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At least 757 records · Page 42Linked to original sources

USGS2017

Historical changes in organic matter input to the muddy sediments along the Zhejiang-Fujian Coast, China over the past 160 years

The burial of sedimentary organic matter (SOM) in the large river-influenced estuarine-coastal regions is affected by hydrodynamic sorting, diagenesis and human activities. Typically, the inner shelf region of the East China Sea is a major carbon sink of the Yangtze River-derived fine-grained sediments. Most of the previous work concentrated on the studies of surface sediments or used a single-proxy in this region. In this study, two cores from the Zhejiang-Fujian Coast were analyzed using bulk (TOC, TN and δ 13 C TOC ) and molecular biomarker ( n -alkane, brassicasterol, dinosterol and glycerol dialkyl glycerol tetraether lipids) techniques to clarify the sources, spatiotemporal distribution and fate of SOM in the Yangtze River Estuary and adjacent shelf. Results from this study indicated that the effects of diagenesis and diffusion on different sedimentary biomarkers resulted in overestimation of the relative contribution of terrestrial organic matter (%OM terr ), compared with those based on δ 13 C TOC . The amounts of terrestrial plant organic matter (OM plant ) and%OM terr in sediments decreased offshore. In contrast, the amounts of marine organic matter (OM marine ) increased offshore, but closer to the Yangtze River mouth, the amounts of soil organic matter (OM soil ) increased. Moreover, the amounts of TOC, OM plant and OM marine biomarkers increased, but OM soil and%OM terr decreased over time in recent decades. Our study suggests that spatial organic matter distribution patterns in marine shelf sediments were controlled primarily by hydrodynamic sorting and nutrient concentrations, and temporally diverse patterns were controlled predominantly by anthropogenic influence (e.g., dam construction and soil conservation, reclamation and agricultural plantations, anthropogenic nutrient input, dust storms, eutrophication, etc) and climate events (e.g., interdecadal climatic jump and heavy rain events) in the geological period.

Zhejiang-Fujian Coast↗

USGS2017

Coastal river plumes: Collisions and coalescence

Plumes of buoyant river water spread in the ocean from river mouths, and these plumes influence water quality, sediment dispersal, primary productivity, and circulation along the world’s coasts. Most investigations of river plumes have focused on large rivers in a coastal region, for which the physical spreading of the plume is assumed to be independent from the influence of other buoyant plumes. Here we provide new understanding of the spreading patterns of multiple plumes interacting along simplified coastal settings by investigating: (i) the relative likelihood of plume-to-plume interactions at different settings using geophysical scaling, (ii) the diversity of plume frontal collision types and the effects of these collisions on spreading patterns of plume waters using a two-dimensional hydrodynamic model, and (iii) the fundamental differences in plume spreading patterns between coasts with single and multiple rivers using a three-dimensional hydrodynamic model. Geophysical scaling suggests that coastal margins with numerous small rivers (watershed areas < 10,000 km 2 ), such as found along most active geologic coastal margins, were much more likely to have river plumes that collide and interact than coastal settings with large rivers (watershed areas > 100,000 km 2 ). When two plume fronts meet, several types of collision attributes were found, including refection, subduction and occlusion. We found that the relative differences in pre-collision plume densities and thicknesses strongly influenced the resulting collision types. The three-dimensional spreading of buoyant plumes was found to be influenced by the presence of additional rivers for all modeled scenarios, including those with and without Coriolis and wind. Combined, these results suggest that plume-to-plume interactions are common phenomena for coastal regions offshore of the world’s smaller rivers and for coastal settings with multiple river mouths in close proximity, and that the spreading and fate of river waters in these settings will be strongly influenced by these interactions. We conclude that new investigations are needed to characterize how plumes interact offshore of river mouths to better understand the transport and fate of terrestrial sources of pollution, nutrients and other materials in the ocean.

