The seismic attenuation structure of the East Pacific Rise

Wilcock, William S. D.

doi:10.1575/1912/5487

Cited by 7 publications

(20 citation statements)

References 128 publications

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“…The seismic network (Figure 1) Solomon, 1977;Duschenes et al, 1981;Trdhu, 1982]. The DOBH has a broad 5-to 80-Hz passband, and the theoretical response is in good agreement with the results of on-bottom self-calibration tests obtained during the experiment [Wilcock, 1992]. The DOBH has a broad 5-to 80-Hz passband, and the theoretical response is in good agreement with the results of on-bottom self-calibration tests obtained during the experiment [Wilcock, 1992].…”

Section: Methodsmentioning

confidence: 67%

“…During the tomography experiment, records from OBS 3 were dominated by a coupling resonance [e.g., Sutton et al, 1981] which precludes meaningful spectral estimates of t*. Extensive tests were conducted to measure the response and linearity of the AOBH below the threshold amplitude for saturation [Wilcock, 1992]. AOBH 8 realfunctioned during the experiment and did not record waveforms suitable for spectral analysis.…”

Section: Methodsmentioning

confidence: 99%

“…The AOBH records continuously with a direct analog tape recorder, and its response must be measured. Because t* estimates are generally obtained over a nearly constant frequency interval for each AOBH, these t* biases can be largely eliminated by a fixed station correction [Wilcock, 1992]. The effective AOBH bandwidth varies among instruments from 5-35 Hz to 5-60 Hz (Figure 2a), and waveforms must generally be discarded at ranges less than 10-15 km because the analog tape saturates.…”

Section: Methodsmentioning

confidence: 99%

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Seismic attenuation structure of the East Pacific Rise near 9°30′N

Wilcock¹,

Solomon²,

Purdy³

et al. 1995

J. Geophys. Res.

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We describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9°32′N on the East Pacific Rise. Over 3500 estimates of t* are obtained from 0.4‐ to 0.7‐s‐long windows aligned with crustal P phases, including diffractions above and below a midcrustal magma chamber and Moho reflections which cross the rise axis at a range of lower crustal depths. We apply a smoothest model inversion algorithm to the t* measurements to derive images of apparent crustal Q−1 both 20 km off axis and within a 16×16 km area centered on the rise crest. The models resolve regions of high attenuation in the uppermost crust and in a low‐Q zone which extends from midcrustal to lower‐crustal depths beneath the rise axis. Off axis, Q values in the upper 1 km average 35–50, while at depths greater than 2–3 km Q is at least 500–1000. The high levels of attenuation in the uppermost crust probably result from the combined effect of factional, fluid flow, and scattering mechanisms. Within 1–3 km of the rise axis, Q increases markedly in the uppermost 1 km to about 65. If the increase in attenuation off axis is entirely due to the ∼300‐m increase in the thickness of layer 2A extrusives required by seismic velocity measurements, then Q in layer 2A must be about 10–20. No measurements of Q are obtained in the immediate vicinity of the 1.6‐km‐deep axial magma lens because no wave paths cross the rise axis through this region. The diffractions beneath the magma chamber and the Moho reflections require a low‐Q region, with minimum Q values of 20–50, which extends from no more than 2.5 km depth to the base of the crust. These values are similar to laboratory measurements of Q obtained at solidus temperatures and constrain the low‐Q region to contain no more than a few percent melt. The axial magma chamber, which comprises a melt lens and an underlying crystal mush zone, must be confined to a narrow, 1‐km‐thick region through which no rise‐crossing paths pass. Inversions for along‐axis structure in the low‐Q anomaly show a 20–25% increase in attenuation at 2–3 km depth north of 9°34′N but resolve no such trend at 4–6 km depth. The along‐axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do not require systematic along‐axis variations in magma supply.

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Section: Methodsmentioning

confidence: 67%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Seismic attenuation structure of the East Pacific Rise near 9°30′N

Wilcock¹,

Solomon²,

Purdy³

et al. 1995

J. Geophys. Res.

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“…(1) using an inversion technique described in detail by Wilcock [1992] and Wilcock et al [1992]. Onedimensional ray-tracing using the velocity models of Christeson et al [submitted] was used to calculate the wave paths for the first arrivals.…”

Section: Ps(f) Pi(f)mentioning

confidence: 99%

“…There are few constraints on densities beneath the mush zone. Delay time and attenuation tomographic inversions suggest that the maximum melt fraction in this region is <3-5%, but a narrow zone of higher melt fraction would not be resolved by these techniques [Toomey et al, 1990;Caress et al, 1992;Wilcock, 1992]. We assumed a density contrast of 3-35 kg/m 3 between the AMC root and the surrounding layer 3 crust; this represents a 1-14% partial melt fraction beneath the mush zone if the melt density is 2700 kg/m 3 and the matrix density is 2950 kg/m 3 .…”

Section: Dike Subsidencementioning

confidence: 99%

Seismic constraints on shallow crustal processes at the East Pacific Rise

Christeson¹

1994

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This thesis is concerned with understanding how oceanic crust is emplaced at mid-ocean ridges. The emphasis is upon fast-spreading ridges, and the use of seismic techniques to image the uppermost several hundred meters of the crust. We present the results of nine on-bottom seismic refraction experiments carried out over young East Pacific Rise (EPR) crust. The experiments are unusual in that both the source and receiver are located within a few meters of the seafloor, allowing high-resolution determinations of shallow crustal structure. Three experiments were located within the axial summit caldera (ASC), over 'zero-age' crust. The seismic structure at these three locations is fundamentally the same, with a thin (<60 m) surficial low-velocity (<2.5 km/s) layer, a 100-150 m thick transition zone with velocities increasing by approximately 2.5 km/s, and a layer with velocities of -5 km/s at a depth below the seafloor of 130-190 m. The surficial low-velocity layer and transition zone are defined as seismic layer 2A, and the -5 km/s layer as layer 2B. Both the surficial low-velocity layer and the transition zone double in thickness within -1 km of the rise axis, with the depth to the 2A/2B boundary increasing from -150 m to 300-350 m over this range. The doubling of layer 2A thickness within 1-2 km of the rise axis is confirmed by multi-channel seismic (MCS) and wide-angle profile (WAP) data, which also indicate that there is no further systematic change in thickness with greater range from the rise axis.Inversions for attenuation structure demonstrate that the layer 2A/2B interface is not only a velocity boundary, but also an attenuation boundary, with Q increasing from 10-20 within layer 2A to >70 in layer 2B.The results of MCS and wide-angle experiments over plausible velocity structures are predicted quantitatively, based on velocity models constructed from on-bottom seismic refraction experiments and expanding spread profiles. We conclude that the accuracy of correlating the prominent shallow reflector observed in MCS and WAP data with the layer 2A/2B boundary is strongly dependent on the structure within layer 2A. If layer 2A consists of a surficial low-velocity layer overlying a steep velocity gradient (our gradient model), then there is an excellent correspondence between the two-way travel times to the shallow reflector and the base of layer 2A. However, the shallow reflector may follow structure within layer 2A if the upper crust contains more than one high-gradient region (our step model). A shallow structure similar to the step model is consistent with onbottom refraction experiments and expanding spread profiles located over zero-age EPR crust. With distance from the rise axis, this step-like structure is apparently destroyed, and is converted into a single steep gradient similar in appearance to our gradient model. Layer 2A is interpreted to be composed of the extrusive section and transition zone, with layer 2B consisting of the sheeted dike complex. This implies that the top of the dikes subs...

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Seismic propagation across the East Pacific Rise: Finite difference experiments and implications for seismic tomography

Wilcock¹,

Dougherty²,

Solomon³

et al. 1993

J. Geophys. Res.

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We use a full-waveform acousto-elastic finite difference technique to investigate seismic propagation across the East Pacific Rise at 9ø30'N for a two-dimensional velocity model based on that proposed by Vera and others (1990). The primary feature of the model is an upper crustal low-velocity region, corresponding to the axial magma charnber, which includes a small magma body located 1.6 km beneath the seafloor at the rise axis. The high velocity gradients in this region result in a complex pattern of propagation which includes considerable scattering of energy above and below the magma chamber. A qualitative comparison of finite difference seismograms with data collected by receivers located 9 km and 20 km off axis during a tomography experiment at 9ø30'N shows generally good agreement. For paths that cross the rise axis, the first arrival in the finite difference solutions diffracts above the magma chamber. This phase has a very low amplitude and at larger offsets falls below the ambient noise levels observed during the tomography experiment. In such cases, the first arrival with significant energy is a diffraction from below the magma chamber. A high-amplitude Mohoturning (PrnP) phase which results from the large velocity change across the Moho beneath the rise axis is apparent in both the finite difference solutions and the observations. Ray-theoretical calculations of the paths of the diffracted arrivals are very unstable, and for the diffractions above the magma chamber no solution can be found with a single-precision algorithm. Synthetic delay-time inversions using an approximate ray-tracing algorithm demonstrate the importance of ensuring that picked arrival times are assigned to paths that pass to the correct side of the magma body. Synthetic inversions of spectral estimates of t* show that Q-1 models are compromised not only if the ray paths are inco•ect but also if t* estimates include significant contributions from more than one phase. Deterministic scattering from the magma chamber may contribute noticeably to spectral estimates of t*, but the results of the finite difference experiments imply that high levels of attenuation observed for phases passing below the magma chamber are predominantly the result of intrinsic attenuation. ], it is necessary to ensure that arrival time picks are assigned to the correct phase.The spectral method of attenuation tomography involves inversions of estimates of apparent attenuation derived from the power spectra of waveforms to yield models of the reciprocal of the quality factor Q. Since the technique requires knowledge of the velocity structure and ray paths, errors in the assumed velocity model will propagate into the attenuation model. Additional errors will result if attenuation estimates are obtained for a time window that includes more than one phase or if estimates are assigned to the incorrect phase. Q-1 models obtained by the spectral technique will inevitably include contributions from effects other than intrinsic attenuation such as scattering [e.g., Cor...

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The seismic attenuation structure of the East Pacific Rise

Cited by 7 publications

References 128 publications

Seismic attenuation structure of the East Pacific Rise near 9°30′N

Seismic attenuation structure of the East Pacific Rise near 9°30′N

Seismic constraints on shallow crustal processes at the East Pacific Rise

Seismic propagation across the East Pacific Rise: Finite difference experiments and implications for seismic tomography

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