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At least 271 records · Page 15Linked to original sources
USGS2007

Oceanic methane hydrate: The character of the Blake Ridge hydrate stability zone, and the potential for methane extraction

Oceanic methane hydrates are mineral deposits formed from a crystalline 'ice' of methane and water in sea-floor sediments (buried to less than about 1 km) in water depths greater than about 500 m; economic hydrate deposits are probably restricted to water depths of between 1.5 km and 4 km. Gas hydrates increase a sediment's strength both by 'freezing' the sediment and by filling the pore spaces in a manner similar to water-ice in permafrost. Concentrated hydrate deposits may be underlain by significant volumes of methane gas, and these localities are the most favourable sites for methane gas extraction operations. Seismic reflection records indicate that trapped gas may blow-out naturally, causing large-scale seafloor collapse. In this paper, we consider both the physical properties and the structural integrity of the hydrate stability zone and the associated free gas deposits, with special reference to the Blake Ridge area, SE US offshore, in order to help establish a suitable framework for the safe, efficient, and economic recovery of methane from oceanic gas hydrates. We also consider the potential effects of the extraction of methane from hydrate (such as induced sea-floor faulting, gas venting, and gas-pocket collapse). We assess the ambient pressure effect on the production of methane by hydrate dissociation, and attempt to predict the likelihood of spontaneous gas flow in a production situation.Oceanic methane hydrates are mineral sits formed from a crystalline `ice' of methane and water in sea-floor sediments (buried to less than about 1 km) in water depths greater than about 500 m; economic hydrate deposits are probably restricted to water depths of between 1.5 km and 4 km. Gas hydrates increase a sediment's strength both by `freezing' the sediment and by filling the pore spaces in a manner similar to water-ice in permafrost. Concentrated hydrate deposits may be underlain by significant volumes of methane gas, and these localities are the most favourable sites for methane gas extraction operations. Seismic reflection records indicate that trapped gas may blow-out naturally, causing large-scale seafloor collapse. In this paper, we consider both the physical properties and the structural integrity of the hydrate stability zone and the associated free gas deposits, with special reference to the Blake Ridge area, SE US offshore, in order to help establish a suitable framework for the safe, efficient, and economic recovery of methane from oceanic gas hydrates. We also consider the potential effects of the extraction of methane from hydrate (such as induced sea-floor faulting, gas venting, and gas-pocket collapse). We assess the ambient pressure effect on the production of methane by hydrate dissociation, and attempt to predict the likelihood of spontaneous gas flow in a production situation.

Journal of Petroleum Geology↗
USGS2019

Lakes as paleoseismic records in a seismically-active, low-relief area (Rieti Basin, central Italy)

Small lakes in low relief areas are atypical candidates for studies on paleoseismicity, but their sediments can contain seismically induced event layers (seismites) generated through strong ground shaking, sediment transport, hydrological reorganization and/or changes in groundwater chemistry and flow. Lakes Lungo and Ripasottile are shallow lakes (<10m deep) located in the tectonically active Rieti Basin in the central Apennines, Italy, where strong normal faulting earthquakes (Mw 6.5 to 7.0) regularly occur. Sediment cores from these lakes provide paleoseismic indicators for the past ~1000 years. Sedimentological and geochemical analysis reveals four event layers identified in both lakes that correspond with documented large-scale earthquakes in 1298, 1349, 1639, and 1703 AD. Chronological correlation between earthquakes and possible seismites is reliable because of the unusually high resolution of sediment dating available for the studied cores. The common physical structure is a physically homogenous bed (homogenite) of re-suspended sediment consisting of a denser, high magnetic susceptibility (MS) clastic base, with organic matter concentrated above. Chemical signatures are associated with some event layers and may represent abrupt or transient shifts to a groundwater-dominated system, or permanent changes in groundwater flow and/or spring discharge. Excursions in δ13Corg may represent disruptions or changes in carbon source. Not all event layers show the same features, a result attributed to differences in seismic processes as well as the lake attributes, and anthropogenic modification. The observations made here may provide a new means of detecting paleoseismicity and may be applied to other low relief lakes in seismically active areas.

