GG 450 April 15, 2008 Refraction Applications

While refraction is used for engineering studies such as depth to basement and depth to the water table, its most important uses are in deeper applications.

A recent study of the Seattle are shows that it sits in a large relatively low-velocity basin:

Grey circular segments show wave fronts of first arrivals. Black arrows show rays.

Generalized Raytrace programs allow for ray paths to be calculated for fairly complex models. The resulting travel times are compared with the data and the model modified to improve the fit.

Much of the earth’s interior has been studied using seismic refraction. Early constant-velocity models for continental crust (top) and oceanic crust (bottom) are based on earthquake data.

The models above were made “BC” (before computers) when seismic inversion and calculation of ray models were restricted to the simplest models. How might “reality” differ from these models?

What are the tectonic characteristics of the regions of thinnest, average, and very thick crust?

Seismic refraction methods yield models of velocity vs. depth. These models aren’t of much use unless we can relate the velocities to composition and other properties of the earth. The figure above shows such parameters for common crustal minerals. The Moho is defined by an abrupt change in P velocity, likely marking a change to ultramafic composition.

Tomographic methods using global data result in models that are no longer required to be radial symmetric. These models allow us to visualize plate tectonic processes and variations in the mantle.

The previous slide shows some of the formal notations for seismic ray paths through the earth. P (and S) are mantle waves, PP is a p-wave that reflects off the earth’s surface. Other rays include PS, which transforms to an s-wave on reflection from the surface, PcS reflects from the core as an s-wave, and PKJKP goes through the inner core as an s-wave.

The core-mantle boundary is a particularly interesting boundary. Describe its characteristics in terms of the changes in density, seismic velocity, state of matter, and composition. The boundary of the core with the inner core is also interesting. Which of the two boundaries might you expect to be the sharpest? What is the nature of the inner core boundary?

Seismic data can be inverted to obtain the likely temperature inside the earth in regions, such as the mantle, where composition is relatively homogeneous.

Tomographic models of the earth constrain temperatures at depth well enough to allow generation of 3-D convection models such as the one above.

Japan’s Ocean Hemisphere Project is establishing geophysical stations around the Pacific Ocean. One of there accomplishments is this preliminary tomographic model of the Hawaiian Ridge showing the low velocity region under Hawaii.

What is EarthScope?EarthScope is a bold undertaking to apply modern observational, analytical and telecommunications technologies to investigate the structure and evolution of the North American continent and the physical processes controlling earthquakes and volcanic eruptions.EarthScope will provide a foundation for fundamental and applied research throughout the United States that will contribute to the mitigation of risks from geological hazards, the development of natural resources, and the public's understanding of the dynamic Earth. Modern digital seismic arrays will produce three-dimensional images of North America's continental crust and the deeper mantle on which it "floats".

The USArray component of the EarthScope experiment is a continental-scale seismic observatory designed to provide a foundation for integrated studies of continental lithosphere and deep Earth structure over a wide range of scales. USArray will provide new insight and new data to address fundamental questions in earthquake physics, volcanic processes, core- mantle interactions, active deformation and tectonics, continental structure and evolution, geodynamics, and crustal fluids (magmatic, hydrothermal, and meteoric)

USArray components are now in place in the western region. Cost? Roughly $200M.