Abstract
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.
Highlights
Lateral variations in density are the driving force behind mass transport in the Earth, from crust to core (e.g. Kennett and Bunge, 2008; Turcotte and Schubert, 2014)
Since scattered waves caused by density heterogeneities must exist, one may conclude that seismograms in general are sensitive to density variations, but this information cannot be contained in direct body wave travel times
A visual waveform comparison suggests that the travel-time differences may be a finitefrequency effect, meaning that waveform differences within short time intervals translate into time shifts when these are measured by cross-correlation within a finite frequency band
Summary
Lateral variations in density are the driving force behind mass transport in the Earth, from crust to core (e.g. Kennett and Bunge, 2008; Turcotte and Schubert, 2014). When a body wave reaches a density perturbation, the resulting scattered wave propagates backwards, meaning that it cannot interfere with the incident wave unless the heterogeneity is located within one wavelength from either source or receiver (Wu and Aki, 1985; Tarantola, 1986; Trampert and Fichtner, 2013) This is in contrast to the scattered wave caused by a velocity heterogeneity, which propagates along with the incident wave, thereby leading to a finite-frequency travel-time shift (e.g. Tong et al, 1998; Marquering et al, 1999; Dahlen et al, 2000). At the long-period end of the seismic spectrum, the gravest normal modes of the Earth are sensitive to long-wavelength density structure as a result of the gravitational restoring force (Woodhouse, 1988; Dahlen and Tromp, 1998; Woodhouse and Deuss, 2007) This may be used to constrain density variations in the lower mantle where a low-degree structure is known to be dominant This may be used to constrain density variations in the lower mantle where a low-degree structure is known to be dominant (e.g. Dziewonski et al, 1977; Becker and Boschi, 2002)
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