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At least 361 records · Page 20Linked to original sources
USGS2015

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

Despite the role of the Alaska-Aleutian megathrust as the source of some of the largest earthquakes and tsunamis, the history of its pre–twentieth century tsunamis is largely unknown west of the rupture zone of the great (magnitude, M 9.2) 1964 earthquake. Stratigraphy in core transects at two boggy lowland sites on Chirikof Island’s southwest coast preserves tsunami deposits dating from the postglacial to the twentieth century. In a 500-m-long basin 13–15 m above sea level and 400 m from the sea, 4 of 10 sandy to silty beds in a 3–5-m-thick sequence of freshwater peat were probably deposited by tsunamis. The freshwater peat sequence beneath a gently sloping alluvial fan 2 km to the east, 5–15 m above sea level and 550 m from the sea, contains 20 sandy to silty beds deposited since 3.5 ka; at least 13 were probably deposited by tsunamis. Although most of the sandy beds have consistent thicknesses (over distances of 10–265 m), sharp lower contacts, good sorting, and/or upward fining typical of tsunami deposits, the beds contain abundant freshwater diatoms, very few brackish-water diatoms, and no marine diatoms. Apparently, tsunamis traveling inland over low dunes and boggy lowland entrained largely freshwater diatoms. Abundant fragmented diatoms, and lake species in some sandy beds not found in host peat, were probably transported by tsunamis to elevations of >10 m at the eastern site. Single-aliquot regeneration optically stimulated luminescence dating of the third youngest bed is consistent with its having been deposited by the tsunami recorded at Russian hunting outposts in 1788, and with the second youngest bed being deposited by a tsunami during an upper plate earthquake in 1880. We infer from stratigraphy, 14 C-dated peat deposition rates, and unpublished analyses of the island’s history that the 1938 tsunami may locally have reached an elevation of >10 m. As this is the first record of Aleutian tsunamis extending throughout the Holocene, we cannot estimate source earthquake locations or magnitudes for most tsunami-deposited beds. We infer that no more than 3 of the 23 possible tsunamis beds at both sites were deposited following upper plate faulting or submarine landslides independent of megathrust earthquakes. If so, the Semidi segment of the Alaska-Aleutian megathrust near Chirikof Island probably sent high tsunamis southward every 180–270 yr for at least the past 3500 yr.

Alaska↗
USGS2015

Slipstream: an early Holocene slump and turbidite record from the frontal ridge of the Cascadia accretionary wedge off western Canada and paleoseismic implications

Slipstream Slump, a well-preserved 3 km wide sedimentary failure from the frontal ridge of the Cascadia accretionary wedge 85 km off Vancouver Island, Canada, was sampled during Canadian Coast Guard Ship (CCGS) John P. Tully cruise 2008007PGC along a transect of five piston cores. Shipboard sediment analysis and physical property logging revealed 12 turbidites interbedded with thick hemipelagic sediments overlying the slumped glacial diamict. Despite the different sedimentary setting, atop the abyssal plain fan, this record is similar in number and age to the sequence of turbidites sampled farther to the south from channel systems along the Cascadia Subduction Zone, with no extra turbidites present in this local record. Given the regional physiographic and tectonic setting, megathrust earthquake shaking is the most likely trigger for both the initial slumping and subsequent turbidity currents, with sediments sourced exclusively from the exposed slump face of the frontal ridge. Planktonic foraminifera picked from the resedimented diamict of the underlying main slump have a disordered cluster of 14 C ages between 12.8 and 14.5 ka BP. For the post-slump stratigraphy, an event-free depth scale is defined by removing the turbidite sediment intervals and using the hemipelagic sediments. Nine 14 C dates from the most foraminifera-rich intervals define a nearly constant hemipelagic sedimentation rate of 0.021 cm/year. The combined age model is defined using only planktonic foraminiferal dates and Bayesian analysis with a Poisson-process sedimentation model. The age model of ongoing hemipelagic sedimentation is strengthened by physical property correlations from Slipstream events to the turbidites for the Barkley Canyon site 40 km south. Additional modelling addressed the possibilities of seabed erosion or loss and basal erosion beneath turbidites. Neither of these approaches achieves a modern seabed age when applying the commonly used regional marine 14 C reservoir age of 800 years (marine reservoir correction Δ R = 400 years). Rather, the top of the core appears to be 400 years in the future. A younger marine reservoir age of 400 years (Δ R = 0 years) brings the top to the present and produces better correlations with the nearby Effingham Inlet paleo-earthquake chronology based only on terrestrial carbon requiring no reservoir correction. The high-resolution dating and facies analysis of Slipstream Slump in this isolated slope basin setting demonstrates that this is also a useful type of sedimentary target for sampling the paleoseismic record in addition to the more studied turbidites from submarine canyon and channel systems. The first 10 turbidites at Slipstream Slump were deposited between 10.8 and 6.6 ka BP, after which the system became sediment starved and only two more turbidites were deposited. The recurrence interval for the inferred frequent early Holocene megathrust earthquakes is 460 ± 140 years, compatible with other estimates of paleoseismic megathrust earthquake occurrence rates along the subduction zone.