Rieti Basin↗
USGS2017

Using strain rates to forecast seismic hazards

One essential component in forecasting seismic hazards is observing the gradual accumulation of tectonic strain accumulation along faults before this strain is suddenly released as earthquakes. Typically, seismic hazard models are based on geologic estimates of slip rates along faults and historical records of seismic activity, neither of which records actively accumulating strain. But this strain can be estimated by geodesy: the precise measurement of tiny position changes of Earth’s surface, obtained from GPS , interferometric synthetic aperture radar ( InSAR ), or a variety of other instruments.

Eos, Earth and Space Science News↗
USGS2002

Analysis of long-period events recorded at Mount Etna (Italy) in 1992, and their relationship to eruptive activity

Seismic activity recorded at Mount Etna during 1992 was characterized by long-period (LP) events and tremor with fluctuating amplitudes. These signals were associated with the evolution of the eruptive activity that began on December 14, 1991. Following the occurrence of numerous volcano-tectonic earthquakes at the onset of the eruption, LP events dominated the overall seismicity starting in January, 1992. The LP activity occurred primarily in swarms, which were temporally correlated with episodic collapses of the crater floor in the Northeast Crater. Source depths determined for selected LP events suggest a source region located slightly east of Northeast Crater and extending from the surface to a depth of 2000 m. Based on the characteristic signatures of the time series, four families of LP events are identified. Each family shares common spectral peaks independent of azimuth and distance to the source. These spectral features are used to develop a fluid-filled crack model of the source. We hypothesize that the locus of the LP events represents a segment of the magma feeding system connecting a depressurizing magma body with a dike extending in the SSE direction along the western wall of Valle del Bove, toward the site of the Mount Etna eruption. We surmise that magma withdrawal from the source volume beneath Northeast Crater may have caused repeated collapses of the crater floor. Some collapse events may have produced pressure transients in the subjacent dike which acted as seismic wave sources for LP events.

Mount Etna, Sicily↗
USGS2011

Estimating seismic site response in Christchurch City (New Zealand) from dense low-cost aftershock arrays

The Mw 7.1 September 2010 Darfield earthquake, New Zealand, produced widespread damage and liquefaction ~40 km from the epicentre in Christchurch city. It was followed by the even more destructive Mw 6.2 February 2011 Christchurch aftershock directly beneath the city&rsquo;s southern suburbs. Seismic data recorded during the two large events suggest that site effects contributed to the variations in ground motion observed throughout Christchurch city. We use densely-spaced aftershock recordings of the Darfield earthquake to investigate variations in local seismic site response within the Christchurch urban area. Following the Darfield main shock we deployed a temporary array of ~180 low-cost 14-bit MEMS accelerometers linked to the global Quake-Catcher Network (QCN). These instruments provided dense station coverage (spacing ~2 km) to complement existing New Zealand national network strong motion stations (GeoNet) within Christchurch city. Well-constrained standard spectral ratios were derived for GeoNet stations using a reference station on Miocene basalt rock in the south of the city. For noisier QCN stations, the method was adapted to find a maximum likelihood estimate of spectral ratio amplitude taking into account the variance of noise at the respective stations. Spectral ratios for QCN stations are similar to nearby GeoNet stations when the maximum likelihood method is used. Our study suggests dense low-cost accelerometer aftershock arrays can provide useful information on local-scale ground motion properties for use in microzonation. Preliminary results indicate higher amplifications north of the city centre and strong high-frequency amplification in the small, shallower basin of Heathcote Valley.

Christchurch↗
USGS1981

Single-channel seismic-reflection profiles and sidescan-sonar records collected on Georges Bank

Subbottom seismic-reflection, echo-sounding, and sidescan-sonar records were collected by the U.S. Geological Survey during two cruises over Georges Bank aboard the R/V OCEANUS (cruise 056, March 6-15, 1979 and cruise 067, August 6-13, 1979) (fig. 1). The data collected during these two cruises contributed to a study of bed-form types and distribution and of sediment transport paths on Georges Bank. The equipment used on these cruises consisted of an EG&G Uniboom 12-kHz echo-sounder, and Klein sidescan-sonar system. The Uniboom data were collected on the OCEANUS 067 cruise along 185 km of trackline (fig. 1), and the 12-kHz echo-sounder and sidescan-sonar data were collected during both cruises along 465 km of trackline. Navigation during both cruises consisted of Loran-C fixes logged at least every 15 minutes. After the cruise, the navigation data were digitized and stored on magnetic tape. The original records can be studied at the U.S. Geological Survey Data Library at Woods Hole, MA 02543. Microfilm copies of the subbottom, echo-sounding, and sidescan-sonar records can be purchased only from the National Geophysical and Solar-Terrestrial Data Center, NOAA/EDIS/NGSDC, Code D621, 325 Broadway, Boulder, CO 80303 (303-497-6338).