Progress in Oceanography↗

USGS2022

An 11,300 yr record of paleoclimatology and paleoceanography of the central California coast in a gravity core from Pioneer Seamount

Diatom, pollen, silicoflagellate, and biogenic opal analyses from a 155 cm-long gravity core from Pioneer Seamount, offshore Santa Cruz, California (PS1410-06 GC, latitude 37.3°N, longitude 123.4°W, water depth 2165 m) are compiled for the last ~11,300 years and compared with those of ODP 1019 and TN062-O550 from northern California. The relative abundance record of the subtropical diatom Fragilariopsis doliolus has similar bimodal Holocene patterns in all three cores, suggesting that sea surface temperatures (SST) were lower during the middle part of the Holocene than they were during the later and earlier parts. The relative abundance of coastal redwood ( Sequoia sempervirens ) pollen, a proxy for fog and coastal upwelling, displays stepwise increases in ODP 1019 and TN062-O550 between ~ 4000 and 3000 cal yr. BP, but its relative abundance in PS1410-06 GC increases gradually throughout the past 10,200 yr without any major steps. Similarly, biogenic silica (opal) displays stepwise increases at ~3600 and 2900 cal yr. BP in ODP 1019 and TN062-O550, respectively, whereas opal increases more gradually in PS1410-06 GC during the past 10,100 yr with relatively minor steps at ~3100 and ~2600 cal yr. BP. Together, coastal redwood and opal argue for a more gradual late Holocene increase in coastal upwelling along the coast of central California compared with that off northern California, where onshore-offshore gradients are more distinct.

California↗

USGS2014

Coastal tectonics on the eastern margin of the Pacific Rim: Late Quaternary sea-level history and uplift rates, Channel Islands National Park, California, USA

The Pacific Rim is a region where tectonic processes play a significant role in coastal landscape evolution. Coastal California, on the eastern margin of the Pacific Rm, is very active tectonically and geomorphic expressions of this include uplifted marine terraces. There have been, however, conflicting estimates of the rate of late Quaternary uplift of marine terraces in coastal California, particularly for the orthern Channel Islands. In the present study, the terraces on San Miguel Island and Santa Rosa Island were mapped and new age estimates were generated using uranium-series dating of fossil corals and amino acid geochronology of fossil mollusks. Results indicate that the 2nd terrace on both islands is ~120 ka and the 1st terrace on Santa Rosa Island is ~80 ka. These ages correspond to two global high-sea stands of the Last Interglacial complex, marine isotope stages (MIS) 5.5 and 51, respectively. The age estimates indicate that San Miguel Island and Santa Rosa Island have been tectonically uplifted at rates of 0.12e0.20 m/ka in the late Quaternary, similar to uplift rates inferred from previous studies on neighboring San Cruz Island. The newly estimated uplift rates for the northern Channel Islands are, however, an order of magnitude lower than a recent study that generated uplift rates from an offshore terrace dating to the Last Glacial period. The differences between the estimated uplift rates in the present study and the offshore study are explained by the magnitude of glacial isostatic adjustment (GIA) effects that were not known at the time of the earlier study. Set in the larger context of northeastern Pacific Rim tectonics, Channel Islands uplift rates are higher than those coastal localities on the margin of the East Pacific Rise spreading center, but slightly lower than those of most localities adjacent to the Cascadia subduction zone. The uplift rates reported here for the northern Channel Islands are similar to those reported for most other localities where strike-slip tectonics are dominant, but lower than localities where restraining bends (such as the Big Bend of the San Andreas Fault) result in crustal shortening.

California↗

USGS2005

Nowcast modeling of Escherichia coli concentrations at multiple urban beaches of southern Lake Michigan

Predictive modeling for Escherichia coli concentrations at effluent-dominated beaches may be a favorable alternative to current, routinely criticized monitoring standards. The ability to model numerous beaches simultaneously and provide real-time data decreases cost and effort associated with beach monitoring. In 2004, five Lake Michigan beaches and the nearby Little Calumet River outfall were monitored for E. coli 7 days a week; on nine occasions, samples were analyzed for coliphage to indicate a sewage source. Ambient lake, river, and weather conditions were measured or obtained from independent monitoring sources. Positive tests for coliphage analysis indicated sewage was present in the river and on bathing beaches following heavy rainfall. Models were developed separately for days with prevailing onshore and offshore winds due to the strong influence of wind direction in determining the river's impact on the beaches. Using regression modeling, it was determined that during onshore winds, E. coli could be adequately predicted using wave height, lake chlorophyll and turbidity, and river turbidity ( R 2 =0.635 , N=94 ); model performance decreased for offshore winds using wave height, wave period, and precipitation ( R 2 =0.320 , N=124 ). Variation was better explained at individual beaches. Overall, the models only failed to predict E. coli levels above the EPA closure limit (235 CFU/100 ml) on five of eleven occasions, indicating that the model is a more reliable alternative to the monitoring approach employed at most recreational beaches.