Slipstream Slump↗
USGS2022

Assigning causality to events in the Holocene record of coral reefs

The uncemented reef-frameworks of Pacific Panamá, which have been dominated throughout the Holocene by branching corals of the genus Pocillopora , experienced a hiatus in vertical accretion lasting c. 2300 years, beginning c. 4100 years ago. The hiatus has been attributed to an increase in variability of the El Niño–Southern Oscillation (ENSO). We tested the alternative hypothesis that the hiatus was solely the result of bioerosion, assuming an acute disturbance halted coral growth 1800 years ago (the time at which reef accretion resumed after the hiatus) and that the entire framework remained in the taphonomically active zone at that time. We calculate that it would have taken 167–511 years for bioerosion to fully remove 2300 years-worth of framework growth under those circumstances. In fact, most of the reef-framework in Panamá is stabilized in sediment that prevents the activity of bioeroders; only the upper c. 1 m of open framework – several decades-worth of growth at most – would have been vulnerable to erosion, greatly increasing the time required to bioerode 2300 years of accumulation. We conclude that the hiatus was not solely an artefact of bioerosion; rather, a long-term increase in ENSO variability suppressed coral growth and vertical reef accretion.

Journal of the Geological Society of London↗
USGS2013

Preliminary report on the Late Pleistocene and Holocene diatoms of Swamp Lake, Yosemite National Park, California, USA

Swamp Lake, Yosemite National Park, is the only known lake in California containing long sequences of varved sediments and thus has the potential to provide a high-resolution record of climate variability. This preliminary analysis of the diatom assemblages from a 947-cm-long composite sediment core (freeze core FZ02–05; 0–67 cm, Livingstone core 02–05; 53–947 cm) shows that the lake has been freshwater, oligotrophic, and circumneutral to alkaline throughout its ~16,000-year-long history. The first sediments deposited in the lake show that the vegetation in the watershed was sparse, allowing organic matter-poor silt and clay to be deposited in the basin. The basin filled quickly to a depth of at least 5 m and remained at least that deep for most of the sediment record. Several short intervals provided evidence of large fluctuations in lake level during the Holocene. The upper 50 cm of the core contains evidence of the Medieval Climate Anomaly and Little Ice Age.

California↗
USGS2012

Holocene behavior of the Brigham City segment: implications for forecasting the next large-magnitude earthquake on the Wasatch fault zone, Utah

The Brigham City segment (BCS), the northernmost Holocene‐active segment of the Wasatch fault zone (WFZ), is considered a likely location for the next big earthquake in northern Utah. We refine the timing of the last four surface‐rupturing (~Mw 7) earthquakes at several sites near Brigham City (BE1, 2430±250; BE2, 3490±180; BE3, 4510±530; and BE4, 5610±650 cal yr B.P.) and calculate mean recurrence intervals (1060–1500 yr) that are greatly exceeded by the elapsed time (~2500 yr) since the most recent surface‐rupturing earthquake (MRE). An additional rupture observed at the Pearsons Canyon site (PC1, 1240±50 cal yr B.P.) near the southern segment boundary is probably spillover rupture from a large earthquake on the adjacent Weber segment. Our seismic moment calculations show that the PC1 rupture reduced accumulated moment on the BCS about 22%, a value that may have been enough to postpone the next large earthquake. However, our calculations suggest that the segment currently has accumulated more than twice the moment accumulated in the three previous earthquake cycles, so we suspect that additional interactions with the adjacent Weber segment contributed to the long elapse time since the MRE on the BCS. Our moment calculations indicate that the next earthquake is not only overdue, but could be larger than the previous four earthquakes. Displacement data show higher rates of latest Quaternary slip (~1.3 mm/yr) along the southern two‐thirds of the segment. The northern third likely has experienced fewer or smaller ruptures, which suggests to us that most earthquakes initiate at the southern segment boundary.

