EARTHQUAKE AND VOLCANIC HAZARDS
Environmental Geoscience (GEOL1133), Dave Stahle
Earthquake and volcano distribution
Earthquake focus and epicenter
Measurement of earthquake location and magnitude
USA Seismic Hazard Map
New Madrid Earthquake, Missouri-Arkansas-Tennessee, 1811-1812
Seismic wave amplification
California quake risk
San Andreas fault (transform plate boundary)
Possible global financial repercussions of a giant earthquake in California?
Strategies to mitigate earthquake risk:
1. building codes
3. earthquake prediction??
Pacific Rim of Fire
Composite or stratovolcano vs. Shield Volcanoes
Hawaiian Islands and hot spots in the aesthenosphere
Nuee Ardente (pyroclastic flow or volcanic avalanche)
Great eruptions in world history
Climatic impact of volcanic eruptions
Bryson, B. 2003. A Short History of Nearly Everything. Broadway Books. 544 p.
Keller, E.A., 2000. Environmental Geology, 8th edition. Prentice Hall, U. Saddle R., NJ.
Thompson, G.R., and J. Turk, 2007. Earth Science and the Environment, 4th edition.
Thomson, Brooks/Cole, Belmont, CA.
Earthquakes are the shaking or trembling of the ground due to passage of seismic waves through the solid earth. Seismic waves transmit energy through solid rock and are caused by sudden rupture of the crust, faulting, and swift subsidence, mostly associated with sudden movements of tectonic plates at plate boundaries. Earthquake focus is the subterranean point in the earth where the rupture occurs (the point of energy release). Earthquake epicenter is the point on the earth surface directly above the focus, and is usually the region where the largest seismic waves and greatest damage occur. However, there can be seismic wave amplification, and this is greatest in unconsolidated sediments like mud or sand. So solid rock can carry a seismic wave, but when it enters softer sediments, the seismic wave amplitude can increase and cause greater damage to structures. This was the case in the Marina District of San Francisco, which was badly damaged during the 1989 Loma Prieta earthquake, even though the epicenter was located many miles south of the city.
Seismometers are used to determine the epicenter and magnitude of earthquakes. There are two basic types of seismic waves generated by earthquakes: body waves and surface waves. Surface waves cause the ground surface to heave up and down, and move laterally from side to side. Surface waves move slower than body waves, but cause most of the damage during earthquakes. Body waves include P-waves and S-waves. P-waves are “primary” body waves and are compressional. P-waves are the fastest seismic wave and will travel through solid, liquid, or gas. P-waves travel incredibly fast (about 8 km per second, compared with the fastest jet airplane which only travels about 0.85 km per second). S-waves are the “secondary” body wave, and are shear waves. They only transmit through solids, and move about 4 km per second.
Because P and S waves propagate at different speeds through the solid earth, seismologists can use the difference in arrival time between P and S waves to determine the distance from a seismometer and the earthquake, and with at least three seismometer stations they can also locate the epicenter of an earthquake. Just like we can determine distance to a lightning strike by counting the seconds between a lightning flash and the subsequent thunder, we can also estimate distance to an earthquake focus by counting the seconds between the observation of a P wave and an S wave. Remember that light travels faster than sound. For measuring lightning, the light is essentially instantaneous, but sound moves at only 331 meters per second (1088 feet per second). So if there is only one second between the lightning flash and the sound of thunder, then the bolt was only 331 meters away. If there is three seconds between the flash and the sound, then the lightning strike was 1 km away.
Because P waves travel through solid and liquid, and S waves travel only through solids, seismologists have been able to estimate a great deal about the composition of the Earth interior, including the mantle and core. Thompson and Turk (2007) use the analogy of thumping on a watermelon to ‘sound’ it and determine if it is solid or mushy.
The amplitude of the S-wave on the seismographic trace can also be used to estimate the magnitude of the earthquake. We will use these relationships in lab to study earthquake locations and magnitudes.
The Measurement of Earthquake Magnitude:
Mercalli Scale, based on structural damage.
Richter Scale, expresses the amount of energy released by the earthquake, calculated from the height of the largest body wave on the seismogram, also referred to as the “local magnitude” (Charles Richter and Beno Gutenberg, Caltech, 1935).
