The imprint of crustal density heterogeneities on regional seismic wave propagation

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.

Seismic waveform tomography of the Central and Eastern Mediterranean upper mantle

“…In previous experiments, we showed that density has a clear imprint on the seismic wavefield and can in principle be inverted for (Płonka et al, 2016;Blom et al, 2017). This is especially the case at longer periods (Takeuchi and Saito, 1972).…”

Seismic waveform tomography of the central and eastern Mediterranean upper mantle

“…It is imaged in various other studies (e.g. Amaru, 2007;Biryol et al, 2011;Portner et al, 2018) and is interpreted as the deeper part of the Cyprian slab. In previous studies, the shallow part of this slab was imaged as near horizontal, stretching northward from the Cyprus trench (e.g.…”

“…ParaView (Ahrens et al, 2005). Amaru, 2007;Biryol et al, 2011;Portner et al, 2018;Hosseini, 2016). In our model, the fast anomaly flattens out towards the 410 km discontinuity and is not imaged in deeper parts (Fig.…”

“…Classical ray tomography has been applied numerous times to study this area, both using body and surface waves (e.g. Spakman et al, 1988;Morelli, 1997, 2003;Amaru, 2007;Biryol et al, 2011;Koulakov et al, 2015;Portner et al, 2018;Snieder, 1988;Zielhuis and Nolet, 1994;Marone et al, 2004;Schivardi and Morelli, 2009;Salaün et al, 2012;Legendre et al, 2012).…”

Full Waveform Inversion Beneath the Central Andes: Insight Into the Dehydration of the Nazca Slab and Delamination of the Back‐Arc Lithosphere

“…Although the parameterization specifies six parameters at each point not all can be resolved independently. In order to reduce the possible bias from a fixed density (Blom et al., 2017; Płonka et al., 2016), we update the density through the iterations but abstain from the interpretations due to the inferior resolution relative to the seismic velocity parameters. The number and type of velocity parameters being inverted for are varied through the stages of the multi‐scale inversion (see Section 3.3).…”