Georges Bank↗
USGS2020

Co-eruptive tremor from Bogoslof volcano: Seismic wavefield composition at regional distances

We analyze seismic tremor recorded during eruptive activity over the course of the 2016–2017 eruption of Bogoslof volcano, Alaska. Only regional recordings of the tremor wavefield exist for Bogoslof, making it a challenge to place the recordings in context with other eruptions that are normally captured by local seismic data. We apply a technique of time-frequency polarization analysis to three-component seismic data to reveal the wavefield composition of Bogoslof eruption tremor. We find that at regional distances, the tremor is dominated by P-waves in the band from 1.5 to 10 Hz. Using this information, along with an enriched Bogoslof earthquake catalog, we obtain estimates of average reduced displacement ( D R ) for eruption tremor during 25 of the 70 Bogoslof events. D R reaches as high as approximately 40 cm 2 for two of the major events, similar to other VEI~3 eruptions in Alaska. Overall, average reduced displacement displays a weak correlation to plume height during the first half of the 9-month-long eruption sequence, with a few notable exceptions. The two events with the highest D R values also generated measurable eruption tremor at very-long-periods (VLP) between 0.05 and 0.15 Hz.

Alaska↗
USGS2026

Subaerially exposed Iceberg Lake sediments: An exceptional record of historical subaqueous earthquake disturbance at the eastern edge of the Alaskan-Aleutian subduction zone

Paleoseismic records are limited in the Yakutat Terrane (eastern edge of the Alaskan-Aleutian subduction zone) due to the extensive ice cover that hinders traditional methods such as trenching of the faults, but lacustrine sediments offer an alternative archive. We investigated lakebed sediments exposed after recent outburst floods (1999 CE) at Iceberg Lake, a glacier-dammed basin whose stratigraphy was revealed after the lake drained and partially eroded. We logged outcrops across the basin and sampled their sediments. Between annually laminated background deposits, we identified sediment gravity flow beds and in-situ soft-sediment deformation structures (convolutions, sand blows, and fractures) interpreted to be earthquake-induced. Our age model links some of the uppermost depositional and deformational events to the 1958, 1964 and 1979 CE earthquakes. These results demonstrate that Iceberg Lake was a sensitive recorder of seismic shaking and its sediments hold strong potential for producing a paleoseismic record for the northern Yakutat Terrane.

Alaska↗
USGS2025

Identifying precursors and tracking pulses of magma ascent in multidisciplinary data during the 2018–2023 phreatomagmatic eruption at Semisopochnoi Island, Alaska

The 2018–2023 phreatomagmatic eruptions at Semisopochnoi Island, Alaska produced abundant long-period (LP) seismicity, harmonic and broadband tremor, and explosion signals over several well-monitored periods of eruption and quiescence. The corresponding dataset provides an excellent opportunity to investigate precursory and syn -eruptive geophysical signals of long-lived phreatomagmatic eruptions using multiparameter observations. We generated explosion and LP event catalogs through novel implementations of the REDPy ( Hotovec-Ellis, 2024 ) repeating event detector in mid-2021 following a network upgrade and the onset of a new phase of the eruption. The hundreds of detected explosions show a high degree of infrasound waveform similarity over more than a year, indicating a repeating source mechanism likely associated with explosive magma-water interaction. The seismic LP catalog shows that events began over a month prior to renewed explosive activity at the beginning of August 2021, and that lower frequency index (FI) LPs were generated in the week prior to the onset of explosions. We applied a recently developed machine learning tool (VOISS-Net, Tan et al., 2024 ) to catalog abundant broadband and harmonic seismic tremor recorded before and during the renewed explosive activity, along with LPs and explosions. The tremor catalogs complement the LP and explosion catalogs by filling out the seismic sequence with the dominant signal types. Together, these catalogs reveal a seismic sequence of renewed unrest that started with several weeks of LP events, followed by LPs with lower FI values and harmonic tremor in the days prior to explosive activity, and finally the onset of discrete explosions and broadband eruption tremor. We interpret this sequence as the ascent of a new pulse of magma that first interacted with the hydrothermal/groundwater system to produce LPs, followed by harmonic tremor, and that ultimately drove explosive magma-water interactions and periods of continuous ash emissions. The 2021 seismic sequence, in combination with long-term records of satellite SO 2 emissions, deformation from interferometric synthetic aperture radar (InSAR) analysis, ash sample analysis, infrasound, and volcano tectonic seismicity, allows us to interpret the entire 9-year period of unrest and eruption that began with an intrusion and earthquake swarm in 2014.