Water Research↗

USGS1998

Chemistry, isotopic composition, and origin of a methane-hydrogen sulfide hydrate at the Cascadia subduction zone

Although the presence of extensive gas hydrate on the Cascadia margin, offshore from the western U.S. and Canada, has been inferred from marine seismic records and pore water chemistry, solid gas hydrate has only been found at one location. At Ocean Drilling Program (ODP) Site 892, offshore from central Oregon, gas hydrate was recovered close to the sediment-water interface at 2–19 m below the seafloor (mbsf) at 670 m water depth. The gas hydrate occurs as elongated platy crystals or crystal aggregates, mostly disseminated irregularly, with higher concentrations occurring in discrete zones, thin layers, and/or veinlets parallel or oblique to the bedding. A 2- to 3-cm thick massive gas hydrate layer, parallel to bedding, was recovered at ∼ 17 mbsf. Gas from a sample of this layer was composed of both CH 4 and H 2 S. This sample is the first mixed-gas hydrate of CH 4 H 2 S documented in ODP; it also contains ethane and minor amounts of CO 2 . Measured temperatures of the recovered core ranged from 2 to −1.8°C and are 6 to 8 degrees lower than in-situ temperatures. These temperature anomalies were caused by the partial dissociation of the CH 4 H 2 S hydrate during recovery without a pressure core sampler.During this dissociation, toxic levels of H 2 S (δ 34 S, +27.4‰) were released. The δ 13 C values of the CH 4 in the gas hydrate, −64.5 to −67.5‰ (PDB) , together with δD values of −197 to −199‰ (SMOW) indicate a primarily microbial source for the CH 4 . The δ 18 O value of the hydrate H 2 O is +2.9‰ (SMOW) , comparable with the experimental fractionation factor for sea-ice. The unusual composition (CH 4 H 2 S) and depth distribution (2–19 mbsf) of this gas hydrate indicate mixing between a methane-rich fluid with a pore fluid enriched in sulfide; at this site the former is advecting along an inclined fault into the active sulfate reduction zone. The facts that the CH 4 H 2 S hydrate is primarily confined to the present day active sulfate reduction zone (2–19 mbsf), and that from here down to the BSR depth (19–68 mbsf) the gas hydrate inferred to exist is a ≥99% CH 4 hydrate, suggest that the mixing of CH 4 and H 2 S is a geologically young process. Because the existence of a mixed CH 4 H 2 S hydrate is indicative of moderate to intense advection of a methane-rich fluid into a near surface active sulfate reduction zone, tectonically active (faulted) margins with organic-rich sediments and moderate to high sedimentation rates are the most likely regions of occurrence. The extension of such a mixed hydrate below the sulfate reduction zone should reflect the time-span of methane advection into the sulfate reduction zone.

