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Environmental and Exploration Geophysics II
Brief Overview of
Earthquake Seismology
tom.h.wilson
[email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
Focal Point, hypocenter and epicenter Distance to focal point
The appearance of a direct P-wave arrival in a
time distance plot also forms a curve whose
slope continually decreases. Seismic waves
traveling greater distances reach greater
depths and travel with higher average velocity
Gradual increase of velocity with depth causes
seismic waves to refract along curved ray paths
that eventually return to the surface.
Velocity variations within the earth’s
interior lead to curved ray-paths and a
variety of refraction paths.
In this time distance plot you will see a variety of curved
raypaths. What do you think the linear blue colored event is
associated with?
6 hours of recording time are shown in the panels below. Most
obvious are the surface waves and P-waves. Note that the
surface waves keep circling the earth and the P-waves bounce
back and forth from one side of the earth to the other.
In the near-surface applications of refraction seismology we
have been able to determine velocity distributions and layer
thickness. In earthquake seismology it is possible to analyze
time distance relationships to uncover the deep structure of
the earth working from similar principles.
As increasing amounts of earthquake observations
become available it has been possible to map out fairly
detailed variations in velocity down to considerable
depth beneath the earth’s surface.
… revealing major and minor structural features within the earth,
The tectonic plates which are carried about on large
mantle convection cells.
Seismic travel times reveal velocity layering
and also more subtle, non-layered regions of
velocity variation.
A puzzle to solve - Given the sums through the various cells
noted along the sides of the square, determine the values A
through D.
Earthquake
tomographic
images much like
CAT scans
reveal subtle
details in the
velocity
structure of the
earth’s interior
Earthquake tomography provides evidence for the
existence of lithosphere graveyards and super plumes.
An increasingly extensive network of seismographs
provides more and more detail views of the earth’s interior
with each passing year.
Relatively hot and cold regions are discerned through relative
differences in velocity. The shapes of these regions reveals information
about plume distribution and irregularities in the cooler materials which
have sunk down to their resting place on the outer core.
For the remainder of the day I want to review
material we’ve covered up to this point that you
may be uncertain about and address any
questions you may have about the problems that
have been assigned.
Homework Problems
1. A reversed seismic refraction survey indicates that a
layer with velocity V1 lies above another layer with velocity V2
and that V2>V1. We examine the travel times at a point located
midway (at C) between the shotpoints (at A and B). The travel
time of the refracted ray from end A to midpoint C is less than
the travel time of the refracted wave from end B to midpoint C.
Show that the apparent velocity determined from the slope of
the travel time curve for refracted waves produced from the
source at A is less that the apparent velocity for refracted waves
produced from the source at B. Toward which end of the layout
does the boundary between the V1 and V2 layers dip? i.e.
where is down-dip? Explain! (Robinson and Coruh, 1988).
2. Suppose that a reversed refraction survey
indicated velocities V1 =1500 m/s and V2 =2500
m/s from one end, and V1 =1500 m/s and
V2=3250 m/s from the other. Find the dip of the
refractor. What would be the changes in velocities
if the refractor had a slope 10 degrees larger than
the one you computed? (Robinson and Coruh,
1988)
In addition - begin working problems 3.2, 3.3 and
3.4 from Chapter 3.
Pb 3.2
First geophone is at 3
meters from the source,
the remainder are
spaced at 10m intervals.
What can you see?
Pb 3.3
First geophone is at 3
meters from the source,
the remainder are
spaced at 10m intervals.
What can you see?
The two problems assigned in class and
problems 3-2, 3-3, and 3-4 will be due next
Tuesday.
There will be no formal recitation section tomorrow,
however, if you have questions I will be in my office
tomorrow from 1-2 and we will also consider additional
questions you may have during Thursday’s class.

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