Abstract. Density heterogeneities are the source of mass transport in the Earth. However, the 3D density structure remains poorly constrained because traveltimes of seismic waves are only weakly sensitive to density. Inspired by recent developments in seismic waveform tomography, we investigate if the visibility of 3D density heterogeneities may be improved by inverting not only traveltimes of specific seismic phases but complete seismograms. As a first step in this direction, we perform numerical experiments to estimate the effect of 3D 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 traveltime shifts of up to ∼ 1 s and amplitude variations of several tens of percent over propagation distances of ∼ 1000 km. Both amplitude and traveltime variations increase with increasing epicentral distance and increasing medium complexity. 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 traveltime differences will often be small, amplitude variations 15 exceeding ±10 % are comparable to those induced by 3D velocity structure and attenuation. While these results certainly encourage more research on the development of 3D density tomography, they also suggest that current full-waveform inversions that use amplitude information may be biased due to the neglect of 3D variations in density.
RegSEM: a versatile code based on the spectral element method to compute seismic wave propagation at the regional scale