Alaska↗
USGS1982

High-resolution seismic-reflection profiles from the R/V Columbus Iselin, cruise CI 7-78-2, over the continental shelf and slope in the Georges Bank area

In September 1978, the U. S. Geological Survey (USGS) collected 5,029 km of single-channel seismic-reflection data from the Georges Bank area of the Atlantic Continental Shelf and Slope during the R/V COLUMBUS ISELIN cruise CI 7-78-2. The purpose of the cruise was to determine the location and frequency of mass sediment movement and other geologic hazards along the Continental Slope. Navigation of the COLUMBUS ISELIN was by LORAN-C; position fixes were automatically recorded at 5-minute intervals and manually plotted and recorded at 15-minute intervals. The navigation equipment included a Northstar 6000 LORAN receiver and a Texas Instruments Silent 700 tape and paper recorder. The seismic equipment consisted of a 40-in 3 airgun, a 5-in 3 airgun, a Teledyne 600-joule mnisparker, and ORE (Ocean Research Equipment Inc.) 3.5-kHz transducer. The seismic profiles obtained were recorded on paper by EPC (EPC Labs Inc.) recorders and on magnetic tape by a 7-channel analog tape recorder. Overall, the data quality is excellent, and penetration and resolution are good although in some areas, the underlying structure was obscured by rough topography. The original records may be viewed at the USGS office in Woods Hole, Massachusetts. Microfilm copies of the data may be purchased only from the National Geophysical and Solar-Terrestrial Data Center, NOAA/EDIS/NGSDC, Code D621, 325 Broadway, Boulder, CO 80303 (Telephone: 303-497-6338).

Open-File Report↗
USGS2024

Application of non-stationary shear-wave velocity randomization approach to predict 1D seismic site response and its variability at two downhole array recordings

Accounting for uncertainties in seismic site response is crucial to improving the performance of one-dimensional (1D) ground response analyses (GRAs) at downhole array recording sites. In addition to site effects, uncertainties in 1D-GRAs can also be contributed from the seismic source and/or path. Though often representing not more than one percent of the distance (path) from the source, site conditions are known to have an enormous influence on ground shaking. In this study, we focus on the site shear-wave velocity ( V S ) structure, which is the main ingredient for estimating the variability of site response. As such, V S can manifest aleatory uncertainties related to the effects of small-scale spatial heterogeneities within the near surface, thus V S can substantially modify ground shaking during earthquakes. We apply a novel V S randomization approach to propagate the small-scale heterogeneities of V S to estimate seismic site response within a non-stationary probabilistic framework. The randomization approach generates samples of V S profiles that are used to perform several 1D-GRAs and obtain an averaged site response and related variability. The proposed method is implemented on data recorded at two downhole array sites with different subsurface soil conditions: a soft soil site on Treasure Island (California, United States of America) and a rock outcrop site in Cadarache (South-East France). We show that synthetic surface-to-borehole transfer functions from 1D-GRAs provide an acceptable fit to the empirical transfer functions from low-motion earthquake records and succeed in reproducing most of the site-specific seismic response variability. The remaining mismatch between transfer functions is likely due to insufficient precision on the seismic bedrock and the impedance contrast. The variability in site response is discussed with emphasis on the role of V S small-scale heterogeneities, attenuation, and input motion incidence angle in ground motion variability for the site and soil conditions at both locations.

