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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 37 records · Page 2Linked to original sources
USGS2013

The complexity of reversals

This chapter contains sections titled: Introduction Some Illustrative Polarity Transition Records The Case for Complexity Conclusions

Geophysical Monograph Series↗
USGS2013

Challenges in making a seismic hazard map for Alaska and the Aleutians

We present a summary of the data and analyses leading to the revision of the time-independent probabilistic seismic hazard maps of Alaska and the Aleutians. These maps represent a revision of existing maps based on newly obtained data, and reflect best current judgments about methodology and approach. They have been prepared following the procedures and assumptions made in the preparation of the 2002 National Seismic Hazard Maps for the lower 48 States, and will be proposed for adoption in future revisions to the International Building Code. We present example maps for peak ground acceleration, 0.2 s spectral amplitude (SA), and 1.0 s SA at a probability level of 2% in 50 years (annual probability of 0.000404). In this summary, we emphasize issues encountered in preparation of the maps that motivate or require future investigation and research.

Alaska↗
USGS2013

A Bayesian method to rank different model forecasts of the same volcanic ash cloud: Chapter 24

Volcanic eruptions often spew fine ash high into the atmosphere, where it is carried downwind, forming long ash clouds that disrupt air traffic and pose a hazard to air travel. To mitigate such hazards, the community studying ash hazards must assess risk of ash ingestion for any flight path and provide robust and accurate forecasts of volcanic ash dispersal. We provide a quantitative and objective method to evaluate the efficacy of ash dispersal estimates from different models, using Bayes theorem to assess the predictions that each model makes about ash dispersal. We incorporate model and measurement uncertainty and produce a posterior probability for model input parameters. The integral of the posterior over all possible combinations of model inputs determines the evidence for each model and is used to compare models. We compare two different types of transport models, an Eulerian model (Ash3d) and a Langrangian model (PUFF), as applied to the 2010 eruptions of Eyjafjallajökull volcano in Iceland. The evidence for each model benefits from common physical characteristics of ash dispersal from an eruption column and provides a measure of how well each model forecasts cloud transport. Given the complexity of the wind fields, we find that the differences between these models depend upon the differences in the way the models disperse ash into the wind from the source plume. With continued observation, the accuracy of the estimates made by each model increases, increasing the efficacy of each model’s ability to simulate ash dispersal.

Geophysical Monograph↗
USGS2013

Intermediate‐depth intraslab earthquakes and arc volcanism as physical expressions of crustal and uppermost mantle metamorphism in subducting slabs

We elaborate on the well-known spatial association between axc volcanoes and Wadati Benioff zones and explore in detail their genetic relationships as dual physical expressions of slab metamorphism of the oceanic crust and uppermost mantle. At hypocentral depths less than 200 km intra slab Wadati-Benioff earthquakes tend to occur near the top surfaces of slabs. Subduction of very young lithosphere (age < 15-25 Ma) with high heat flow (> 75 mW/m 2) produces mainly shallow earthquakes and spaxse or absent arc volcanism. Subduction of older crust with normal heat flow (50-65 mW/m 2) produces markedly deeper intraslab earthquakes and generally normal volcanic vigor. Seismological observations show that the low-seismic-velocity gabbroic mineralogy of the crust may persist to depths of as much as 150 km in old, cold lithosphere but only to depths of 50-60 km in young, warm slabs. Metamorphic processes in the crust and shallow upper mantle of subducting slabs and the reactivation of faults originally created at shallow depths in the ocean basins probably control the occurrence of intraslab earthquakes to depths of as much as 350 kin. A conceptual model for this metamorphism incorporates the likely effects of water liberated by dehydration. Such dehydration facilitates both brittle faulting by fault reactivation and promotes the kinetics of the transformation of the anhydrous gabbro component of the crust to eclogite. Finite-element modelling shows that densification to eclogite is expected to produce extensional stresses in transformed crust and a smaller compression in the underlying mantle. This model helps explain why most intermediate-depth intraslab earthquakes occur just below the top surfaces of slabs and why many have focal mechanisms indicating down-dip extension. Young, warm slabs have mostly shallow intraslab earthquakes and sparse axc volcanoes because dehydration and eclogite formation largely cease before such slabs axe in contact with asthenosphere. These processes are evidently delayed by kinetic hindrance and the high pressure stability of hydrous phazes at low temperatures in older, colder subducting crust, and thus earthquake activity and asthenospheric-wedge melting tend to be focused at depths of 100-170 kin. Anomalous behavior correlated with the subduction of island-and seamount chains appears to be associated with anomalou shallow intraplate faulting and with perturbations of slab metamorphism by these chains.