Earth and Planetary Science Letters↗

USGS2003

Winter-time circulation and sediment transport in the Hudson Shelf Valley

The Hudson Shelf Valley is a bathymetric low that extends across the continental shelf offshore of New York and New Jersey. From December 1999 to April 2000 a field experiment was carried out to investigate the transport of sediment in the shelf and valley system. Near-bed tripods and water-column moorings were deployed at water depths from 38 to 75 m in the axis of the shelf valley and at about 26 m on the adjacent shelves offshore of New Jersey and Long Island, New York. These measured suspended sediment concentrations, current velocities, waves, and water column properties. This paper analyzes observations made during December 1999 and January 2000, and presents the first direct near-bed measurements of suspended sediment concentration and sediment flux from the region. Sediment transport within the Hudson Shelf Valley was coherent over tens of kilometers, and usually aligned with the axis of the shelf valley. Down-valley (off-shore) transport was associated with energetic waves, winds from the east, moderate current velocities (5-10 cm/s), and sea level setup at Sandy Hook, NJ. Up-valley (shoreward) transport occurred frequently, and was associated with winds from the west, low wave energy, high current velocities (20-40 cm/s), and sea level set-down at the coast. Within the shelf valley, net sediment flux (the product of near-bed concentration and velocity) was directed shoreward, up the axis of the valley. Current velocities and suspended sediment fluxes on the New York and New Jersey continental shelves were lower than within the shelf valley, and exhibited greater variability in alignment. Longer term meteorological data indicate that wind, setup, and wave conditions during the study period were more conducive to up-valley transport than seasonal data suggest as average. To relate the observed up-valley sediment flux to observed accumulation of contaminants within the Hudson Shelf Valley requires consideration of transport over longer timescales than those observed here, and methods that account for the region's complex bathymetry, sediment distribution, and circulation. ?? 2003 Elsevier Science Ltd. All rights reserved.

New Jersey, New York↗

USGS1994

The influence of ice on southern Lake Michigan coastal erosion

Coastal ice does not protect the coast but enhances erosion by displacing severe winter wave energy from the beach to the shoreface and by entraining and transporting sediment alongshore and offshore. Three aspects of winter ice in Lake Michigan were studied over a 3-year period and found to have an important influence on coastal sediment dynamics and the coastal sediment budget: (1) the influence of coastal ice on shoreface morphology, (2) the transport of littoral sediments by ice, and (3) the formation of anchor and underwater ice as a frequent and important event entraining and transporting sediment. Coastal lake ice includes a belt of mobile brash (ice blocks) and slush and a dynamic nearshore ice complex consisting of an icefoot, a lakeward sequence of wave-generated ice ridges, and intervening ice lagoons. Our studies indicate that the nearshore ice complex contains a sediment load (0.2 - 1.2 t/m of coast) that is roughly equivalent to the average amount of sand eroded from the coastal bluffs and to the amount sand ice-rafted offshore to the deep lake basin each year. Up to 0.28 t/m of coast can be entrained by ice in a single anchor-ice event, and separate events occurred on 15 days in January 1991. The brash/slush belt is the most important system component responsible for ice-induced sediment transport. Estimates of longshore ice drift, ice volume, and ice-borne sediment load suggest that 0.36 to 4.14 × 10 3 t/d are transported alongshore.

Journal of Great Lakes Research↗

USGS1995

Availability of lake trout reproductive habitat in the Great Lakes

A decades-long program to reestablish self-sustaining stocks of lake trout ( Salvelinus namaycush ) in the four lower Great Lakes produced excellent fisheries supported by stocked fish. These fish spawned widely and small numbers of their offspring were collected intermittently from Lakes Michigan, Huron, and Ontario, but no self-sustaining stocks were established. Irt this paper we address habitat sufficiency as a factor in the failure of stocked lake trout to established self-sustaining populations in the four lower Great Lakes. We present the previously unpublished results of lake trout spawning habitat surveys conducted at seven sites in the Great Lakes since 1987 and we compare them with the published results of similar surveys conducted at 24 other sites in the four lower lakes since 1981. Our evaluation indicates all but two of these sites can support the production of viable fry from spawnings by the shallow-water strains of lake trout that are stocked in the Great Lakes. However, some of the best spawning, egg, and fry habitat in the lower Great Lakes seems to be at deeper offshore sites that may be unattractive to these shallow-water strains. Thus, we suggest also stocking the lower four lakes with strains from Lake Superior that might more fully exploit the best spawning habitat at these deeper, offshore sites.