Utah↗
USGS2008

A 1500-year holocene caribbean climate archive from the Blue Hole, lighthouse reef, belize

Sediment cores (up to 6 m in length) from the bottom of the Blue Hole, a 125 m deep Pleistocene sinkhole located in the lagoon of Lighthouse Reef Atoll, Belize, consist of undisturbed, annually layered biogenic carbonate muds and silts with intercalated coarser grained storm beds. The sedimentation rate of the layered sections is 2.5 mm/y on average, and the long cores span the past 1500 years. Oxygen isotopes of laminated sediment provide a late Holocene climate proxy: A high-resolution ??18O time series traces the final Migration Period Pessimum, the Medieval Warm Period, the Little Ice Age, and the subsequent temperature rise. Carbon isotopes (??13C) decrease up core and show the impacts of the decline of the Mayan culture and the Suess effect. Time series analyses of ??18O and ??13C content reveal 88-, 60-, 52-, and 32-year cyclicities, and suggest solar forcing. Storm event beds are most common during AD 650-850, around AD 1000, during AD 1200-1300, and AD 1450-1550. Major storm beds are rare during the past 500 years BP.

Journal of Coastal Research↗
USGS2008

High-resolution foraminiferal, isotopic, and trace element records from holocene estuarine deposits of San Francisco Bay, California

A 3.5-m gravity core (DJ6-93SF-6) from San Francisco Bay reveals a complex paleoclimatic history of the region over the last 3870 cal YBP. A polynomial equation based on 11 AMS 14C ages provides an excellent age model for the core, and environmental proxies for water temperature and salinity are derived from various foraminiferal abundances, stable carbon and oxygen isotopes, and Mg/Ca ratios. Two foraminiferal associations were identified by Q-mode cluster analysis: a colder-water Elphidium excavatum association and the warmer-water Ammonia beccarii-Elphidium gunteri association. The E. excavatum association dominates the core for all but about 600 years out of the last four millennia. At 3870 cal YBP, water temperatures were warm (13.9??C) and freshwater inflow was reduced compared with today. From 3590 to 2860 cal YBP, temperatures dropped 0.5??C and the climate remained dry. This was followed by a period of pronounced lower ??13C values, indicating that conditions became considerably wetter from 2860 to 2170 cal YBP. During this interval, the temperature oscillated frequently, peaking at 13.9??C at 2710 cal YBP, then dropping shortly thereafter to 12.8??C at 2420 cal YBP. Freshwater inflow gradually decreased between 2170 and 1950 cal YBP with a minimal rise in temperature, then changed quickly to colder and wetter conditions at 1900 cal YBP. Drier conditions then prevailed until 1480 cal YBP with water temperatures fluctuating between 13.1??C and 13.8??C, followed by wetter climate from 1480 to 1320 cal YBP. A significant faunal shift from the E. excavatum association to the A. beccarii-E. gunteri association occurred from 1250 to 650 cal YBP, possibly due to regional warming, decreased oxygen availability, and/or a change in the phyto-plankton community. Associated with this change in faunal composition were warm and dry conditions, representative of the Medieval Warm Period (Medieval Climatic Anomaly). A climatic shift coincident with the onset of the Little Ice Age (LIA I and LIA II) is evident from 650 to 280 cal YBP, with the return of the E. excavatum association and an extreme drop in ??18O values, all indicating increased precipitation and fresh water inflow. This was followed by generally drier conditions to the present, except for a brief wet period around 100 cal YBP, and fairly consistent water temperatures in the middle 13??C, except for a drop to 12.8??C at 200 cal YBP. Two significant faunal changes occur near the top of the core. First, there is the reappearance of the A. beccarii-E. gunteri association, suggesting that, once again, regional warming has taken place, oxygen availability has declined, and/or environmental conditions changed such that diatoms have become a scarce food source. Second, there is the first appearance of the invasive Japanese foraminifera Trochammina hadai Uchio, a species that commonly lives in highly polluted areas and is an indicator of eutrophication in its native estuaries. At the same time, freshwater inflow decreased, which may be explained by global warming during the last 100 years, or more likely due to modern water diversion for agriculture in the central valley of California. ADDITIONAL INDEX WORDS: Climate, San Francisco Bay, Holocene, foraminifers, isotopes, trace metals, Medieval Warm Period, Little Ice Age.

