CRUST 5.1

A New Global Crustal Model at 5x5 Degrees

Authors: Walter Mooney (USGS), Gabi Laske, Guy Masters

Description of the model

The global crustal model uses a type key to assign crustal structure at a 5 x 5 deg scale. Data were gathered from seismic experiments and averaged globally for similar geological and tectonic settings (such as Archean, early Proterozoic, rifts etc.). These averages were used to assigned structure to regions without seismic information (e.g. most of Africa, South America). Ice and sediment thicknesses where gathered from published maps. Bathymetry and topography is that of ETOPO5. The global model is composed of 139 key 1d-profiles (the crustal types) where one of these profiles is assigned to each 5 x 5 degree cell.
Each individual profile is a 7 layer 1D-model with

1. ice
2. water
3. soft sediments
4. hard sediments
5. upper crust
6. middle crust
7. lower crust

Why do we need another crustal model?

Seismic tomography has been extensively used in various forms to determine the three-dimensional velocity structure of the Earth's mantle. The data used in these studies are surface wave (waveforms and dispersion measurements) and free oscillation (frequency shift) data as well as body wave travel times (arrival times from catalogues such as the ISC catalog and specially hand-picked long-period arrivals). For the majority of these studies, the crust has a significant impact on the observed seismic data but, at the same time, is too thin to be resolved by them. Most authors handle this by applying an assumed "crustal correction" to the data before an inversion for mantle structure. Since the inversion techniques can erroneously map crustal structure down to great depth (at least 250km), accurate crustal corrections to the data sets are extremely important.

Another aspect is the seismic monitoring of nuclear explosions. Key elements of the monitoring effort are location and size estimation. The location accuracy strongly depends on the chosen velocity model (i.e. crustal structure), especially when seismic sources lie outside the monitoring network.
Monitoring on regional scale includes the modelling of short-period surface waves and the propagation of Lg waves which are very sensitive to variations of crustal structure. Obviously, an accurate image of the Earth's crust would tremendously improve the understanding of wave propagation of such phases.

What makes our model so special?

• The thickness and seismic/density structure of sedimentary basins is accounted for more completely.
• The velocity structure of sedimentary basins is estimated using a significantly larger database of crustal structure.
• The compressional wave, shear wave, and density structure have been explicitly specified using newly available constraints from field and laboratory studies.
• The use of statistics to predict crustal structure in areas without field measurements. The velocity structure of the crystalline crust and uppermost mantle is estimated from the statistical average of regions with a similar crustal age and tectonic setting.
• The model is extremely transportable. The files provided are small ASCII files (smaller than 25KByte) and fit on a low-density diskette (postscript files not included). The files are not compressed. Hence, the model can be handled on any computer and can be obtained extremely fast.

Publication:

Further info is available at the USGS site.

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