Slide1: Zhang Heng invented the seismoscope, or Di Dong Yi (Earth Motion Instrument) for measuring earthquakes. A similar device did not reach Europe until sixteen hundred years later, when a seismoscope was "invented" again in France.  Heng's device was made of bronze in the shape of an urn eight feet in diameter.  Although Heng's original seismoscope was destroyed, Wang Zhenduo reconstructed it in 1951.   Inside the seismoscope is a copper pendulum shaft connected to eight copper arms.  Outside the device are eight dragon heads around the top, each with a ball in its mouth. Around the bottom, eight frogs squat with open mouths, each directly under a dragon head. When an earthquake occurred, a ball fell out of a the dragon's mouth into a frog's mouth, making a noise that was supposedly loud enough to wake the Emperor's household, alerting them of the earthquake.  Then, all anyone had to do was look to see which ball had dropped to determine which direction the earthquake was coming from.            


Slide2: According to the History of the Later Han Dynasty, one day in February, 138 A.D., Zhang Heng and several others found that the dragon facing west had dropped its copper ball.  In response, Heng made a report to the emperor saying that an earthquake had happened somewhere to the west  of the capital,  Louyang. However, on that particular day, nothing unusual occurred around Louyang, nor was there any news about an earthquake elsewhere. This led to various suspicions and accusations to the effect that Zhang Heng's seismograph was a fraud and he was a liar.  However, within two days, several men riding on horseback were seen galloping in the direction of Luoyang along the road that led west from the capital. The message they brought to the emperor was that a violent earthquake had taken place about 500 kilometers to the west in western Gansu and had caused landslides. This anecdote is evidence that the seismograph was not only very sensitive, but also accurate.


Slide3: By the end of the 19th century, several European inventors had constructed different seismographs. Most were electromagnetic and operated by suspending a magnetic mass, or pendulum, within an electric coil.


Slide4: Because a magnet moving inside a coil creates a current within the coil, the movement of the ground during an earthquake can be converted into an electrical signal. This signal could then be used to modify the projection of light onto photographic paper, or to move a needle across paper and trace out the wiggles of the Earth's shaking.


Slide5: Today's high-technology, digital seismographs record ground shaking over a large band of frequencies and seismic amplitudes. Today's seismometers are called broadband because they are able to sense ground motion over a wide range of frequencies, from thousands of seconds to less than a hundredth of a second. The amplitude of the signals recorded by old seismometers was limited by the amount of movement possible between the mass, or pendulum, and the seismometer housing. Today's seismometers operate by measuring the amount of electrical energy needed to keep the mass centered in the housing in the presence of strong ground shaking. Modern seismometers can record a wide range of seismic signals, both very small and very large.


Slide8: SOUTHERN BOLIVIA. Magnitude 6.1 Depth 288 km 3/17/2004


Slide9: http://www.pbs.org/wnet/savageearth/animations/index.html Earthquake Animations from “Savage Earth” aka PBS Online! http://neic.usgs.gov/ National Earthquake Information Center


Slide10: When an earthquake occurs, one of the first questions is "where was it?" An earthquake's location may tell us what fault it was on and where damage (if any) most likely occurred. Unfortunately, the earth is not transparent and we can't just see or photograph the earthquake disturbance like meteorologists can photograph clouds. When an earthquake occurs, it generates an expanding wavefront from the earthquake hypocenter at a speed of several kilometers per second. We observe earthquakes with a network of seismometers on the earth's surface. The ground motion at each seismometer is amplified and recorded electronically at a central recording site. As the wavefront expands from the earthquake, it reaches more distant seismic stations.


Slide11: When an earthquake occurs, we observe the times at which the wavefront passes each station. We must find the unknown earthquake source knowing these wave arrival times. Here is a map of U.S. Geological Survey seismic stations in the San Francisco Bay Area and 6 seismograms from an earthquake: We want to find the location, depth and origin time of an earthquake whose waves arrive at the times measured on each seismogram. We want a straightforward and general procedure that we can also program in a computer.


Slide12: The procedure is simple to state: guess a location, depth and origin time; compare the predicted arrival times of the wave from your guessed location with the observed times at each station; then move the location a little in the direction that reduces the difference between the observed and calculated times. Then repeat this procedure, each time getting closer to the actual earthquake location and fitting the observed times a little better. Quit when your adjustments have become small enough and when the fit to the observed wave arrival times is close enough.


Slide13: You can try to fit an earthquake location on the map just to see how the procedure goes. Note that the earthquake arrives first on station C, thus C is a good first guess for the location. Many earthquakes in California occur between 2 and 12 kilometers depth and we will guess a 6 km. depth. The origin time should be a few seconds before the time of the wave at the first station. Let's guess an origin time of 10 seconds, measured on the same clock that made the time scale at the bottom of the figure and timed the seismograms. Then we can list the tentative travel times by subtracting the origin time from the observed arrival times:


Slide14: Note the scale at the left of the figure. It shows travel times for waves from an earthquake at a depth of 6 kilometers. The scale starts at 1.3 seconds because the wave reaches the surface 1.3 seconds after the earthquake origin time. You can make a tracing of the scale and move the earthquake on the map until the tentative travel times match the travel times from the scale.