Journal of Great Lakes Research↗

USGS1998

Maturity schedules of lake trout in Lake Michigan

We determined maturity schedules of male and female lake trout (Salvelinus namaycush) in Lake Michigan from nearshore populations and from an offshore population on Sheboygan Reef, which is located in midlake. Gill nets and bottom trawls were used to catch lake trout in fall 1994 and 1995 from two nearshore sites and Sheboygan Reef. Each lake trout was judged immature or mature, based on visual examination of gonads. Probit analysis, coupled with relative potency testing, revealed that age-at-maturity and length-at-maturity were similar at the two nearshore sites, but that lake trout from the nearshore sites matured at a significantly earlier age than lake trout from Sheboygan Reef. However, length at maturity for the nearshore populations was nearly identical to that for the offshore population, suggesting that rate of lake trout maturation in Lake Michigan was governed by growth rather than age. Half of the lake trout males reached maturity at a total length of 580 mm, whereas half of the females were mature at a length of 640 mm. Over half of nearshore males were mature by age 5, and over half the nearshore females matured by age 6. Due to a slower growth rate, maturity was delayed by 2 years on Sheboygan Reef compared with the nearshore populations. Documentation of this delay in maturation may be useful in deciding stocking allocations for lake trout rehabilitation in Lake Michigan.

Journal of Great Lakes Research↗

USGS1998

Depth and substrate as determinants of distribution of juvenile flathead sole (Hippoglossoides elassodon) and rock sole (Pleuronectes bilineatus), in Kachemak Bay, Alaska

Three transects in Kachemak Bay, Alaska, were sampled in September 1994, May and August 1995, and February, May, and August 1996. Juvenile flathead sole, Hippoglossoides elassodon, and rock sole, Pleuronectes bilineatus, were the most abundant flatfishes, comprising 65-85% of all fiatfishes captured at any period. Collections of fish and sediments were made at regular depth contour intervals of l0 m. Habitat distribution was described by depth at 10 m increments and sediment percent weights of gravel, sand, and mud. Year-round habitat of flathead sole age-0 was primarily from 40 to 60 m, and age-1 habitat was primarily from 40 to 80 m. Summer habitat of rock sole age-0 and -1 was from 10 to 30 m, and in winter they moved offshore to depths of up to 150 m. Both age classes of flathead sole were most abundant on mixed mud sediments, while age-1 were also in high abundance on muddy sand sediments. Rock sole age-0 and -1 were most abundant on sand, though age-1 were also found on a variety of sediments both finer and coarser grained than sand. Flathead sole and rock sole had distinctive depth and sediment habitats. When habitat overlap occurred between the species, it was most often due to rock sole moving offshore in the winter. Abundances were not significantly different among seasons for age-1 flatfishes.

Journal of Sea Research↗

USGS2022

Review of past gas Production attempts from subsurface gas hydrate deposits and necessity of long-term production testing

This paper summarizes the conditions, applied techniques, results, and lessons of major field gas production attempts from gas hydrates in the past and the necessity of longer term production testing with the scale of years to fulfill the gap between the currently available information and the knowledge required for commercial development. The temporal and spatial scales of field production test projects employing depressurization have expanded since 2002. The results from the projects have proved the applicability of these techniques in both onshore and offshore conditions. However, many technical and reservoir condition-related issues have emerged in gas production, and the gap between current status and industrial requirements is still large. Sand control, artificial lift, and related flow assurance issues are common technical issues that impact onshore and offshore production testing operations. Different reservoir responses were observed well by well, and discrepancy between model predictions and actual field measurements were seen, although reasonable matches were made for short-term behaviors. Those observations suggest that temporal change of the wellbore and near-wellbore conditions and reservoir heterogeneity that cannot be fully modeled have caused complex short-term responses to the depressurization operations. To ensure the long-term operational stability and reliability of the prediction technologies for production behaviors that are essential for commercialization of gas hydrate resources, gas hydrate production testing with comparable duration with commercial operations are necessary. Due to the locality of geological conditions in gas hydrate reservoirs, numerous gas production tests will be required to understand the factors controlling gas production.