California↗
USGS2012

Observations constraining near-source ground motion estimated from locally recorded seismograms

To estimate the seismic hazard to underground facilities or operations in the environs of a mining-induced tremor or a natural earthquake, it is useful to be able to relate locally recorded seismic waveforms to peak ground velocity and slip at the causative fault. For this purpose, far-field S wave pulses are analyzed to define the faulting slip D and near-fault peak ground velocity D /2 that give rise to the most significant ground motion. This most intense region of faulting, an assumed circular asperity, has radius r within a broader source zone of radius r 0 , which is traditionally calculated from the corner frequency of the S wave spectrum. In developing relationships between peak far-field velocity v and peak acceleration a, and the source processes of the asperity, D and D , as well as its radius r , the key model assumption is that r = k β/ω, where ω is the angular frequency of the sinusoidal velocity pulse of maximum amplitude, β is the sheaf wave speed, and k is a constant. Observations in deep-level gold mines of fault slip and slip velocity as well as laboratory observations of slip rate as a function of stress drop for stick-slip failure support a choice of about k = 2.34, the value commonly used for estimating r 0 using the Brune model. In particular, observations of fault slip up to 410 mm for mining-induced tremors in the moment magnitude range 4–5 are consistent with D = 8.1 R v/β, where R is hypocentral distance. Moreover, estimates based on underground damage of near-fault ground velocities ranging up to 3.5 m/s are in accord with D /2 = 1.28(β/μ) ρ R a, where μ is the modulus of rigidity and ρ is the density. Alternatively, the average slip velocity 〈 D 〉 can be expressed in terms of the stress drop Δσ a of the asperity as 〈 D 〉 = 0.51 β Δσ a /μ, and the agreement of this relationship with measurements made during stick-slip failure in the laboratory is good. To the extent that seismic slip exterior to the asperity is a consequence of preevent suppression of slip due to the asperity, the broader-scale( r 0 ) slip can be related to that of the asperity. Just as the asperity radius r can be estimated from r = 2.34 βv/a, an alternative estimate for r 0 is given by r 0 = ρ R a M 0 /[75.8ρμ( R v) 2 ], the results of which are generally in good agreement with estimates based on the spectral corner frequency method.

Journal of Geophysical Research↗
USGS1992

Seismic images of a Grenvillian terrane boundary

A series of gently dipping reflection zones extending to mid-crustal depths is recorded by seismic data from Lakes Ontario and Erie. These prominent reflection zones define a broad complex of southeast-dipping ductile thrust faults in the interior of the Grenville orogen. One major reflection zone provides the first image of a proposed Grenvillian suture&mdash;the listric boundary zone between allochthonous terranes of the Central Gneiss and Central Metasedimentary belts. Curvilinear bands of reflections that may represent "ramp folds" and "ramp anticlines" that originally formed in a deep crustal-scale duplex abut several faults. Vertical stacking of some curvilinear features suggests coeval or later out-of-sequence faulting of imbricated and folded thrust sheets. Grenvillian structure reflections are overlain by a thin, wedge-shaped package of shallow-dipping reflections that probably originates from sediments deposited in a local half graben developed during a period of post-Grenville extension. This is the first seismic evidence for such extension in this region, which could have occurred during terminal collapse of the Grenville orogen, or could have marked the beginning of pre-Appalachian continental rifting.

Lake Erie, Lake Ontario↗
USGS1962

Crustal structure in Nevada and southern Idaho from nuclear explosions

The time of first arrival of seismic waves generated by 4 underground nuclear explosions at the Nevada Test Site (NTS) and recorded along a line extending north into southern Idaho is expressed as T 0 = 0. 00 + &Delta;/3.0 (assumed), T 1 = 0 .40 + &Delta;/6.03, and T 2 = 6.15 + &Delta;/7.84, where time is in seconds and the shot-detector distance (&Delta;) is in km. Assuming constant velocities and horizontal layers, crustal thickness in the vicinity of NTS was determined to be 28 km. Delays in the traveltime segment T 2 , which represents P n , indicate that the crust may thicken to 32 km in northern Nevada. A third phase, expressed as T 3 = 14.48 + &Delta;/7.84, was also recognized and has arrival times appropriate for SPS. Amplitudes of P n were determined at 7 places from recordings of seismic waves from one underground nuclear explosion (ANTLER).

Nevada;Idaho↗