Slide15: Where do you think the earthquake was? Are the times for each station systematically early or late, requiring a shift in the origin time? The earthquake was near station C. The depth was about 6 km and the origin time was about 10 seconds. (We guessed very well!) A real magnitude 3.4 earthquake occurred at this location on April 29, 1992. It was felt by many people who were sitting or at rest.


Slide16: Mathematically, the problem is solved by setting up a system of linear equations, one for each station. The equations express the difference between the observed arrival times and those calculated from the previous (or initial) hypocenter, in terms of small steps in the 3 hypocentral coordinates and the origin time. We must also have a mathematical model of the crustal velocities (in kilometers per second) under the seismic network to calculate the travel times of waves from an earthquake at a given depth to a station at a given distance. The system of linear equations is solved by the method of least squares which minimizes the sum of the squares of the differences between the observed and calculated arrival times. The process begins with an initial guessed hypocenter, performs several hypocentral adjustments each found by a least squares solution to the equations, and iterates to a hypocenter that best fits the observed set of wave arrival times at the stations of the seismic network.


Slide17: How Do I Locate That Earthquake's Epicenter? To figure out just where that earthquake happened, you need to look at your seismogram and you need to know what at least two other seismographs recorded for the same earthquake. You will also need a map of the world, a ruler, a pencil, and a compass for drawing circles on the map. (From Bolt, 1978) One minute intervals are marked by the small lines printed just above the squiggles made by the seismic waves (the time may be marked differently on some seismographs). The distance between the beginning of the first P wave and the first S wave tells you how many seconds the waves are apart. This number will be used to tell you how far your seismograph is from the epicenter of the earthquake. Here's an example of a seismogram:


Slide18: Measure the time between the first P wave and the first S wave. In this case, the first P and S waves are 24 seconds apart. Find the point for 24 seconds on the left side of the chart below and mark that point. According to the chart, this earthquake's epicenter was 215 kilometers away. Finding the Distance to the Epicenter and the Earthquake's Magnitude


Slide19: Finding the Epicenter You have just figured out how far your seismograph is from the epicenter and how strong the earthquake was, but you still don't know exactly where the earthquake occurred. Draw a circle with a radius equal to the distance to the earthquake. The center of the circle will be the location of your seismograph. The epicenter of the earthquake is somewhere on the edge of that circle. Do the same thing for the distance to the epicenter that the other seismograms recorded (with the location of those seismographs at the center of their circles). All of the circles should overlap. The point where all of the circles overlap is the approximate epicenter of the earthquake.


Slide20: One of Dr. Charles F. Richter's most valuable contributions was to recognize that the seismic waves radiated by all earthquakes can provide good estimates of their magnitudes. He collected the recordings of seismic waves from a large number of earthquakes, and developed a calibrated system of measuring them for magnitude. Richter showed that, the larger the intrinsic energy of the earthquake, the larger the amplitude of ground motion at a given distance. He calibrated his scale of magnitudes using measured maximum amplitudes of shear waves on seismometers particularly sensitive to shear waves with periods of about one second. The records had to be obtained from a specific kind of instrument, called a Wood-Anderson seismograph. Although his work was originally calibrated only for these specific seismometers, and only for earthquakes in southern California, seismologists have developed scale factors to extend Richter's magnitude scale to many other types of measurements on all types of seismometers, all over the world. In fact, magnitude estimates have been made for thousands of Moon-quakes and for two quakes on Mars. Earthquake Magnitude


Slide21: The diagram below demonstrates how to use Richter's original method to measure a seismogram for a magnitude estimate in Southern California: The scales in the diagram above form a nomogram that allows you to do the mathematical computation quickly by eye.


Slide22: Earthquake Magnitude Scale


Slide23: Earthquake Magnitude Classes Earthquakes are also classified in categories ranging from minor to great, depending on their magnitude.


Slide24: Seismic Energy: Both the magnitude and the seismic moment are related to the amount of energy that is radiated by an earthquake. Richter, working with Dr. Beno Gutenberg, early on developed a relationship between magnitude and energy. Their relationship is: logES = 11.8 + 1.5M giving the energy ES in ergs from the magnitude M. Note that ES is not the total “intrinsic” energy of the earthquake, transferred from sources such as gravitational energy or to sinks such as heat energy. It is only the amount radiated from the earthquake as seismic waves, which ought to be a small fraction of the total energy transferred during the earthquake process.


Slide25: Magnitude Energy Yield (approximate) -1.5 6 ounces (TNT) Breaking a rock on a lab table 1.0 30 pounds Large Blast at a Construction Site 1.5 320 pounds 2.0 1 ton Large Quarry or Mine Blast 2.5 4.6 tons 3.0 29 tons 3.5 73 tons 4.0 1,000 tons Small Nuclear Weapon 4.5 5,100 tons Average Tornado (total energy) 5.0 32,000 tons 5.5 80,000 tons 6.0 1 million tons 6.5 5 million tons Northridge, CA Quake, 1994 7.0 32 million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest Thermonuclear Weapon 7.5 160 million tons Landers, CA Quake, 1992 8.0 1 billion tons San Francisco, CA Quake, 1906 8.5 5 billion tons Anchorage, AK Quake, 1964 9.0 32 billion tons Chilean Quake, 1960 10.0 1 trillion tons (San-Andreas type fault circling Earth) 12.0 160 trillion tons (Fault Earth in half through center, OR Earth's daily receipt of solar energy)