Conference Paper↗
USGS1999

Liquefaction evidence for at least two strong Holocene paleo-earthquakes in central and southwestern Illinois, USA

Two strong mid-Holocene earthquakes in Illinois have been documented by paleoliquefaction features such as clastic dikes, sills, and detachments of fine-grained sediment that sunk into liquefied sand. At least one paleo-earthquake occurred in central Illinois about 35 km NE of Springfield, probably sometime between 5,900 and 7,400 yr BP. Dike widths are as much as 0.4 m near the energy center of the earthquake. Outward from this center, dike widths attenuate and ultimately disappear at about 35 km. More than one paleo-earthquake is probably represented by liquefaction features near Springfield. Another paleo-earthquake that appears to have been centered about 65 km ESE of St. Louis, Missouri, occurred near 5,700 yr BP. The energy center is inferred as being in Illinois, and most likely near lowermost Shoal Creek where the meizoseismal region is defined by dikes as wide as 0.5 m and by a regional abundance of dikes. Dikes from this earthquake probably extend at least as far as 35 km from its inferred energy center. The earthquake near Shoal Creek and one earthquake near Springfield almost certainly exceeded M 6. The paleomagnitudes can be more closely bracketed by geotechnical testing and analysis, when used in conjunction with existing data.

Illinois↗
USGS1983

Late Holocene ice wedges near Fairbanks, Alaska, USA: Environmental setting and history of growth

Test trenches excavated into muskeg near Fairbanks in 1969 exposed a polygonal network of active ice wedges. The wedges occur in peat that has accumulated since about 3500 yr BP and have grown episodically as the permafrost table fluctuated in response to fires, other local site conditions and perhaps regional climatic changes. Radiocarbon dates suggest one or two episodes of ice-wedge growth between about 3500 and 2000 yr BP as woody peat accumulated at the site. Subsequent wedge truncation evidently followed a fire that charred the peat. Younger peat exhibits facies changes between sedge-rich components that filled troughs over the ice wedges and woody bryophytic deposits that formed beyond the troughs. A final episode of wedge development took place within the past few hundred years. Pollen data from the site indicate that boreal forest was present throughout the past 6000 yr, but that it underwent a gradual transition from a predominantly deciduous to a spruce-dominated assemblage. This change may reflect either local site conditions or a more general climatic shift to cooler, moister summers in late Holocene time. The history of ice-wedge growth shows that wedges can form and grow to more than 1 m apparent width under mean annual temperatures that probably are close to those of the Fairbanks area today (-3.5°C) and under vegetation cover similar to that of the interior Alaskan boreal forest. The commonly held belief that ice wedges develop only below mean annual air temperatures of -6 to -8°C in the zone of continuous permafrost is invalid.

Alaska↗
USGS1985

Newly discovered Holocene volcanic vents, Port Moller and Stepovak Bay quadrangles, Alaska Peninsula: A section in The United States Geological Survey in Alaska: Accomplishments during 1983

Reconnaissance mapping in 1982-83 suggests previously unreported Holocene volcanic activity in the area 14-24 km southwest of Kupreanof Volcano (fig. 45). One young volcanic vent has been observed along the divide between Stepovak Bay and Bear Lake, and the locations of two others are inferred from the position of morphologically young lava flows mapped in valleys draining into Clark Bay, Grub Gulch, and Ramsey Bay. Other morphologically young flows extending northwest from the divide are evident on aerial photographs.

Alaska↗
USGS2000

Bibliography of literature from 1990-1997 pertaining to Holocene and fumarolic Pleistocene volcanoes of Alaska, Canada, and the conterminous United States