Journal of Energy and Fuels↗

USGS2004

Use of 87 Sr/ 86 Sr and δ 11 B to Identify Slag-Affected Sediment in Southern Lake Michigan

Slag is a ubiquitous byproduct of the iron-smelting industry and influences geochemistry and water quality in adjacent geologic units, ground and surface water. Despite extensive slag deposition along the Indiana shoreline of Lake Michigan, definitive evidence that slag has affected lakebed sediments has not been established. Concerns for the protection of water and ecosystem resources in the Great Lakes motivated this study to determine if strontium and boron isotopes could be used to identify and delineate slag-affected bed sediment in Lake Michigan. Sixty-five samples of bed sediment were acquired from the southern lobe of Lake Michigan and analyzed for 87Sr/86Sr and ??11B. Samples immediately offshore from Indiana steel mills and slag-disposal sites contained higher median 87Sr/86Sr values (0.70881) than shoreline sediments collected elsewhere in the basin (0.70847) and uniquely decreased with increasing distance from the shoreline. The highest ??11B values occurred in sediments from the Indiana shoreline (+12.9 to 16.4???) but were also elevated in sediments collected offshore from three Lake Michigan cities (+11.7 to 12.7???). Contoured isotope data indicated that 82-154 km2 of bed sediment along the Indiana shoreline had elevated 87Sr/86Sr and ??11B values relative to shoreline sediments elsewhere in southern Lake Michigan.

Environmental Science & Technology↗

USGS2004

A bootstrap approach to computing uncertainty in inferred oil and gas reserve estimates

This study develops confidence intervals for estimates of inferred oil and gas reserves based on bootstrap procedures. Inferred reserves are expected additions to proved reserves in previously discovered conventional oil and gas fields. Estimates of inferred reserves accounted for 65% of the total oil and 34% of the total gas assessed in the U.S. Geological Survey’s 1995 National Assessment of oil and gas in US onshore and State offshore areas. When the same computational methods used in the 1995 Assessment are applied to more recent data, the 80-year (from 1997 through 2076) inferred reserve estimates for pre-1997 discoveries located in the lower 48 onshore and state offshore areas amounted to a total of 39.7 billion barrels of oil (BBO) and 293 trillion cubic feet (TCF) of gas. The 90% confidence interval about the oil estimate derived from the bootstrap approach is 22.4 BBO to 69.5 BBO. The comparable 90% confidence interval for the inferred gas reserve estimate is 217 TCF to 413 TCF. The 90% confidence interval describes the uncertainty that should be attached to the estimates. It also provides a basis for developing scenarios to explore the implications for energy policy analysis.

Natural Resources Research↗

USGS2004

Suspended sediment and hydrodynamics above mildly sloped long wave ripples

We investigate the spatial and temporal distribution of suspended sediment and the associated hydrodynamics over mildly sloped long wave ripples on the inner shelf. These bedforms had wavelengths of approximately 1 m and heights of approximately 5 cm, in a mean water depth of 4 m. The vertical and temporal structures of the suspended sediment concentration (SSC) are consistent with the entrainment of sediment on the offshore flank of the ripple, and rapid vertical mixing at the time of flow reversal, followed by advection onshore by the onshore fluid motion. This work confirms that the mechanism for sediment suspension above low-amplitude, long wave ripples is similar to the vortex formation process expected over steeper vortex ripples. Numerical simulations of the flow using the Dune2d model indicate that a separated rotational flow structure is generated at the flank of the ripple on the seaward side of the ripple crest, near the time of flow reversal. The simulations indicate that only one vortex is formed during each wave period, in agreement with the field observations. This asymmetry is due mainly to the presence of an offshore mean near-bed current of approximately 6 to 8 cm/s. The SSC is calculated by the model and compared to the field observations. A hydraulic bed roughness of 10 to 15 median grain diameters ( d 50 ) was used in order to match the model prediction to the observed SSC approximately 1 cm above the seabed (cab). However, the modeled SSC and turbulent kinetic energy were significantly lower than the field observations at elevations exceeding approximately 2 cab.

North Carolina↗

USGS2009

Northern Monterey Bay upwelling shadow front: Observations of a coastally and surface-trapped buoyant plume

