The tectonic diversity of the North American continent makes it an ideal region to investigate the structure and dynamics of the continental upper mantle. Investigations of timely geophysical questions, such as the relation to geological age of the variations in the lithospheric thickness, the relation of upper-mantle anisotropy to present day asthenospheric flow and past tectonic events, the nature and strength of the lithosphere/asthenosphere coupling and the driving mechanisms of plate motions, are contingent upon obtaining high-resolution 3-D tomographic models of the isotropic and anisotropic mantle structure of the continent.
The USGS 3-D Geologic and Seismic Velocity Models of the San Francisco Bay region provide a three-dimensional view of the geologic structure and physical properties of the region down to a depth of 45 km (28 miles). Construction of this 3D Bay Area model has been a joint effort of the USGS Earthquake Hazards Program and the USGS National Cooperative Geologic Mapping Program.
The purpose of the Three-Dimensional Community Velocity Model for Southern California is to provide a unified reference model for the several areas of research that depend of the subsurface velocity structure in their analysis. These include strong motion modeling, seismicity location, and tomographic velocity modeling. It is also hoped that the geologic community will find the basin models useful because they are based on structures and interfaces that are largely derived from geologic structure models. The deeper sediment velocities themselves are obtained from empirical relationships that take into account age of the formation and depth of burial. The coefficients of these relationships are calibrated to sonic logs taken from boreholes in the region. Shallow sediment velocities are taken from geotechnical borehole measurements. Hardrock velocities are based on tomographic studies.
Compared to what we know about earthquake hazards in California, less is known in the Central US. The main reason for this difference is the relatively few numbers of large earthquakes in the Central US compared to California. The more frequent occurrence of small and larger earthquakes in California gives scientists data that can be used to infer the effects of historical earthquakes in the region and estimate the effects of future large earthquakes. A lack of large earthquakes in the Central US means a lack of data on stronger ground motions from big earthquakes. When there is little observational data, scientists use models to generate estimated data. A model for estimating the effects of an earthquake includes an earthquake source and the earth through which the seismic waves travel. A research paper on this modeling effort (4.7MB PDF) was published in the August 2015 issue of the Bulletin of the Seismological Society of America.
The first major global tomographic study was made by Professor Dziewonski in early 70's. The idea of this study was that the travel time anomalies observed for many ray paths, criss-crossing the Earth between various points near the Earth's surface and reaching different depths in its interior, could be resolved formally into a three-dimensional (3-D) model. This is now called `seismic tomography', as it conceptually resembles the medical CAT-scan. The early results were reported orally in 1974 and 1975 by Dziewonski and a full report was published in January 1977 (Dziewonski et al., 1977). A more detailed description can be found a statement written by Professor Dziewonski. The motivation for studying 3-D structure of the Earth's interior is that it may offer the best information on the dynamic processes in the deep interior of the Earth. As the seismic wave speeds change with temperature, it is plausible to obtain 3-D snapshots of the convection pattern in the Earth. By performing waveform and travel time inversions using long period seismic records, we have obtained global models for the long wavelength 3-D velocity structure of the Earth.
Mantle seismic velocity tomography is a powerful tool for exploring the Earth's deep structure. Interpreting the velocity anomalies as thermal anomalies allows us a snapshot glimpse of the dynamics of a convecting mantle.
Isotropic seismic velocities independent of direction are a simplifying assumption made in most tomography models. Velocity anisotropy, however, is often interpreted to exist in many areas. These range from the upper crust due to sedimentary layering, to the upper mantle as a result of viscous interaction between the underlying convecting mantle and the rigid lithospheric slabs, down to the core-mantle boundary, where chemical interactions with the molten outer core or a chemically distinct layer of old slabs may introduce anisotropy.
Anisotropy can have many causes. Often, it is a result of alignment of crystal orientation, or possibly alignment of fractures or pockets of melt within a strain field. In general, while isotropic velocities may only give us snapshots of the current thermal and chemical conditions of the mantle, anisotropy can give us a more dynamic picture by giving us additional information about the stress and strain present in the mantle.
With the increasing reliability of the details in elastic global models, it may be possible, in the near future, to accurately predict elastic effects on the amplitudes of globally traveling long period surface and body waves, thereby gaining access to more accurate estimates of the lateral variations of Q in the earth.
Several well-known tomographic studies published in the 1990's (van der Hilst et al., 1991; Fukao et al., 1992) captured a large-scale subhorizontal high velocity anomaly at the bottom of the upper mantle in the northwestern Pacific. These models suggest that a large volume of subducted slab is stagnant in the transition zone.