The 1980's and 1990's were marked by frequent worldwide destructive volcanic eruptions. Significant loss of life and property during eruptions, and extensive media coverage of volcanic events around the globe have brought volcanic processes and hazards increasingly to the forefront of scientific, governmental, and public attention (e.g., Casadevall, 1994; Pringle, 1994). A result of these circumstances has been an increasing number of scientific and popular publications dealing with active volcanism. The purpose of this bibliography is to provide a comprehensive source of literature published between 1990 and 1997 relevant to all of the Holocene and fumarolic Pleistocene volcanoes in Alaska, the conterminous United States, and Canada. The period between 1990 and 1997 was chosen for two reasons. First, it was necessary to define a precise period of time to keep the work tractable. Second, research for this project began during the summer of 1996 and lasted through 1998, with minor additions into 1999. Thus, 1997 was chosen as the younger cut-off and 1990 for the older cut-off. Literature pertaining to Hawaiian volcanoes has been assembled into a database maintained by the Hawaiian Volcano Observatory (Wright and Takahashi, 1984; Wright and Takahashi, 1998) and has been excluded from this work. We hope that this work will be useful in three ways. First, researchers interested in a specific volcano can use the search capability of the electronic version to readily find literature relevant to their work or determine what aspects of the volcano are in need of study. Second, references cited within the included references are guides to older literature. Third, during times of volcanic crisis this reference list can save time for responding scientists.

Open-File Report↗
USGS2004

Distribution of Holocene Sediment in Chesapeake Bay as Interpreted from Submarine Geomorphology of the Submerged Landforms, Selected Core Holes, Bridge Borings and Seismic Profiles

Overview -- We have interpreted the geomorphology of the submerged landforms to show thick Holocene sediment that accumulated from three different sources during on-going sea level rise that began 10,000 - 12,000 years ago at the end of Pleistocene. We used a variety of subsurface data from the literature and unpublished information to document thicknesses, materials, dates and duration of processes. Although the details of the true extent and thicknesses are unknown, the deposits of different sources have affinity for particular geographic and submerged geomorphic regions of the Chesapeake Bay and its tidal tributaries. During the last Pleistocene glacial event (Wisconsian), the area now occupied by the Chesapeake Bay was exposed, sea level being about 100 m lower than present. The Susquehanna River valley extended beyond the Bay well out on the exposed Atlantic Shelf. The Susquehanna transported glacial outwash from northern Pennsylvania and New York; the glacio-fluvial deposits were graded to the edge of the continental shelf (Colman et al., 1990; Hack, 1957). Other Piedmont and Appalachian Rivers including the Potomac and James Rivers transported large volumes of sediment to confluence with the Susquehanna channel. Locally, across the encompasing coastal plain landscape, intensive headward erosion, gullies, and slope failure, generated extensive debris flows, sheet wash, and terraces of braided alluvial channel deposits. Large volumes of sediment were moved through the river system to the continental shelf. This was accomplished by a cold, wet climate that included much freezing and thawing; steep eroding slopes resulted from the lowering of sea level from the previous high stand (Stage 5e) between glacial events. Across the Delmarva peninsula extensive wind-blown deposits of sand and loess were recycled onto low terraces and uplands from the unvegetated glacio-fluvial sediments moving through the system (Denny et al., 1979). The volume and distribution of sediment eroded and transported from the watershed surrounding the area of the Bay was several orders of magnitude greater than generally observed in transport and storage on the present day landscape.

Open-File Report↗
USGS2007

Granulometry of pebble beach ridges in Fort Williams Point, Greenwich Island, Antarctic Peninsula; a possible result from Holocene climate fluctuations

We present a granulometric study of emerged pebble beach ridges in the Fort Williams Point, Greenwich Island, Antarctic Peninsula. We studied 8 beach ridges from the shore up to 13.5 m above current sea level. The beach ridges are made of volcanic material from the surrounding relief, but also include glacially transported gneiss and granodiorite pebble and cobble. Based on granulometric distribution analysis of 2100 samples from 39 locations we identified evidence of 4 sequences of 1 to 3 ridges. Most of the material seems to be reworked from a till. Pavement formation by iceberg between the sequences of beach ridges suggests periods of lower temperature. The interpretation suggests that sequences of beach ridge construction formed during warmer periods of the late Holocene. This occurs in the framework of an isostatic postglacial uplift allowing the progressive mobilization of periglaciar material.

Open-File Report↗
USGS2008

Chemical Analyses of Pre-Holocene Rocks from Medicine Lake Volcano and Vicinity, Northern California

Chemical analyses are presented in an accompanying table (Table 1) for more than 600 pre-Holocene rocks collected at and near Medicine Lake Volcano, northern California. The data include major-element X-ray fluorescence (XRF) analyses for all of the rocks plus XRF trace element data for most samples, and instrumental neutron activation analysis (INAA) trace element data for many samples. In addition, a limited number of analyses of Na2O and K2O by flame photometry (FP) are included as well assome wet chemical analyses of FeO, H2O+/-, and CO2. Latitude and longitude location information is provided for all samples. This data set is intended to accompany the geologic map of Medicine Lake Volcano (Donnelly-Nolan, in press); map unit designations are given for each sample collected from the map area.

