Earthquake activity near Ascension Island, South Atlantic Ocean, as seen by a combined seismic/hydrophone array

Hanson, J. A.; Given, Holly K.; Berger, Jonathan

doi:10.1016/0375-6505(96)00015-6

Cited by 5 publications

(3 citation statements)

References 18 publications

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“…9), and a Moho depth of z 12 km is predicted. A receiver-function analysis of data from the recently installed broadband seismometer on the island (Hanson, Given & Berger 1996) indicates a Moho depth of z 11 km at a radius of w 5 km from the seismometer (J. Hanson, personal communication, 1996), which is slightly shallower than, but broadly in agreement with, our result, given the uncertainties involved.…”

Section: Flexure Of the Lithospheresupporting

confidence: 89%

“…Although the ridge axis appears to have a morphology more typical of fast-spreading ridges in a region to the south of Ascension Island(Brozena & White 1990), at the latitude of Ascension Island a deep rift valley is present, indicating that the lithosphere has considerable flexural Flexure surface predicted for an elastic thickness of 3 km and a load and infill density of 2500 kg m-3. Filled circle marks the location of the 3 km borehole Ascension #l; triangle marks the location of the broadband seismic station ASCN(Hanson et al 1996).…”

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confidence: 99%

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Gravity anomalies and flexure of the lithosphere at Ascension Island

Minshull

Brozena

1997

Geophysical Journal International

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S U M M A R YAscension Island, in the northern South Atlantic, forms the summit of a volcanic edifice 60 km in diameter which places a substantial load on the underlying young oceanic lithosphere. An analysis of a combined data set of recent and historical surface-gravity and bathymetry measurements on and around the island suggests that the lithosphere responds to this load by flexure equivalent to that of an elastic plate only z 3 km thick, and that the mean density of the volcanic edifice is ~2 5 0 0 kg m-3. A steep gravity gradient across the island cannot be explained by a simple flexural model and must be attributed to lateral density variations within the volcano itself. The effective elastic thickness is considerably less than the expected z 12 km mechanical thickness of the ~6 Ma lithosphere loaded by the volcano, and less even than zero-age elastic thicknesses commonly observed at slow-spreading ridges with axial rift valleys. The unusually small elastic thickness may be attributed to the combined effects of the high curvature beneath the island, which produces bending stresses that are limited by the yield stress envelope, localized heating of the lithosphere during emplacement of the island, and crustal thickening. When these factors are taken into account, the observed flexure is consistent with rheological models based on experimental rock mechanics.

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Section: Flexure Of the Lithospheresupporting

confidence: 89%

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confidence: 99%

Gravity anomalies and flexure of the lithosphere at Ascension Island

Minshull

Brozena

1997

Geophysical Journal International

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“…Data from oceanic hydrophone arrays deployed mainly for military reconnaissance purposes are increasingly being applied to studies of underwater geophysical phenomena such as earthquakes, volcanic eruptions, and the timing of signals for climate studies. For example, Fox et al (1995) observed an underwater volcanic event using military hydrophone arrays, and in a previous study Hanson et al (1996) examined seismic activity on a mid-ocean ridge using military hydrophones in combination with a seismic station. As one component of the International Monitoring System, a global network of various types of sensors to be established under the Comprehensive Test Ban Treaty, hydrophones will monitor the ocean basins for underwater and atmospheric nuclear explosions and natural events such as earthquakes that could be confused with explosions.…”

Section: Introductionmentioning

confidence: 99%

Accurate azimuth estimates from a large aperture hydrophone array using T‐phase waveforms

Hanson

Given

1998

Geophysical Research Letters

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Abstract. A simple method is described whereby stationto-source azimuths are estimated by fitting a plane wave to envelope functions of T-phases observed on a 5-element hydrophone array around Ascension Island, South Atlantic Ocean. When applied to a data set of 55 earthquakes of known location ranging between 2 and 45 degrees distance from Ascension Island, estimated azimuths have a standard deviation of 3.3 degrees from reference azimuths when 3 or more hydrophone elements are used. The standard deviation decreases to 1.8 degrees if T-phase data from all 5 hydrophone elements are used. We also investigate variations in predicted errors for different array geometries and arrival azimuths. This simple method is amenable to automation and can easily be incorporated into a global monitoring system.

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Magma Storage at Ocean Islands

Barker

Rydeblad

Silva

2021

Geophysical Monograph Series

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The Cape Verde archipelago is a group of Ocean Islands in the Central Atlantic that forms two chains of islands trending Northwest and Southwest. Several of the islands are considered to be volcanically active, with frequent eruptions on Fogo. We examine the mineral chemistry and thermobarometry of the southern islands; Santiago, Fogo and Brava together with the Cadamosto Seamount. Our objective is to explore the magmatic storage system and implications for volcanic eruptions and associated hazards at Cape Verde. The volcanic rocks at Cape Verde are alkaline and dominantly mafic, whereas the island of Brava and the Cadamosto Seamount are unusually felsic. Clinopyroxene compositions range from 60 to 90 Mg# at Santiago and Fogo. In contrast, at Brava and the Cadamosto Seamount the clinopyroxene compositions are 5 to 75 Mg#. Mineral chemistry and zonation records fractional crystallization, recharge, aggregation of crystals, magma mixing and variations in thermal conditions of the magma at temperatures from 925 to 1250ºC. Magma storage depths at Santiago, Fogo, Brava and the Cadamosto Seamount are between 12 and 40 km, forming deep sub-Moho magma storage zones. Transient magma storage in the crust is suggested by fluid inclusion re-equilibration and pre-eruption seismicity. A global compilation of magma storage at Ocean Islands suggests deep magma storage is a common feature and volcanic eruptions are often associated with rapid magma ascent through the crust. Shallow magma storage is more variable and likely reflects local variations in crustal structure, sediment supply and tectonics. Petrological constraints on the magma plumbing system at Cape Verde and elsewhere are vital to integrate with deformation models and seismicity in order to improve understanding and mitigation of the volcanic hazards.

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Earthquake activity near Ascension Island, South Atlantic Ocean, as seen by a combined seismic/hydrophone array

Cited by 5 publications

References 18 publications

Gravity anomalies and flexure of the lithosphere at Ascension Island

Gravity anomalies and flexure of the lithosphere at Ascension Island

Accurate azimuth estimates from a large aperture hydrophone array using T‐phase waveforms

Magma Storage at Ocean Islands

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