15:45 〜 16:00
[SIT16-07] Radially anisotropic 3-D shear wave structure of the upper mantle beneath North America by multi-mode eikonal tomography
キーワード：表面波、北米、高次モード、アイコナールトモグラフィー
The complex structural features of the North American continent include tectonically active regions in the west and stable cratons in the east. The high-density Transportable Array (USArray) deployment in the last two decades has facilitated seismological studies to unravel the crust and mantle structure in North America. To delineate the deep root of the cratonic lithosphere and the lithosphere-asthenosphere transition (LAT) at a depth of 200 km or deeper, the use of higher-mode surface waves is inevitable.
In this study, we construct a new 3-D radially anisotropy S-wave model in North America using a hybrid approach incorporating the single-station method of multi-mode dispersion measurements with a fully nonlinear waveform fitting (Yoshizawa & Ekström, 2010) and the eikonal tomography (Lin et al., 2009). At first, average phase speeds along source-receiver paths for teleseismic events (Mw≧5.8, occurred in 2007-2015, 710 events) and all USArray stations and some other seismic networks in the U.S. are used to reconstruct travel-time (phase-front tracking) fields for each mode and period. The phase speed distributions for each event derived from the lateral gradient of travel-time fields are stacked for all events to reconstruct multi-mode phase speed models.
Then, the radially anisotropic 3-D model down to a depth of 400 km was inverted from a set of multi-mode phase speed maps of both Rayleigh and Love waves simultaneously, using the iterative nonlinear inversion method by Tarantola & Valette (1982). Following an earlier work by Yoshizawa (PEPI, 2014), we determined the upper and lower boundaries of the LAT, based on the negative peak of the S wave velocity gradient and the slowest S wave velocity below the lithosphere, respectively.
We could obtain models with a higher lateral resolution for the SV- and SH-wave speed structures shallower than 200 km, constrained mainly by the fundamental-mode surface waves. The upper mantle at such depth beneath the U.S. is characterized by faster SH wave speed than SV, particularly in the shallow lithosphere. The small-scale pattern of the heterogeneity found in the eikonal model well reflects geologic features such as Snake River Plain, Colorado Plateau, Rio Grande Rift, and New Madrid Seismic Zone.
The velocity contrast between the western and eastern U.S. at a depth shallower than 100–150 km has been imaged clearly in our model. We can see significant lateral changes in the LAT across the Rocky Mountains; e.g., the LAT tends to be shallower at around 100–200 km depth in the tectonically active western region but becomes deeper around 150–300 km depth in the eastern stable cratonic areas. Some local tectonic features can also be seen in the LAT, e.g., the lithospheric thinning with the shallow upper bound in the New Madrid Seismic Zone. Since the eikonal tomography enables us to use the relatively small number of higher-mode measurements effectively, both the lateral and vertical resolutions have been improved in our model, allowing us to image the localized lateral variations that reflect surface tectonics and cratonic roots.
For the radially anisotropic parameter, the entire continent at depths shallower than 100 km is characterized by ξ > 1, representing faster SH-wave speed than SV-wave speed. Frozen-in anisotropy is one possible interpretation of the existence of ξ > 1 beneath the continent. The contrast of ξ beneath the eastern U.S. suggests that the lithospheric keel of the Superior craton, which corresponds to the LAB, can be estimated at a depth of 200–250 km. Below 300 km depth, the two zones of ξ < 0.95 (faster SV) are noticeable; beneath the Cascadia Range and the Superior Craton. Such lower ξ can be interpreted as the mantle up- or the down-welling. The one beneath the Cascadia Range may reflect slab subduction, while the other beneath the Superior Craton may indicate the delamination or basal drag of the cratonic root.
In this study, we construct a new 3-D radially anisotropy S-wave model in North America using a hybrid approach incorporating the single-station method of multi-mode dispersion measurements with a fully nonlinear waveform fitting (Yoshizawa & Ekström, 2010) and the eikonal tomography (Lin et al., 2009). At first, average phase speeds along source-receiver paths for teleseismic events (Mw≧5.8, occurred in 2007-2015, 710 events) and all USArray stations and some other seismic networks in the U.S. are used to reconstruct travel-time (phase-front tracking) fields for each mode and period. The phase speed distributions for each event derived from the lateral gradient of travel-time fields are stacked for all events to reconstruct multi-mode phase speed models.
Then, the radially anisotropic 3-D model down to a depth of 400 km was inverted from a set of multi-mode phase speed maps of both Rayleigh and Love waves simultaneously, using the iterative nonlinear inversion method by Tarantola & Valette (1982). Following an earlier work by Yoshizawa (PEPI, 2014), we determined the upper and lower boundaries of the LAT, based on the negative peak of the S wave velocity gradient and the slowest S wave velocity below the lithosphere, respectively.
We could obtain models with a higher lateral resolution for the SV- and SH-wave speed structures shallower than 200 km, constrained mainly by the fundamental-mode surface waves. The upper mantle at such depth beneath the U.S. is characterized by faster SH wave speed than SV, particularly in the shallow lithosphere. The small-scale pattern of the heterogeneity found in the eikonal model well reflects geologic features such as Snake River Plain, Colorado Plateau, Rio Grande Rift, and New Madrid Seismic Zone.
The velocity contrast between the western and eastern U.S. at a depth shallower than 100–150 km has been imaged clearly in our model. We can see significant lateral changes in the LAT across the Rocky Mountains; e.g., the LAT tends to be shallower at around 100–200 km depth in the tectonically active western region but becomes deeper around 150–300 km depth in the eastern stable cratonic areas. Some local tectonic features can also be seen in the LAT, e.g., the lithospheric thinning with the shallow upper bound in the New Madrid Seismic Zone. Since the eikonal tomography enables us to use the relatively small number of higher-mode measurements effectively, both the lateral and vertical resolutions have been improved in our model, allowing us to image the localized lateral variations that reflect surface tectonics and cratonic roots.
For the radially anisotropic parameter, the entire continent at depths shallower than 100 km is characterized by ξ > 1, representing faster SH-wave speed than SV-wave speed. Frozen-in anisotropy is one possible interpretation of the existence of ξ > 1 beneath the continent. The contrast of ξ beneath the eastern U.S. suggests that the lithospheric keel of the Superior craton, which corresponds to the LAB, can be estimated at a depth of 200–250 km. Below 300 km depth, the two zones of ξ < 0.95 (faster SV) are noticeable; beneath the Cascadia Range and the Superior Craton. Such lower ξ can be interpreted as the mantle up- or the down-welling. The one beneath the Cascadia Range may reflect slab subduction, while the other beneath the Superior Craton may indicate the delamination or basal drag of the cratonic root.