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Solid Earth An interactive open-access journal of the European Geosciences Union
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Volume 7, issue 6
Solid Earth, 7, 1591–1608, 2016
https://doi.org/10.5194/se-7-1591-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Solid Earth, 7, 1591–1608, 2016
https://doi.org/10.5194/se-7-1591-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 Nov 2016

Research article | 29 Nov 2016

The imprint of crustal density heterogeneities on regional seismic wave propagation

Agnieszka Płonka1, Nienke Blom1, and Andreas Fichtner2 Agnieszka Płonka et al.
  • 1Department of Earth Sciences, Utrecht University, Utrecht, the Netherlands
  • 2Department of Earth Sciences, ETH Zurich, Zurich, Switzerland

Abstract. Density heterogeneities are the source of mass transport in the Earth. However, the 3-D density structure remains poorly constrained because travel times of seismic waves are only weakly sensitive to density. Inspired by recent developments in seismic waveform tomography, we investigate whether the visibility of 3-D density heterogeneities may be improved by inverting not only travel times of specific seismic phases but complete seismograms.

As a first step in this direction, we perform numerical experiments to estimate the effect of 3-D crustal density heterogeneities on regional seismic wave propagation. While a finite number of numerical experiments may not capture the full range of possible scenarios, our results still indicate that realistic crustal density variations may lead to travel-time shifts of up to  ∼ 1 s and amplitude variations of several tens of percent over propagation distances of  ∼ 1000 km. Both amplitude and travel-time variations increase with increasing epicentral distance and increasing medium complexity, i.e. decreasing correlation length of the heterogeneities. They are practically negligible when the correlation length of the heterogeneities is much larger than the wavelength. However, when the correlation length approaches the wavelength, density-induced waveform perturbations become prominent. Recent regional-scale full-waveform inversions that resolve structure at the scale of a wavelength already reach this regime.

Our numerical experiments suggest that waveform perturbations induced by realistic crustal density variations can be observed in high-quality regional seismic data. While density-induced travel-time differences will often be small, amplitude variations exceeding ±10 % are comparable to those induced by 3-D velocity structure and attenuation. While these results certainly encourage more research on the development of 3-D density tomography, they also suggest that current full-waveform inversions that use amplitude information may be biased due to the neglect of 3-D variations in density.

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Imaging the inside of the Earth requires knowledge of how the Earth's properties affect seismic recordings. However, certain properties, such as density, affect seismograms in a way that is not fully known. Using numerical simulations, we can calculate a synthetic seismogram for a medium with and without density heterogeneities, and then compare the two signals. That way, we quantify the density effect on a seismogram. We also show that it is visible and growing as the wavefield gets scattered.
Imaging the inside of the Earth requires knowledge of how the Earth's properties affect seismic...
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