Moment Magnitude Scale, expresses the amount of movement and the total surface area of movement. Moment magnitude gives a better estimate of the total energy released during an earthquake, and is now in wide use. The Richter and Moment Magnitude scales are base-10 logarithmic scales. The energy released on both scales is also a log scale, but the base is 32, so the energy released increases by a factor of 32 for each full increment (a magnitude 7 is 32 times more powerful than a magnitude 6, and over 1000 times more powerful than a magnitude 5).
To get an idea for the magnitudes of energy in nature, consider the following energy scale, using ergs. An erg is a standard energy unit:
1 x 100 ergs = 1 erg, which is incredibly tiny.
3 x 1012 ergs = 3 trillion ergs, the energy needed for 100-watt light bulb for one hour.
1 x 1016 ergs = 2.389 x 108 calories, or 238,900,000 calories (a calorie is the amount of
heat needed to raise one gram of water 1 degree C, at standard atmospheric pressure).
5 x 1016 ergs = typical lightning bolt.
1 x 1027 ergs = annual energy consumption in the USA.
1 x 1029 ergs = daily reception of solar energy by Earth (so theoretically at least, solar
energy could ultimately be the solution to the energy needs of society.)
The most powerful earthquakes usually occur at subduction boundaries between the tectonic plates. Having said this, it is true that the most powerful earthquakes ever to strike the lower 48 states are believed to have occurred in SE Missouri and NE Arkansas in 1811-1812, a continental interior region far removed from modern plate boundaries. These were the famous New Madrid Earthquakes, and it is now believed that there is a buried rift zone in the crust below the sediments of the lower Mississippi Valley, and this ancient rift is capable of generating powerful earthquakes.
California is actually located at a transform boundary, the famous San Andreas fault. Transform boundaries are usually not as dangerous as subduction boundaries in terms of quake risk, but they can certainly generate powerful quakes. There is great concern for the possible recurrence of a great earthquake in California, especially were it to happen in Los Angeles or San Francisco. Those of us located in Arkansas may feel a sense of security and perhaps detachment from this quake risk to California. But our economy is tightly integrated across the country and even around the world, and a severe earthquake in California could have a major financial impact here in Arkansas as well. So everyone really shares in earthquake risk, and we all have a stake in trying to minimize the consequences to society. What can be done to lessen earthquake hazard?
1. Building codes. Some beautiful historical structures in California were never built to withstand seismic ground motions. These buildings need to be remodeled to improve their resistance to quake damage. New construction in California must meet strict building codes written with earthquakes in mind. These codes help prevent damage and death in quakes. How might building codes, or construction standards, help mitigate earthquake damage? There are hundreds of possible examples. Consider the water heater in your home. It stands in a closet or under the house full of heated water. A rigid gas line is attached (if it is heated with natural gas). In earthquakes they can easily topple over, ripping the gas line from the heater, and allowing natural gas to vent into the room where it might be ignited to start a fire that burns down your house and perhaps the entire neighborhood. To lessen the likelihood of this disaster, one can strap the water heater to the wood framing with stout metal straps, so the thing will not fall over even in a heavy earthquake. One can also use a flexible hose connection for the gas line that will give but not break in an earthquake. These are simple solutions, and if they are mandated for all structures in an earthquake prone area, then there will be far less death and destruction. It is often said that earthquakes don't kill people, buildings and heavy objects do the killing. So we should build and furnish our houses and offices with this in mind.
2. Zoning laws. Science knows where earthquake damage will be most severe, simply by mapping unconsolidated sediments where seismic waves can be amplified. Zoning laws then dictate that no high rise apartment complexes be built there, for example, because that would put the occupants at undue risk to quake damage.
3. Earthquake prediction. There have been a very few notable successes in earthquake forecasting, but most quakes have not been predicted in advance. The 1975 quake in Haicheng, China was forecast five hours in advance and 6 million people were evacuated. Hundreds were killed, but thousands would have perished without this advance warning. The Haicheng example was a big success, but there are many more failures in quake forecasting than successes. The USGS said there was a 95% chance for a magnitude 6 earthquake between 1987 and 1992 at Parkfield, California, but it has yet to happen.
At present, long lead time prediction of earthquakes is not possible, but there are seismologists who are optimistic that this may be possible in the future, using geophysical monitoring of variables like radon gas releases, magnetic anomalies, electromagnetic radiation anomalies, crustal strain, minor seismic activity, and many others. Even bizarre animal behavior has been observed before some earthquakes. Whether this will bear fruit remains to be seen. However, certain extremely short term earthquake protection schemes may be possible. For example, if a sensor detects a seismic wave it could then trigger an electrical signal to shut down electrical grids, gas pipelines, and other utilities to minimize subsequent earthquake damage. This may be possible because electrical signals travel faster than seismic waves. The Japanese have just installed a system to detect P-waves and then trigger television and radio warnings seconds prior to the arrival of the slower moving but more damaging S-waves. This may save lives, though there is some worry that people might panic and crush each other trying to escape buildings.
Long lead time earthquake prediction is not routinely possible, but there have been some famous examples of bogus earthquake predictions. For Arkansas the most notorious would be Iben Browning, who predicted an earthquake for Dec. 4, 1990 on the New Madrid Seismic Zone in NE Arkansas and SE Missouri. This caused incredible publicity, and expensive school and industrial closings. This occurred in spite of the fact that Browning was a well known crackpot, and was basing his prediction on tidal strain which has been thoroughly investigated for many years and found to be of no value in earthquake forecasting. The predicted earthquake did not happen, of course, and it exposed what can only be described as a massive failure of objective journalism. The major networks and others presented Browning's predictions with little serious criticism, even though 99% of the scientific community disagreed with Browning. They gave him a voice, evidently for sensational reasons. When the quake failed to materialize, the public justifiably became more skeptical of the whole science of quake prediction, even though Browning was not an expert.
Volcanoes are conical or dome-shaped mountains or hills constructed from deposits of lava flows and tephra (i.e., pyroclastic material with solidified magma, ash, and clastics of all sizes, blown out under pressure). There are two broad categories of volcanoes, composite and shield volcanoes. Shield volcanoes are accumulations of basalt lava flows, largely “Mafic” or “Sima” in composition (i.e., rich in iron, Fe, and magnesium, Mg). Sheild volcanoes are often found at spreading boundaries or over hot spots in the aesthenosphere. They are characterized by a low dome-like profile, are low in gas content, and often produce river-like eruptions of flowing lava. The most famous example would probably Kilauea crater in Hawaii. The shield volcanoes of Hawaii are a bit peculiar because the island is located in the middle of the Pacific plate, far from any plate boundaries. But the aesthenosphere is especially hot under the lithosphere of Hawaii, a so-called hot spot, and it has essentially burned holes through the crust to form the shield volcanoes we know as the Hawaiian Islands. In fact, because the Pacific plate is drifting slowly to the northwest over this stationary hot spot, the Big Island of Hawaii (in the southeast) is the youngest island in the archipelago, and the islands increase in age to the northwest.
Composite volcanoes are typically found at subduction boundaries, and are the most dangerous type of volcano because they tend to erupt explosively. Composite volcanoes (also called stratovolcanoes) are built from alternating layers of lava and tephra, and are largely Sial in composition (felsic minerals dominated by Fe and aluminum, Al). Composite volcanoes are cone-like in profile, contain huge quantities of heated water and compressed gasses in the magma chambers, and can explode in violent eruptions. Because subduction boundaries ring much of the Pacific basin, this great arc of subduction zones is often referred to as the Pacific Rim of Fire, referring to the dangerous composite volcanoes also found at these subduction zones. Composite eruptions have killed thousands of people, and famous examples of composite volcanoes include Mt. Fuji, Japan, Mt. St. Helens, USA, Mt. Vesuvius, Italy. The Nuee Ardente, also called a superheated pyroclastic flow, is a real killer in the eruption of composite volcanoes. A Nuee Ardente is a volcanic avalanche of glowing, searing, white-hot ash (1800 degrees F), dust, and poisonous gases which literally explode at high speed downhill, engulfing everything in its path and causing total death and destruction. Pompeii was engulfed in a Nuee Ardente from Vesuvius, as was the town of St. Pierre on the island of Martinique in 1902 where 29,933 people were killed almost instantly, and only four people in the town survived, all terribly burned.