Open-File Report↗
USGS2011

Geochemical characteristics of Holocene laminated sapropel (unit II) and underlying lacustrine unit III in the Black Sea

eg 1 of the 1988 R/V Knorr expeditions to the Black Sea recovered 90 gravity and box cores. The longest recovery by gravity cores was about 3 meters, with an average of about 2.5 meters, recovering all of the Holocene and upper Pleistocene sections in the Black Sea. During the latest Pleistocene glaciation, sea level dropped below the 35-meters-deep Bosporus outlet sill of the Black Sea. Therefore throughout most of its history the Black Sea was a lake, and most of its sediments are lacustrine. The oldest sediments recovered (older than 8,000 calendar years) consist of massive to coarsely banded lacustrine calcareous clay designated as lithologic Unit III, generally containing less than 1 percent organic carbon (OC). The base of overlying Unit II marks the first incursion of Mediterranean seawater into the Black Sea, and the onset of bottom-water anoxia about 7,900 calendar years. Unit II contains as much as 15 percent OC in cores from the deepest part of the Black Sea (2,200 meters). The calcium carbonate (CaCO3) remains of the coccolith Emiliania huxleyi form the distinctive white laminae of overlying Unit I. The composition of Unit III and Unit II sediments are quite different, reflecting different terrigenous clastic sources and increased contributions from hydrogenous and biogenic components in anoxic Unit II sapropel. In Unit II, positive covariance between OC and three trace elements commonly concentrated in OC-rich sediments where sulfate reduction has occurred (molybdenum, nickel, and vanadium) and a nutrient (phosphorus) suggest a large marine source for these elements although nickel and vanadium also have a large terrigenous clastic source. The marine sources may be biogenic or hydrogenous. A large biogenic source is also suggested for copper and cobalt. Because abundant pyrite forms in the water column and sediments of the Black Sea, we expected to find a large hydrogenous iron component, but a strong covariance of iron with aluminum suggests that the dominant source of iron is from terrigenous clastic material. Most elements in lacustrine Unit III sediments have a strong covariance with Al indicating a very dominant terrigenous source. In Unit II, some elements, especially nickel, molybdenum, vanadium, and zinc, do not correlate with aluminum and have concentrations well above terrigenous clastic material, indicating a marine source.

Open-File Report↗
USGS2012

Deep-Sea Turbidites as Guides to Holocene Earthquake History at the Cascadia Subduction Zone—Alternative Views for a Seismic-Hazard Workshop

This report reviews the geological basis for some recent estimates of earthquake hazards in the Cascadia region between southern British Columbia and northern California. The largest earthquakes to which the region is prone are in the range of magnitude 8-9. The source of these great earthquakes is the fault down which the oceanic Juan de Fuca Plate is being subducted or thrust beneath the North American Plate. Geologic evidence for their occurrence includes sedimentary deposits that have been observed in cores from deep-sea channels and fans. Earthquakes can initiate subaqueous slumps or slides that generate turbidity currents and which produce the sedimentary deposits known as turbidites. The hazard estimates reviewed in this report are derived mainly from deep-sea turbidites that have been interpreted as proxy records of great Cascadia earthquakes. The estimates were first published in 2008. Most of the evidence for them is contained in a monograph now in press. We have reviewed a small part of this evidence, chiefly from Cascadia Channel and its tributaries, all of which head offshore the Pacific coast of Washington State. According to the recent estimates, the Cascadia plate boundary ruptured along its full length in 19 or 20 earthquakes of magnitude 9 in the past 10,000 years; its northern third broke during these giant earthquakes only, and southern segments produced at least 20 additional, lesser earthquakes of Holocene age. The turbidite case for full-length ruptures depends on stratigraphic evidence for simultaneous shaking at the heads of multiple submarine canyons. The simultaneity has been inferred primarily from turbidite counts above a stratigraphic datum, sandy beds likened to strong-motion records, and radiocarbon ages adjusted for turbidity-current erosion. In alternatives proposed here, this turbidite evidence for simultaneous shaking is less sensitive to earthquake size and frequency than previously thought. Turbidites far below a channel confluence, instead of representing the merged flows from two tributaries, monitor the dominant tributary only. Sandy beds low in the turbidites, instead of matching from channel to channel, permit divergent stratigraphic correlations; and rather than approximating strong-motion seismograms, the sandy beds more likely record processes internal to the generation and transformation of subaqueous mass movements. The age adjustments, instead of supporting other evidence that all the northern ruptures were long, are uncertain enough to accord with variation in rupture mode, and this variation improves agreement with onshore paleoseismology. Many of the turbidites counted as evidence for frequent earthquakes on the southern Cascadia plate boundary may instead reflect nearness to steep slopes. This report is meant to aid in the updating of national maps of seismic hazards in Canada and the United States. It offers three main conclusions for consideration at a U.S. hazard-map workshop slated for March 21-22, 2012: If giant earthquakes are the norm for the plate boundary offshore southern Washington, the strongest paleoseismic evidence for this rupture mode is the average earthquake-recurrence interval of about 500 years that is evidenced both offshore in lower Cascadia Channel and onshore at estuaries of southern Washington and northernmost Oregon. The plate boundary offshore southern British Columbia and northern Washington may be capable of producing great earthquakes at an average interval as short as 300 years that is evidenced mainly onshore. Review of more of the turbidite evidence now in press may clarify implications for the hazard maps. Further work on the deep-sea turbidites could target sedimentary processes and chronological uncertainties that may affect the turbidites' sensitivity to fault-rupture lengths and recurrence rates.

