Week 2 Project Gib Lucas EDU 653 Dr. Merrill 6/28/2012
EarthquakesEarthquake – Sudden slip on a fault, and resultingground shaking and radiated seismic energy caused bythe slip, or by volcanic or magmatic activity, or othersudden stress changes in the earth.
Earthquake TermsAmplitude – The size of the wiggles on an earthquake recording.Epicenter – The point on the earth’s surface vertically above the (hypocenter )focus, point in the crust where a seismic rupture begins.
Earthquake TermsHypocenter – The point within the earth where an earthquake rupture starts.The epicenter is the point directly above it at the surface of the earth. Alsocommonly termed the focus.
Earthquake TermsMagnitude – A number that characterizes the relative size of an earthquake.Magnitude is based on measurement of the maximum motion recorded by aseismograph.P wave, or compression wave, is a seismic body wave that shakes the groundback and forth in the same direction and the opposite direction as the directionthe wave is moving.S wave, or shear wave, is a seismic body wave that shakes the ground back andforth perpendicular to the direction the wave is moving
Earthquake TermsSeismogram – A record written by a seismograph in response to ground motionsproduced by an earthquake, explosion, or other ground-motion sources.
Earthquake TermsThe Richter magnitude scale was developed in 1935 by Charles F. Richter of theCalifornia Institute of Technology as a mathematical device to compare the size ofearthquakes. On the Richter Scale, magnitude is expressed in whole numbers anddecimal fractions. For example, a magnitude 5.3 might be computed for a moderateearthquake, and a strong earthquake might be rated as magnitude 6.3. As an estimateof energy, each whole number step in the magnitude scale corresponds to the release ofabout 31 times more energy than the amount associated with the preceding wholenumber value.
OBJECTIVESReading a SeismogramCalculating the Magnitude of an EarthquakeFinding the Epicenter of an Earthquake
How Are Earthquake Magnitudes Measured? The magnitude of most earthquakes is measured on .the Richter scale, invented by Charles F. Richter in 1934. For each whole number you go up on the Richter scale, the amplitude of the ground motion recorded by a seismograph goes up ten times.CHARLES RICHTER STUDYING A Using this scale, a magnitude 5 earthquake wouldSEISMOGRAM result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released).
Earthquake Energy Release To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: A magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people.
A TYPICAL SEISMOGRAM When you look at a seismogram, there will be wiggly lines allacross it. These are all the seismic waves that the seismograph hasrecorded.Most of these waves were so small that nobody felt them. Theycan be caused by heavy traffic near the seismograph, waves hittinga beach, the wind, and any number of other ordinary things thatcause some shaking of the seismograph.There may also be some little dots or marks evenly spaced alongthe paper. These are marks for every minute that the drum of theseismograph has been turning.
What do the Wiggles mean?When you look at a seismogram, there will be wiggly lines all across it.So which wiggles are the earthquake? The P wave will be the first wiggle that is bigger than the rest of the littleones . Because P waves are the fastest seismic waves, they will usually bethe first ones that your seismograph records.The next set of seismic waves on your seismogram will be the S waves.These are usually bigger than the P waves.
USE THE AMPLITUDE TO DERIVE THE MAGNITUDE OF THE EARTHQUAKE AND THEDISTANCE FROM THE EARTHQUAKE TO THE STATION.To find the magnitude:1. Measure the distance between the 1st P wave and 1st S wave (24 seconds).2. Find the point for 24 secs. On the chart and mark the spot. The Epicenter is 215 kilometers away.3. Measure the amplitude (height) of the strongest wave (23 mm).4. Using a strait edge, draw a line between the 2 marks you made. The point where your line crosses the middle line on the chart is the magnitude (strength) of the earthquake. In this example the earthquake had a magnitude of 5.0
How Do I Locate That Earthquakes Epicenter?You have figured out how far your seismograph is from the epicenter and how strong theearthquake was, but you still don’t know where the earthquake occurred.To figure out just where that earthquake happened, you need to look at your seismogram and youneed to have seismograms from at least two other seismographs recorded for the sameearthquake.For our example we will use seismograms from Detroit, Minneapolis, and Charleston.You will also need a U.S. map, a ruler, a pencil, and a compass for drawing circles on the map.
FINDING THE EPICENTER1. Check the scale on your map.2. If 1 cm = 100 km, then an epicenter of 215 kilometers away = 2.15 cm on map3. Use the compass to draw a circle with a radius of the number From step 2 (radius is the distance from the center of the circle to its edge. The center of the circle will be location of your seismograph.4. Do the same thing for the other seismograms. The point where all of the circles overlap is the epicenter of the earthquake. THE POINT WHERE THE THREE CIRCLES INTERSECT IS THE EPICENTER OF THE EARTHQUAKE. THIS TECHNIQUE IS CALLED TRIANGULATION.