Download Lecture 27 April 3, 2006

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.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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, coremantle 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.