BAD VIBRATIONS (Earthquakes/Tsunamis)

Transcription

1 Society of Exploration Geophysicists Youth Education Committee BAD VIBRATIONS (Earthquakes/Tsunamis) ACTIVITY RESOURCES Primary and Secondary (4th 12th) Includes web links on using a laptop as an oscilloscope Author: William D. Underwood, PhD. Geophysicist Chesapeake Energy Chair Geosciences, Oklahoma School of Science and Mathematics Oklahoma City, Oklahoma, USA SEG Youth Education Committee Member NATIONAL SCIENCE EDUCATION STANDARDS (USA) ADDRESSED: Earth science: using observation, interaction, experimentation, identification, interpretation, communication, and inquiry to discover processes that affect earth s crust: faults, and moving plates, due to stresses inside the earth. World geography, Pangea and plate tectonics. One geological event can trigger another geological event (tsunami); importance of early warning systems; better constructions practices for housing, buildings, bridges SEG BUSINESS OFFICE: P.O. BOX TULSA, OK USA STREET ADDRESS: 8801 S. YALE, STE. 500 TULSA, OK USA FAX:

2 Earthquakes or Bad Vibrations INTRODUCTION Do you remember what happened December 26, 2004? A major earthquake off the coast of Sumatra, Indonesia, generated a devastating tsunami (often incorrectly called a tidal wave) that did damage as far away as the East coast of Africa and was measurable in London, England. This earthquake, which measured 9.1 on the Richter scale, was the third largest since 1900 (Table 1). Information on tsunamis can be found on the United States Geological Survey (USGS) website at EARTHQUAKES Earthquakes are a common occurrence on Earth. According to one source there are approximately 900,000 per year, or 1 every 35 seconds. The USGS web site (listed below under references) shows the probability of a particular magnitude earthquake occurring each year. According to this list there are 3.3 million earthquakes each year. Why the difference in numbers? The USGS website lists the occurrence of earthquakes magnitude 1 or above; the smallest earthquake on the other list is notably larger. The USGS provides weekly lists of earthquakes from around the world at earthquake.usgs.gov/eqcenter/recenteqsww. A list of US earthquakes is available at Information on latitude and longitude can be found at stargaze/slatlong.htm. Exercise 1: locate the earthquakes in Table 1 on a world map using latitude and longitude Exercise 2: locate the 10 largest earthquakes on this week s USGS world earthquake list on a world map using latitude and longitude RING OF FIRE If you plot all the earthquakes that occur over a long period of time, say a couple of years, you will notice a pattern emerging. Figure 1 below, top image, shows all earthquakes for a 5 year period. Notice that most earthquakes are concentrated together, in lines and clusters around the world. In fact, the 26 December 2004 earthquake is on one of those lines. One such grouping is around the Pacific Ocean. We call this the Ring of Fire. Why? The red circles on the map in Figure 1, bottom image, represent volcanoes, and volcanoes are easier to find than earthquakes. Before we had sophisticated instruments, the Pacific Rim was called the Ring of Fire. Combine the information from the top and bottom images in Figure 1, and we see earthquakes and volcanoes are occurring in about the same place. Do earthquakes cause volcanoes? No. Do volcanoes cause earthquakes? No. Well, sometimes. For example, most volcanoes cause small earthquakes as the magma moves around. Some scientists are trying to use these earthquake swarms to predict when volcanoes will erupt. (An earthquake swarm is a lot of small earthquakes close together, like a swarm of bees is a lot of bees close together.) Consider a pot of water, boiling on the stove. It shakes and rattles. In the same way, the ground shakes when volcanoes are rumbling. Information on the ring of fire can be found on wikipedia at wiki/pacific_ring_of_fire

