Arctic lithospheric structure from delays of teleseismic P-wave reflections

Stewart, Ian

doi:10.1016/0040-1951(80)90126-2

Cited by 6 publications

(3 citation statements)

References 67 publications

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“…Some of the errors present in the ISC data set which hinder differential travel time studies based on this data are discussed by Darragh [1985], who used the ISC data to study a number of regions of the globe. Other studies using ISC data have concentrated on more localized regions, such as Atlantic Canada [Stewart, 1976], the Arctic [Stewart, 1980], and the central Atlantic [Dorbath and Dorbath, 1981].…”

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confidence: 99%

Global upper mantle structure from long‐period differential travel times

Woodward¹,

Masters²

1991

J. Geophys. Res.

180

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We have made over ten thousand measurements of PP‐P and SS ‐ S differential travel times from long‐period Global Digital Seismograph Network recordings of all events with mb ≥ 5.5 which occurred during the years 1976 to 1986. We have obtained reasonable global coverage, with very good sampling of several regions of the world. The residuals range from −5 to 5 s for PP‐P and −12 to 12 s for SS‐S and reveal a large‐scale pattern of heterogeneity when plotted at the reflected phase's bounce point. Our experiments indicate that lower‐mantle structure and source‐receiver structure can each contribute approximately ±0.5 s to our measured PP – P residuals so there is considerable signal to be explained. The pattern observed in the PP – P measurements is similar to the pattern observed in the SS‐S measurements, with the SS‐S residuals 2 to 4 times larger in magnitude. Comparisons of our measured residuals to those predicted by the upper‐mantle models of Woodhouse and Dziewonski show that the overall patterns are quite similar but the amplitude of the model residuals is roughly a factor of 2 too small. Comparisons with the predictions of a whole‐mantle model of Tanimoto again shows that the predicted pattern of residuals is reasonably consistent with the observations but now the predicted residuals are too large by about a factor of 2. This variation in predicted amplitudes is probably mainly due to differences in the near‐surface structure of the models. We have also binned our measurements according to the tectonic regionalization GTR1 of Jordan and find a qualitative correlation of average residual with tectonic region. In particular, Precambrian shields show a strong anomaly, and there is a correlation of residual size with the age of oceanic crust at the bounce point. For all tectonic regions the ratio of SS ‐ S to PP ‐ P residuals is approximately 2. This ratio is consistent with a thermal origin for the observed signal. Finally, our measurements show no compelling evidence for azimuthal anisotropy which might be related to fossil spreading direction or the direction of absolute plate motion. Such a signal is probably overwhelmed by the large signal from three‐dimensional structure.

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confidence: 99%

Global upper mantle structure from long‐period differential travel times

Woodward¹,

Masters²

1991

J. Geophys. Res.

180

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“…Times measured from records after compensating for the n / 2 phase shift are presumably reliable estimates of the traveltimes for the geometric ray path. However the onset times of PP read directly from seismograms are regularly reported in bulletins and are assumed to be reliable enough to be used (without compensation for non-causality) to determine anomalies in wavespeed in the upper mantle in the vicinity of the reflection point (Stewart 1976(Stewart , 1980Stewart & Keen 1978;Dorbath & Dorbath 1981;Darragh 1985). Perhaps surprisingly this assumption appears to be justified.…”

Section: On the Onset Of Mini-mar Phases 397mentioning

confidence: 99%

On the onset ofPPand other mini-max phases

Douglas

Sheehan

Stewart

1992

Geophysical Journal International

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S U M M A R Y Theory predicts that pulses that propagate on stationary non-minimum time paths, such as the P P path, undergo phase shifts such that the onset is emergent and the observed pulse is non-causal relative to the arrival time predicted by ray theory. The phase shift is n / 2 radians, at least at high frequencies, so that the observed pulses should approximate to the Hilbert transform of the pulse recorded over a minimum time path. Although onsets of phases that follow non-minimum time paths (usually called mini-max phases) are predicted to be emergent, those observed often seem clear and no more difficult to read than those of arrivals that follow minimum time paths. An example of a P P seismogram which is clearly emergent is shown here. Simulations obtained by Hilbert transforming the impulse responses of conventional long-period and short-period seismographs convolved with attenuation operators, show that the first half-cycle of mini-max phases which should have the emergent onset will usually be of low amplitude relative to the second half-cycle. This suggests that on observed seismograms the first half-cycle of mini-max phases may be obscured by earlier arrivals. The second half-cycle will then be taken as the first motion and could appear to have a well-defined onset. The effects of phase shifts that result in emergent onsets can be corrected for to some extent, and ideally onset times of mini-max phases would be read from such corrected records. However, onset times of P P read directly from conventional seismograms are reported in bulletins and these times have been assumed by some to be reliable enough to be used to determine anomalies in wavespeed in the upper mantle in the vicinity of the reflection point. Perhaps surprisingly the assumption appears to be justified in that the apparent onset time of a mini-max phase on a conventional short-period seismograph may be little different from the onset time read after correction for the n / 2 phase shifts.Key words: Hilbert transform, mini-max phases, P P onsets.By considering the radiation from a point disturbance in a uniform compressible liquid sphere, Jeffreys & Lapwood (1957) show that P P travels by a stationary non-minimum time path (commonly referred to as a mini-max path),

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“…A result of Gregersen (1970) is that the bedrock is right under the ice layer, within the resolution of the seismic surface waves, so the average thickness of the sedimentary layers is small under the ice cap. Stewart (1980) has studied the seismic waves, which are reflected from below the earth's surface (PP-waves) in the earth's crust and upper mantle under Greenland. These PP-wave reflections indicate that the interior of Greenland is typical continental shield in contrast to Baffin Bay and the area in eastern Greenland, which is covered by Tertiary basalt, so there is no trace of an upper mantle structure, which could connect the areas of volcanics in eastern and western Greenland (fig.…”

Section: Earthquakes and Tectonicsmentioning

confidence: 99%

Earthquakes in Greenland

1982

bgsd

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Data on earthquakes in Greenland from the international and Canadian seismological bulletins have been checked against the seismograms of the seismological stations in Greenland. A few new earthquakes have also been located based on seismograms from Greenland and Canada. A total of 103 reliable earthquakes have been confirmed, located and relocated. The earthquakes occur mainly along the coasts of eastern, northern and western Greenland. The largest earthquakes in Greenland have magnitudes around 5. There is no tectonic plate boundary in Greenland. The intraplate earthquake zones in north-eastern and in northern Greenland are situated as linear continuations of the plate boundaries near the bend of the mid ocean ridge close to Station Nord, between Spitzbergen and Greenland. Under the ice cap only a few earthquakes have occurred. In eastern and in northern Greenland a few swarms of earthquakes have been found. In western Greenland a sequence of seismic signals is noticed at a distance of 17 5 km from Godhavn. Its origin may be small earthquakes. The time sequence of the earthquakes in Greenland shows two time intervals of increased earthquake activity after the two largest earthquakes. This indicates that stress adjustments in the largest earthquakes give rise to stress adjustments in the smaller earthquakes more than 1000 km away, in other parts of Greenland. There is only limited correlation between earthquake activity and surface geology. It can not be determined whether the main cause of the intraplate earthquakes in Greenland is isostatic uplift following the latest ice age or tectonic plate motion in connection with sea floor spreading in the Norwegian-Greenland Sea and in the Arctic Ocean.

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Arctic lithospheric structure from delays of teleseismic P-wave reflections

Cited by 6 publications

References 67 publications

Global upper mantle structure from long‐period differential travel times

Global upper mantle structure from long‐period differential travel times

On the onset ofPPand other mini-max phases

Earthquakes in Greenland

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