Project Report S e i s m i c W a v e s 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 19 th, 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 Mid- Atlantic 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, 3 rd Edition, Harcourt Inc. Fundamentals of Physics, Halliday, Resnick, Walker, 5 th 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