EENS 3050 | Natural Disasters |
Tulane University | Prof. Stephen A. Nelson |
Earthquake Prediction, Control and Mitigation |
Earthquake Prediction Long-Term Forecasting
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The map below shows the southern coast of Mexico. Here the Cocos plate is subducting beneath the North American Plate along the Acapulco Trench. Prior to September of 1985 it was recognized that within recent time there had been major and minor earthquakes on the subduction zone in a cluster pattern. For example, there were clusters of earthquakes around a zone that included a major earthquake on Jan 30, 1973, another cluster around an earthquake of March 14, 1979, and two more cluster around earthquakes of July 1957 and January, 1962. Between these clusters were large areas that had produced no recent earthquake activity. The zones with low seismically are called seismic gaps. Because the faulting had occurred at other places along the subduction zone it could be assumed that strain was building in the seismic gaps, and earthquake would be likely in such a gap within the near future. Following a magnitude 8.1 earthquake on September 19, 1985, a magnitude 7.5 aftershock on Sept. 21, and a magnitude 7.3 aftershock on Oct. 25, along with thousands of other smaller aftershocks, the Michoacan Seismic gap was mostly filled in. Note that there still exists a gap shown as the Guerrero Gap and another farther to the southeast. Over the next 5 to ten years we may expect to see earthquakes in these gaps. |
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Short-Term Prediction
Among the precursor events that may be important are the following:
P-wave Warning Systems Because P-waves are generally less destructive than S-waves and Surface waves, and because P-waves always travel faster than S and Surface waves, recent efforts are being used to exploit these factors to develop a P-wave warning system. Such a warning system would not be very effective near the epicenter of an earthquake because the time delay between the arrival of the first P-wave and the first S-wave would be too short. But, at greater distance from the epicenter, the delay becomes larger and might at least give people time to protect themselves. Also, devices that shut down gas and electrical lines could do so on detection of the arrival of P-waves to that potential damage from gas and electricity could be avoided when the S- and Surface- waves arrive. Systems that rely on this principle are currently being tested. |
Controlling Earthquakes
Although no attempts have yet been made to control earthquakes, earthquakes have been known to be induced by human interaction with the Earth. This suggests that in the future earthquake control may be possible. Examples of human induced earthquakes
In the first two examples the increased seismicity was apparently due to increasing fluid pressure in the rocks which resulted in re-activating older faults by increasing strain. The problem, however, is that of the energy involved. Remember that for every increase in earthquake magnitude there is about a 30 fold increase in the amount of energy released. Thus, in order to release the same amount of energy as a magnitude 8 earthquake, 30 magnitude 7 earthquakes would be required. Since magnitude 7 earthquakes are still very destructive, we might consider generating smaller earthquakes. If we say that a magnitude 4 earthquake might be acceptable, how many magnitude 4 earthquakes are required to release the same amount of energy as a magnitude 8 earthquake? Answer 30 x 30 x 30 x 30 =810,000! Still, in the future it may be possible to control earthquakes either with explosions to gradually reduce the stress or by pumping fluids into the ground. Mitigation Against Earthquake Damage As we discussed previously "earthquakes don't kill people, buildings do". Thus, if we can construct buildings and other structures in such a way that they do not collapse of fail during an earthquake, we can reduce the casualties and damage from earthquakes. This has proven evident from comparison of earthquakes in areas with and without earthquake resistant building codes. Again, there are many examples to show this, including - the January 12, 2010 earthquake of Moment Magnitude 7.0 occurred in Haiti, where most of the construction was poorly reinforced concrete. The destruction was massive with an estimated 250,000 deaths. This is in contrast to the February 27, 2010 Moment Magnitude 8.8 earthquake in Chile, a country where earthquake resistant building codes were enforced. The death toll from this larger earthquake was about 520, again, proving the effectiveness of building codes. In order to withstand an earthquake, buildings must be designed and built with the following characteristics of earthquakes in mind:
To elaborate on some these points: Duration of Shaking The duration of shaking largely depends on the size of the earthquake. |
Richter Magnitude |
Duration of Shaking (seconds) |
8 - 8.9 |
30 - 180 |
7 - 7.9 |
20 - 130 |
6 - 6.9 |
10 - 30 |
5 - 5.9 |
2 - 15 |
4 - 4.9 |
0 - 5 |
In the 2011 Moment Magnitude 9.0 earthquake in Japan, severe shaking continued for 3 to 5 minutes. (See http://www.scientificamerican.com/article.cfm?id=fast-facts-japan) Building Resonance Consider that Seismic waves cause the ground to vibrate at frequencies between 20 and 0.002 cycles per second which translates to periods (time to complete one cycle) of between 0.05 seconds and several minutes. All structures have natural frequencies or periods of vibration. If a structure has a period of vibration similar to a seismic wave it will resonate, and the longer it resonates, the more likely it will fail. Resonance can be eliminated by:
Horizontal Shaking P-waves, as they arrive at the surface, generally cause the ground to move up and down. But S-waves and Surface Waves cause the ground to move from side to side as well as up and down. The horizontal movement, causes different parts of building to move in different directions. Problems with buildings can be eliminated by using shear walls – designed to receive horizontal forces from floors, roofs and trusses and transmit them to the ground. Building a house of cards illustrates this point well. A house of cards will stand on its own, but will fall apart of you shake the foundation. If the cards are taped together, so that the vertical components and horizontal components are connected together, the house of cards will not fall over if you shake the foundation. The structure and also be isolated from the underlying ground by base isolation, wherein devices on the ground or within structure are placed to absorb part of earthquake energy. This can be done by installing wheels, ball bearings, shock absorbers, ‘rubber doughnuts’, etc. to isolate building from worst shaking. Concrete and Steel Construction Materials Large buildings are often constructed with concrete reinforced by steel rebar. Concrete is strong under compression, but weak under extension or shearing. Steel reinforcement of concrete can help prevent failure, but must be done in such a way to prevent motion both horizontally and vertically. For example, bridges supported by vertically embedded rebar in the support structures can easily fail when subjected to horizontal vibrations, as illustrated by the freeway collapse in the 1989 Loma Prieta earthquake near San Francisco. But, if the vertical rebar is surrounding by rebar spiraling around the vertical rebar, the structure will have more resistance to horizontal movement. Bridges Cantilever type bridges, like a portion of the Bay Bridge from Oakland to San Francisco or the Crescent City Connection in New Orleans where towers support the bridge are more susceptible to damage than suspension bridges, like the Golden Gate Bridge in San Francisco, where more flexible steel cables support the bridge deck. In cantilever type bridges, since the towers are fixed, they can move apart during an earthquake, causing the bridge deck to collapse. The more flexible steel cables of a suspension bridge allow the deck to sway with the vibrations and therefore resist failure. Utilities Water supplies are often cut off as water pipes break during and earthquake. Failure of the water supply prevents control of secondary fires and slows recovery. Constructing the water supply system wi ht flexible joints can prevent breakage of pipes. Gas mains and electrical meters can be fitted with devices that shut off natural gas automatically at the first sign of shaking. This will also aid in the prevention of secondary fires. Houses Modern 1 and 2-story wood-frame houses generally perform well in earthquakes, but they can perform better if additional support is provided by building shear walls, bracing, and tying walls, foundations and roof together. Even if the house does not fall down during an earthquake, interior furnishings can cause considerable damage and injury as interior items are thrown about during the shaking. Thus, it is essential to bolt down water heaters, air conditioning/heating unites, ceiling fans, cabinets, bookshelves, and electronics, basically anything that can fall or collapse and cause injury. Summary While architecture & building codes reduce risk, not all kinds of behavior can be predicted. For example: different earthquakes show different frequencies and durations of ground shaking, & different vertical & horizontal ground accelerations. Old buildings cannot cost-effectively be brought up to code, especially with yearly refinements to code. And, even with construction to earthquake code, buildings fail for other reasons - like poor quality materials, poor workmanship, etc., that are not discovered until after an earthquake. What To Do Before and During an Earthquake Every person living in areas susceptible to earthquakes should be educated in steps they can take to minimize risk, before, during and after an earthquake. These steps are discussed in detail on FEMA web pages - see https://www.ready.gov/earthquakes and are briefly summarized here, but you should check the website for important details. Before an Earthquake
During an Earthquake Be aware that some earthquakes are actually foreshocks and a larger earthquake might occur. Minimize your movements to a few steps to a nearby safe place and if you are indoors, stay there until the shaking has stopped and you are sure exiting is safe. If indoors - drop and cover - get under a sturdy table, hold on until the shaking stops, crouch in an inside corner if no other protection is available, stay away from glass and anything else that might shatter or fall. DO NOT use elevators. If outdoors - Stay there. Move away from buildings, streetlights, and utility wires or anything else that may fall on you. If in a moving vehicle - Stop as quickly as safety permits and stay in the vehicle. Avoid stopping near or under buildings, trees, overpasses, and utility wires. Proceed cautiously once the earthquake has stopped. Avoid roads, bridges, or ramps that might have been damaged by the earthquake. If trapped under debris - Do not light a match.
Do not move about or kick up dust.
Cover your mouth with a handkerchief or clothing.
Tap on a pipe or wall so rescuers can locate you. Use a whistle if one is available. Shout only as a last resort. Shouting can cause you to inhale dangerous amounts of dust. After an Earthquake
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Examples of questions on this material that could be asked on an exam
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