Factors Affecting Seismic Amplitudes*

S U M M A R YWe examine along-axis variations in melt content of the axial magma lens (AML) beneath the fast-spreading East Pacific Rise (EPR) using an amplitude variation with angle of incidence (AVA) crossplotting method applied to multichannel seismic data acquired in 2008. The AVA crossplotting method, which has been developed for and, so far, applied for hydrocarbon prospection in sediments, is for the first time applied to a hardrock environment. We focus our analysis on 2-D data collected along the EPR axis from 9• 29.8 N to 9• 58.4 N, a region which encompasses the sites of two well-documented submarine volcanic eruptions (1991-1992 and 2005-2006). AVA crossplotting is performed for a ∼53 km length of the EPR spanning nine individual AML segments (ranging in length from ∼3.2 to 8.5 km) previously identified from the geometry of the AML and disruptions in continuity. Our detailed analyses conducted at 62.5 m interval show that within most of the analysed segments melt content varies at spatial scales much smaller (a few hundred of metres) than the length of the fine-scale AML segments, suggesting high heterogeneity in melt concentration. At the time of our survey, about 2 yr after the eruption, our results indicate that the three AML segments that directly underlie the 2005-2006 lava flow are on average mostly molten. However, detailed analysis at finer-scale intervals for these three segments reveals AML pockets (from >62.5 to 812.5 m long) with a low melt fraction. The longest such mushy section is centred beneath the main eruption site at ∼9• 50.4 N, possibly reflecting a region of primary melt drainage during the 2005-2006 event. The complex geometry of fluid flow pathways within the crust above the AML and the different response times of fluid flow and venting to eruption and magma reservoir replenishment may contribute to the poor spatial correlation between incidence of hydrothermal vents and presence of highly molten AML. The presented results are an important step forward in our ability to resolve small-scale characteristics of the AML and recommend the AVA crossplotting as a tool for examining mid-ocean ridge magma-systems elsewhere.

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“…The thin bed (Widess 1973;Sheriff 1975;Kallweit & Wood 1982) configuration is relevant for AML studies, since most results for the northern EPR 9…”

Section: A (Intercept) Versus B (Slope) Crossplotting Interpretation mentioning

confidence: 99%

Distribution of melt along the East Pacific Rise from 9°30′ to 10°N from an amplitude variation with angle of incidence (AVA) technique

et al. 2015

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“…Intrinsic attenuation is treated above, but it is only one mechanism. Sheriff summarizes the possible mechanisms which include [86] 1. Intrinsic attunuation -This is the attenuation caused by conversion of acoustic energy to heat as a wave passes through a material.…”

Section: Sources Of Attenuationmentioning

confidence: 99%

Investigation of ocean acoustics using autonomous instrumentation to quantify the water-sediment boundary properties

Carey

Holmes

2006

The Journal of the Acoustical Society of America

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Several noise criteria methods commonly used in architectural acoustics have been quantitatively related to noise perception and task performance under a variety of ventilation systems-induced background noise conditions. Noise criteria, balanced noise criteria, room criteria, room criteria mark II, and A-weighted equivalent sound pressure level were examined. The first phase of the project included noise conditions controlled to be non-time-varying and nontonal, with neutral, rumbly, roaring, or hissy characteristics. An intermediate study examined exposure time length and types of performance tasks used. The final phase included noise conditions containing various levels of discrete tones from 120 to 595 Hz. Under each noise, subjects completed performance tasks and perception questionnaires. Results indicate task performance was significantly affected by perception of noise, but this relationship was not fully demonstrated by the criteria systems analyzed. The five criteria were generally well suited in describing subjective loudness perception, but some discrepancies in criteria spectral quality ratings and subjective perception existed. Finally, perception of annoyance changed based on the frequency and prominence of tones in noise, but these changes were not reflected in the criteria level or spectral quality ratings. Modifications to the existing criteria are recommended.

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“…Reflectivity results suggest that only when a strong mineral segregation occurs in mafic mylonites a good reflectivity could be reached between layers with pyroxene and layers with plagioclase (R z 0.04; Sheriff, 1975). Mechanical segregation of phases could be favored in high-grade shear zones affecting mafic protoliths because of the contrasting mechanical behavior of e.g.…”

Section: Reflectivitymentioning

confidence: 99%

Texture and elastic anisotropy of a mylonitic anorthosite from the Morin Shear Zone (Quebec, Canada)

Barreiro

Wenk

Vogel

2015

Journal of Structural Geology

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a b s t r a c tA sample of anorthosite from the granulite facies Morin Shear Zone (Quebec, Canada) was investigated for crystal preferred orientation and elastic anisotropy. Time-of-flight neutron diffraction data obtained with the HIPPO diffractometer at LANSCE were analyzed with the Rietveld method to obtain orientation distribution functions of the principal phases (plagioclase, clinopyroxene and orthopyroxene). Texture and microstructures are compatible with the plastic deformation of the aggregate under high-T conditions. All mineral phases depict a significant preferred orientation that could be related to the general top-to-the north shearing history of the Morin Shear Zone. Texture patterns suggest that (010)[001] in plagioclase and (110)[001] in clinopyroxene are likely dominant slip systems. Using preferred orientation data P-and S-waves velocities and elastic anisotropy were calculated and compared with previous studies to explore elastic properties of rocks with different pyroxene-plagioclase mixtures. P-wave velocity, S-wave splitting and anisotropy increase with clinopyroxene content. Seismic anisotropy is linked to the texture symmetry which can lead to large deviations between actual anisotropy and that measured along Cartesian XYZ sample directions (lineation/foliation reference frame). This is significant for the prediction and interpretation of seismic data, particularly for monoclinic or triclinic texture symmetries.

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Factors Affecting Seismic Amplitudes*

Abstract: SHERIFF,R. E.

Cited by 113 publications

References 3 publications

Distribution of melt along the East Pacific Rise from 9°30′ to 10°N from an amplitude variation with angle of incidence (AVA) technique

Distribution of melt along the East Pacific Rise from 9°30′ to 10°N from an amplitude variation with angle of incidence (AVA) technique

Investigation of ocean acoustics using autonomous instrumentation to quantify the water-sediment boundary properties

Texture and elastic anisotropy of a mylonitic anorthosite from the Morin Shear Zone (Quebec, Canada)

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