During the upwelling season in central California, northwesterly winds along the coast produce a strong upwelling jet that originates at Point A??o Nuevo and flows southward across the mouth of Monterey Bay. A convergent front with a mean temperature change of 3.77 ?? 0.29??C develops between the warm interior waters and the cold offshore upwelling jet. To examine the forcing mechanisms driving the location and movement of the upwelling shadow front and its effects on biological communities in northern Monterey Bay, oceanographic conditions were monitored using cross-shelf mooring arrays, drifters, and hydrographic surveys along a 20 km stretch of coast extending northwestward from Santa Cruz, California, during the upwelling season of 2007 (May-September). The alongshore location of the upwelling shadow front at the northern edge of the bay was driven by: regional wind forcing, through an alongshore pressure gradient; buoyancy forces due to the temperature change across the front; and local wind forcing (the diurnal sea breeze). The upwelling shadow front behaved as a surface-trapped buoyant current, which is superimposed on a poleward barotropic current, moving up and down the coast up to several kilometers each day. We surmise that the front is advected poleward by a preexisting northward barotropic current of 0.10 m s -1 that arises due to an alongshore pressure gradient caused by focused upwelling at Point A??o Nuevo. The frontal circulation (onshore surface currents) breaks the typical two-dimensional wind-driven, cross-shelf circulation (offshore surface currents) and introduces another way for water, and the material it contains (e.g., pollutants, larvae), to go across the shelf toward shore.Copyright 2009 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗

USGS2011

The 25 October 2010 Mentawai tsunami earthquake, from real-time discriminants, finite-fault rupture, and tsunami excitation

The moment magnitude 7.8 earthquake that struck offshore the Mentawai islands in western Indonesia on 25 October 2010 created a locally large tsunami that caused more than 400 human causalities. We identify this earthquake as a rare slow‐source tsunami earthquake based on: 1) disproportionately large tsunami waves; 2) excessive rupture duration near 125 s; 3) predominantly shallow, near‐trench slip determined through finite‐fault modeling; and 4) deficiencies in energy‐to‐moment and energy‐to‐duration‐cubed ratios, the latter in near‐real time. We detail the real‐time solutions that identified the slow‐nature of this event, and evaluate how regional reductions in crustal rigidity along the shallow trench as determined by reduced rupture velocity contributed to increased slip, causing the 5–9 m local tsunami runup and observed transoceanic wave heights observed 1600 km to the southeast.

Mentawai Islands↗

USGS2011

Spatiotemporal evolution of dike opening and décollement slip at Kīlauea Volcano, Hawai'i

Rapid changes in ground tilt and GPS positions on Kīlauea Volcano, Hawai'i, are interpreted as resulting from a shallow, two‐segment dike intrusion into the east rift zone that began at 1217 UTC (0217 HST) on 17 June 2007 and lasted almost 3 days. As a result of the intrusion, a very small volume of basalt (about 1500 m 3 ) erupted on 19 June. Northward tilt at a coastal tiltmeter, subsidence of south flank GPS sites, southeastward displacements at southwestern flank GPS sites, and a swarm of flank earthquakes suggest that a slow slip event occurred on the décollement beneath Kīlauea's south flank concurrent with the rift intrusion. We use 4 min GPS positions that include estimates of time‐dependent tropospheric gradients and ground tilt data to study the spatial and temporal relationships between the two inferred shallow, steeply dipping dike segments extending from the surface to about 2 km depth and décollement slip at 8 km depth. We invert for the temporal evolution of distributed dike opening and décollement slip in independent inversions at each time step using a nonnegative least squares algorithm. On the basis of these inversions, the intrusion occurred in two stages that correspond spatially and temporally with concentrated rift zone seismicity. The dike opening began on the western of the two segments before jumping to the eastern segment, where the majority of opening accumulated. Dike opening preceded the start of décollement slip at an 84% confidence level; the latter is indicated by the onset of northward tilt of a coastal tiltmeter. Displacements at southwest flank GPS sites began about 18 h later and are interpreted as resulting from slow slip on the southwestern flank. Additional constraints on the evolution of the intrusion and décollement slip come from inversion of an Envisat interferogram that spans the intrusion until 0822 UTC on 18 June 2007, combined with GPS and tilt data. This inversion shows that up to 0822 UTC on 18 June, décollement slip is only required in a limited region offshore of Ka'ena Point. A similar inversion of the complete event, which includes GPS and tilt data up to 21 June and a second Envisat interferogram spanning the complete intrusion until 21 June, shows décollement slip spread westward across the south flank. This may suggest westward migration of the décollement slip as the event progressed.

Hawaii↗