Washington;Oregon;California↗
USGS2014

Precipitation variability of the Grand Canyon region, 1893 through 2009, and its implications for studying effects of gullying of Holocene terraces and associated archeological sites in Grand Canyon, Arizona

A daily precipitation dataset covering a large part of the American Southwest was compiled for online electronic distribution ( http://pubs.usgs.gov/of/2014/1006/ ). The dataset contains 10.8 million observations spanning January 1893 through January 2009 from 846 weather stations in six states and 13 climate divisions. In addition to processing the data for distribution, water-year totals and other statistical parameters were calculated for each station with more than 2 years of observations. Division-wide total precipitation, expressed as the average deviation from the individual station means of a climate division, shows that the region—including the Grand Canyon, Arizona, area—has been affected by alternating multidecadal episodes of drought and wet conditions. In addition to compiling and analyzing the long-term regional precipitation data, a second dataset consisting of high-temporal-resolution precipitation measurements collected between November 2003 and January 2009 from 10 localities along the Colorado River in Grand Canyon was compiled. An exploratory study of these high-temporal-resolution precipitation measurements suggests that on a daily basis precipitation patterns are generally similar to those at a long-term weather station in the canyon, which in turn resembles the patterns at other long-term stations on the canyon rims; however, precipitation amounts recorded by the individual inner canyon weather stations can vary substantially from station to station. Daily and seasonal rainfall patterns apparent in these data are not random. For example, the inner canyon record, although short and fragmented, reveals three episodes of widespread, heavy precipitation in late summer 2004, early winter 2005, and summer 2007. The 2004 event and several others had sufficient rainfall to initiate potentially pervasive erosion of the late Holocene terraces and related archeological features located along the Colorado River in Grand Canyon.

Arizona↗
USGS1979

A Late Pleistocene and Holocene pollen record from Laguna de las Trancas, northern coastal Santa Cruz County, California

A 2.1-m core from Laguna de las Trancas, a marsh atop a landslide in northern Santa Cruz County, Calif., has yielded a pollen record for the period between about 30,000 B.P. and roughly 5,000 B.P. Three pollen zones are recognized; the earliest is characterized by high frequencies of pine pollen and is correlated with a mid-Wisconsinan interstade of the mid-continent. The middle zone contains high frequencies of both pine and fir (Abies, probably A. grandis) pollen and is correlated with the last full glacial interval (upper Wisconsinan). The upper zone is dominated by redwood (Sequoia) pollen and represents latest Pleistocene to middle Holocene. The past few thousand years are not represented in the core. The pollen evidence indicates that during the full glacial period the mean annual temperature at the site was about 2 ° C to 3 ° C lower than it is today. We attribute this small difference to the stabilizing effect of marine upwelling on the temperature regime in the immediate vicinity of the coast. Precipitation may have been about 20 percent higher as a result of longer winter wet seasons.

California↗