeosynclinal sediments caused by under-thrusting of oceanic crust beneath an island arc. Magma generation for the Cretaceous and Tertiary plutons in the northern part of the batholith does not appear related to the early Mesozoic subduction zone, for it would require that the zone shift inland, or toward the continent, from its position during the Jurassic and then shift away from the continent once again to its present position.

GSA Bulletin