Project Report
Seismic Waves
PORTLAND STATE UNIVERSITY
PHYSICS 213
SPRING TERM – 2005
Instructor: Dr. Andres La Rosa
Student Name: Prisciliano Peralta-Ramirez
Table Of Contents
1. Cover Sheet
2. Table Of Contents
3. Abstract
4. Introduction
5. Content
6. Conclusion
7. References
Abstract
This time I want to talk about a theme that concerns to the whole word,
because of its potential, and that concerns me as a Physics 213 student.
Seismic Waves are important to our daily life on Earth. The impact they
can cause in our lives is transcendental.
On September 19th, 1985, a great earthquake (8.1 on the Richter Scale)
occurred on the western coast of Mexico, impacting Mexico City, and collapsing
buildings. Most of the buildings collapsed were those of median height, but
shorter and taller buildings remained standing due to a higher resonant angular
frequency, and lower resonant angular frequency, respectively.
More recently, the tsunami in Asia was caused by Seismic Waves that
occurred far away from the coast, at the bottom of the ocean, but even though
they were weak, their amplitude must have grown greatly and caused the
tsunami that destroyed buildings, and unfortunately lives.
Introduction
Seismic Deformation
When an earthquake fault ruptures, it causes two types of deformation:
static; and dynamic. Static deformation is the permanent displacement of the
ground due to the event. The earthquake cycle progresses from a fault that is
not under stress, to a stressed fault as the plate tectonic motions driving the
fault slowly proceed, to rupture during an earthquake and a newly-relaxed but
deformed state.
Seismic Waves
The second type of deformation, dynamic motions, are essentially sound waves
radiated from the earthquake as it ruptures, named Seismic Waves.
Content
Most of the plate-tectonic energy driving fault ruptures is taken up by
static deformation, up to 10% may dissipate immediately in the form of seismic
waves.
The mechanical properties of the rocks that seismic waves travel through quickly
organize the waves into two types. Compress ional waves, also known as
primary or P waves, travel fastest, at speeds between 1.5 and 8 kilometers per
second in the Earth's crust. Shear waves, also known as secondary or S waves,
travel more slowly, usually at 60% to 70% of the speed of P waves.
P waves shake the ground in the direction they are propagating, while S waves
shake perpendicularly or transverse to the direction of propagation.
Although wave speeds vary by a factor of ten or more in the Earth, the ratio
between the average speeds of a P wave and of its following S wave is quite
constant. This fact enables seismologists to simply time the delay between the
arrival of the P wave and the arrival of the S wave to get a quick and reasonably
accurate estimate of the distance of the earthquake from the observation station.
Just multiply the S-minus-P (S-P) time, in seconds, by the factor 8 km/s to get
the approximate distance in kilometers.
Seismographs and Seismograms
Sensitive seismographs are the principal tool of scientists who study
earthquakes. Thousands of seismograph stations are in operation throughout the
world, and instruments have been transported to the Moon, Mars, and Venus.
Fundamentally, a seismograph is a simple pendulum. When the ground shakes,
the base and frame of the instrument move with it, but inertia keeps the
pendulum bob in place. It will then appear to move, relative to the shaking
ground. As it moves it records the pendulum displacements as they change with
time, tracing out a record called a seismogram.
One seismograph station, having three different pendulums sensitive to the
north-south, east-west, and vertical motions of the ground, will record
seismograms that allow scientists to estimate the distance, direction, Richter,
and type of faulting of the earthquake. Seismologists use networks of
seismograph stations to determine the location of an earthquake, and better
estimate its other parameters. It is often revealing to examine seismograms
recorded at a range of distances from an earthquake:
On this example it is obvious that seismic waves take more time to arrive
at stations that are farther away. The average velocity of the wave is just the
slope of the line connecting arrivals, or the change in distance divided by the
change in time. Variations in such slopes reveal variations in the seismic
velocities of rocks. Note the secondary S-wave arrivals that have larger
amplitudes than the first P waves, and connect at a smaller slope.
Locating Earthquakes
The principal use of seismograph networks is to locate earthquakes. Although it
is possible to infer a general location for an event from the records of a single
station, it is most accurate to use three or more stations. Locating the source of
any earthquake is important, of course, in assessing the damage that the event
may have caused, and in relating the earthquake to its geologic setting.
Given a single seismic station, the seismogram records will yield a measurement
of the S-P time, and thus the distance between the station and the event.
Multiply the seconds of S-P time by 8 km/s for the kilometers of distance.
Drawing a circle on a map around the station's location, with a radius equal to
the distance, shows all possible locations for the event. With the S-P time from a
second station, the circle around that station will narrow the possible locations
down to two points. It is only with a third station's S-P time that you can draw a
third circle that should identify which of the two previous possible points is the
real one:
This example uses stations in Boston, Edinborough, and Manaus. With the
distances shown, all three circles can intersect only at a single point on the MidAtlantic Ridge spreading center.
Conclusions
Earthquakes come in many different sizes which you generally hear
described by their magnitude. The magnitude measures how much the ground
shakes. There are other ways to measure the size of an earthquake, for instance
we can measure how long the fault was that slipped during the earthquake.
Using borehole geophysical measurements in conjunction with laboratory studies,
scientists study heat flow, stress, fluid pressure, and the mechanical behavior of
fault-zone materials at seismogenic depths to yield improved models of the
earthquake cycle
Geologists learn about future earthquakes by looking at evidence left behind by
past earthquakes, such as fault traces (the "footprints" of fault motion at the
Earth's surface), offset natural and man-made features (which mark relative
movement across a fault), and the disruption of sedimentary layers (as revealed
in trenches).
References
“Waves”, Jerry D. Wilson, Technical College Physics, 3rd Edition, Harcourt Inc.
“Fundamentals of Physics”, Halliday, Resnick, Walker, 5th Edition, John Wiley and
Sons, Inc.
//www/seismo.unr.edu/ftp/pub/luoie/class/100/seismic-waves.html
http://quake.usgs.gov/info, Andy Michael, USGS, and Daniel Russ