3 Figure 1

4 PLATE TECTONICS Most earthquakes, especially the big ones, are caused by plate motion. Figure 1, bottom image, shows the plates on Earth. Compare this with the earthquake distribution in Figure 1, top image. As plates slide past each other (e.g. in California) or one slides under the other (e.g. Japan, Alaska, Sumatra) they stick, then slip. When they slip, this is an earthquake. When one plate slides under another, such as in Alaska, the descending plate melts and generates magma. This magma makes its way to the surface as a volcano. Information on plate tectonics, including the ring of fire, can be found on the USGS website at nformation on volcanoes can be found on the USGS website at gov/gip/volc (this is an excellent source of volcano information with description of sources and types). The Incorporated Research Institutions for Seismology (IRIS) website at has much information about earthquakes, seismometers, and seismic education. Not all earthquakes occur at known plate boundaries. In 1811 and 1812 three to five strong earthquakes occurred near New Madrid in the Mississippi River valley in the interior of the North American plate. These earthquakes, of magnitude 7.2 to 8.1, caused the Mississippi River to appear to flow upstream for a short time, woke people as far away as Pittsburgh, PA, and Norfolk, VA, and rang bells in Boston, MA, and could be felt throughout the eastern United States. The cause of these earthquakes, and when (or if) they will recur, are topics of current research and debate. Information on the New Madrid fault system can be found on the USGS website at gov/regional/states/events/ php THE RICHTER SCALE The Richter scale, developed by Dr. Charles Richter in 1935, is a measure of the amplitude of a seismic wave. T logarithmic scale, each number represents an increase in amplitude of 10 from the previous number. That is, a 3 on the Richter scale has 10 times the amplitude of a 2, and 100 times the amplitude of a 1. In most cases energy is approximately the square of the amplitude. On the Richter scale energy increases by about 30 as each magnitude number increases by 1. Thus a 3 has a little more than 30 times the energy of a 2, and about 1000 times the energy of a 1. Table 2 shows the energy of earthquakes compared to TNT. Information on the Richter scale can be found at or

5 Table 1: The top 12 earthquakes since 1900 (from USGS RICHTER EQUIVALENT APPROXIMATE MAGNITUDE TNT ENERGY YIELD ounces Breaking a rock on a lab table pounds Large blast at a construction site pounds ton Large quarry or mine blast tons tons tons 4.0 1,000 tons Small nuclear weapon 4.5 5,100 tons Average tornado (total energy) ,000 tons ,000 tons Little Skull Mtn., NV quake, million tons Double Spring Flat, NV quake, million tons Northridge, CA quake, million tons Hyogo-Ken Nanbu, Japan quake, 1995; Largest Thermonuclear Weapon million tons Landers, CA quake, billion tons San Francisco, CA quake, billion tons Anchorage, AK quake, billion tons Chilean Quake, trillion tons (San-Andreas type fault circling Earth) trillion tons (Fault Earth in half through center, OR Earth s daily receipt of solar energy) Table 2: Equivalent Richter magnitudes (from Univ. Nevada at Reno

6 Figure 2 Earthquakes Energy Equivalents This educational material is reproduced with permission from the IRIS Consortium. SEEING GROUND MOTION It is quite easy to see seismic waves in the classroom by hooking up an old (or new) geophone to an oscilloscope. Oscilloscope software is available for download for free or a small fee from several sources. Geophones may be available from a local geophysical contractor or oil company. After hooking the geophone to the oscilloscope have the smallest person stamp his or her foot. If the scope is properly adjusted the trace will deflect. If several people stamp their feet at once the deflection will be larger. Geophones are very sensitive. A geophone placed on a table will detect a stamped foot across the room in most cases. In spite of their sensitivity, they do not produce a large amount of current. A geophone s output can be fed directly into the sound card of a computer without fear of damage. Earthquake seismograms can be found on may websites. Several plans are available to make inexpensive seismometers. A good place to start is the Incorporated Research Institutions for Seismology (IRIS) web site. The IRIS website contains a large amount of earthquake information in one site, and links to many others. Earthquake information and links are available at the USGS. Free and inexpensive oscilloscope software can be found at downloads/pc/index.html The (IRIS) website at has much information about earthquakes, seismometers, and seismic education. The USGS website at contains a vast array of information on earthquakes, volcanoes, and other geologic topics, including hazards and hazard mitigation.

7 REFERENCES Near real time earthquake list (and much other earthquake information): WaveWindow shareware (378kb) COURTESY OF: