Synthetic seismogram images of upper mantle structure: No evidence for a 520‐km discontinuity
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Abstract:
Seismological data used by Shearer (1990, 1991) to infer the existence of a seismic discontinuity at 520 km depth are compared with complete long‐period body wave seismograms calculated for the International Association of Seismology and Physics of the Earth's Interior 1991 (Iaspei91) Earth model. The Iaspei91 model does not contain a seismic discontinuity at or near 520 km depth. The observed P and SH multiples caused by topside reflections and SS precursors caused by underside reflections from the 410‐km and 660‐km discontinuities are well reproduced by the synthetic stacks. The synthetics exhibit clear “phase extrema” between these reflections that correlate well with similar features in the observed waveform stacks. Shearer interpreted these “phase extrema” as separate reflections from a seismic discontinuity at 520 km depth. Cross‐correlation analysis of synthetic seismograms gives an apparent discontinuity depth of about 520 km for P and SH multiples as well as SS precursors. Similarly, amplitude analysis of synthetic upper mantle reflections is in reasonable agreement with the observations reported by Shearer (1991). The phase extrema seen in the synthetics are the result of an extended, multicycle wavelet which is composed of depth phases and other structural phases such as reverberations from the crustal layer, convolved with the instrument response of long‐period stations of the Global Seismograph Network. The comparison of observational data, presented by Shearer (1990, 1991), with the synthetic seismogram stacks of this paper shows that the claim of good seismological evidence for a 520‐km seismic discontinuity as a global feature is not compelling.Keywords:
Seismogram
Classification of discontinuities
Discontinuity (linguistics)
Synthetic seismogram
Earth structure
Seismometer
Core–mantle boundary
Abstract If energy emitted by a seismic source such as an earthquake is recorded on a suitable backbone array of seismometers, source‐receiver interferometry (SRI) is a method that allows those recordings to be projected to the location of another target seismometer, providing an estimate of the seismogram that would have been recorded at that location. Since the other seismometer may not have been deployed at the time at which the source occurred, this renders possible the concept of “retrospective seismology” whereby the installation of a sensor at one period of time allows the construction of virtual seismograms as though that sensor had been active before or after its period of installation. Here we construct such virtual seismograms on target sensors in both industrial seismic and earthquake seismology settings, using both active seismic sources and ambient seismic noise to construct SRI propagators , and on length scales ranging over 5 orders of magnitude from ∼40 m to ∼2500 km. In each case we compare seismograms constructed at target sensors by SRI to those actually recorded on the same sensors. We show that spatial integrations required by interferometric theory can be calculated over irregular receiver arrays by embedding these arrays within 2‐D spatial Voronoi cells, thus improving spatial interpolation and interferometric results. The results of SRI are significantly improved by restricting the backbone receiver array to include approximately those receivers that provide a stationary‐phase contribution to the interferometric integrals. Finally, we apply both correlation‐correlation and correlation‐convolution SRI and show that the latter constructs fewer nonphysical arrivals.
Seismogram
Seismometer
Seismic interferometry
Synthetic seismogram
Seismic Noise
Seismic array
Interpolation
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Seismogram
Synthetic seismogram
Earth structure
Reflection
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Waveform modelling is a numerical technique used for minimizing the difference between observed and synthetic seismograms. The matching between observed and synthetic seismogram can be achieved by adjusting some controlling parameters, while generating the synthetic seismogram. Earthquakes are recorded using seismometer/accelerometers, and synthetic seismograms can be generated by approximating earthquake source, medium properties and instrument response. In most cases, instrument response is readily available by the instrument manufacturers, whereas earthquake source and medium properties are modelled mathematically. Synthetic seismograms can be generated either by adjusting the earth structure or the source parameters of the observed event. If the earth structure is given, earthquake source parameters can be extracted from the observed seismogram using the moment tensor inversion technique. In contrast, earth's subsurface information like velocity, density and attenuation, can be extracted from the observed seismic waves with the knowledge of earthquake source parameters using seismic waveform tomography.
Seismogram
Synthetic seismogram
Seismometer
Microseism
Earth structure
Moment tensor
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Seismogram
Synthetic seismogram
Seismic velocity
Velocity gradient
Seismic refraction
Earth structure
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Seismogram
Synthetic seismogram
Seismometer
Earth structure
Earth model
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