Geophysical Monograph↗
USGS2015

Hawaiian volcanoes: From source to surface

Hawaiian Volcanoes, From Source to Surface is the outcome of an AGU Chapman Conference held on the Island of Hawaii in August 2012. As such, this monograph contains a diversity of research results that highlight the current understanding of how Hawaiian volcanoes work and point out fundamental questions requiring additional exploration.

Hawaii↗
USGS2010

Glacier modeling in support of field observations of mass balance at South Cascade Glacier, Washington, USA

The long-term USGS measurement and reporting of mass balance at South Cascade Glacier was assisted in balance years 2006 and 2007 by a new mass balance model. The model incorporates a temperature-index melt computation and accumulation is modeled from glacier air temperature and gaged precipitation at a remote site. Mass balance modeling was used with glaciological measurements to estimate dates and magnitudes of critical mass balance phenomena. In support of the modeling, a detailed analysis was made of the "glacier cooling effect" that reduces summer air temperature near the ice surface as compared to that predicted on the basis of a spatially uniform temperature lapse rate. The analysis was based on several years of data from measurements of near-surface air temperature on the glacier. The 2006 and 2007 winter balances of South Cascade Glacier, computed with this new, model-augmented methodology, were 2.61 and 3.41 mWE, respectively. The 2006 and 2007 summer balances were -4.20 and -3.63 mWE, respectively, and the 2006 and 2007 net balances were -1.59 and -0.22 mWE. PDF version of a presentation on the mass balance of South Cascade Glacier in Washington state. Presented at the American Geophysical Union Fall Meeting 2010.

Washington↗
USGS2010

Two-dimensional hydrodynamic modeling to quantify effects of peak-flow management on channel morphology and salmon-spawning habitat in the Cedar River, Washington

The Cedar River in Washington State originates on the western slope of the Cascade Range and provides the City of Seattle with most of its drinking water, while also supporting a productive salmon habitat. Water-resource managers require detailed information on how best to manage high-flow releases from Chester Morse Lake, a large reservoir on the Cedar River, during periods of heavy precipitation to minimize flooding, while mitigating negative effects on fish populations. Instream flow-management practices include provisions for adaptive management to promote and maintain healthy aquatic habitat in the river system. The current study is designed to understand the linkages between peak flow characteristics, geomorphic processes, riverine habitat, and biological responses. Specifically, two-dimensional hydrodynamic modeling is used to simulate and quantify the effects of the peak-flow magnitude, duration, and frequency on the channel morphology and salmon-spawning habitat. Two study reaches, representative of the typical geomorphic and ecologic characteristics of the Cedar River, were selected for the modeling. Detailed bathymetric data, collected with a real-time kinematic global positioning system and an acoustic Doppler current profiler, were combined with a LiDAR-derived digital elevation model in the overbank area to develop a computational mesh. The model is used to simulate water velocity, benthic shear stress, flood inundation, and morphologic changes in the gravel-bedded river under the current and alternative flood-release strategies. Simulations of morphologic change and salmon-redd scour by floods of differing magnitude and duration enable water-resource managers to incorporate model simulation results into adaptive management of peak flows in the Cedar River. PDF version of a presentation on hydrodynamic modelling in the Cedar River in Washington state. Presented at the American Geophysical Union Fall Meeting 2010.

Washington↗