A selected chronological history of great volcanic eruptions (all composite volcanoes):
1. Long Valley Caldera, Mammoth Lakes, California, erupted 700,000 years ago, and as recently as 100,000 years ago. Deposited the Bishop tuff, and is responsible for the largest volcanic eruption known in North America. Deposited ash across the continent, including Maine.
2. Mt. Mazama erupted 6600 years ago and formed Crater Lake, Oregon. Formed the Mazama Ash, which is an important stratigraphic dating horizon across much of North America
3. Santorini on the island of Thera in the Mediterranean (near Cyprus) exploded ca. 1628BC in one of the greatest known eruptions in world history, and may have contributed to the end of the Minoan Civilization. Evidence of the climatic impact of this eruption has been found in frost rings formed in high elevation bristlecone pine of California, suggesting a global scale climatic impact of large magnitude volcanic eruptions. Paradoxically, the climatic impact of these big eruptions is cooling for two or three years following the eruption, because dust and aerosols get lofted into the stratosphere, blown worldwide by stratospheric winds, and the dust then blocks sunlight to cool the surface temperature of earth.
4. Vesuvius, Italy, in AD 79 exploded and buried Pompeii and Herculaneum with pyroclastic flow, killing over 2000 people. Recorded by Roman archivists, this is the most famous volcanic eruption in world history.
5. Mt. Etna, Scicily, erupted in AD 1669, killing 13,000 people.
6. Tambora, Indonesia exploded in 1815 in the largest volcanic eruption during the historic era. 25 cubic miles of rock was ejected into the atmosphere by this epic explosion. Killed 12,000 people and disrupted the weather worldwide. Resulted in the "Year Without a Summer" in 1816 over the northeastern USA. Snowstorms occurred in June over New York, frosts throughout the summer in the Midwest.
7. Krakatoa, Indonesia, exploded in 1883 in the Sunda Straits. Wiped out the island and blew a hole in the ocean floor 1100' deep and 6.5 miles wide! 70 pound boulders (volcanic "bombs") were thrown onto islands 50 miles distant. 130' tsunamis were generated, and over 20,000 people were killed. 17.5 cubic miles of volcanic material was ejected into the atmosphere, again lowering global temperatures for two or three years.
8. Mt. St. Helens, Washington, May 18, 1980. 2.5 cubic miles of rock was atomized and ejected in this explosion, comparable to the eruption of Vesuvius in AD 79. No nearby cities of any size, and only 74 people were killed. The growing threat of eruption was monitored and most people in the area were evacuated ahead of time. The incredible explosion wiped out the upper 1300' of the mountain, and blasted a crater 2 miles wide by one mile deep on the north side of the mountain. The shock wave of the eruption flattened some of the most beautiful forests in America in a fan shaped area out 30 miles from the volcano. There were 156 square miles of total destruction. Dave Johnston, a USGS volunteer scientist monitoring the volcano way too close was instantly vaporized. Elk, deer, bear, mountain lion, and countless other wildlife perished in the blast and under the scalding hot ash at 800 degrees C. The river water became so hot that salmon left the water to die on the ground. The eruption cloud blew in the westerlies across the USA. The eruption paralyzed air travel in the area, and the Columbia River was shut down by the flood debris from the eruption.
Volcanic impacts on human society include: 1. A source of geothermal energy; 2. A cause of volcanic catastrophes; 3. Volcanic eruptions can change the global climate. Volcanoes can inject fine particulate matter into the stratosphere, veil the planet with these fine suspended ‘aerosols,’ block incoming sunlight and reduce global temperatures (like the ‘Year without a Summer’ following the 1815 eruption of Tambora). Volcanoes can also emit huge quantities of carbon dioxide gas. Carbon dioxide is a heat trapping gas and can warm the planet. So massive volcanic eruptions over Earth history have also been implicated in long term temperature changes. The mid-Cretaceous superplume 120 million years ago, which included massive basaltic flows rich in carbon dioxide emissions, may have contributed to global warming of the Cretaceous that reached several degrees higher than the current global average temperature. Violent composite eruptions which inject aerosols into the stratosphere tend to be associated with global cooling episodes (typically short term, maybe 2 to 4 years). Massive and long-lasting shield eruptions rich in carbon dioxide tend to be associated with global warming episodes. However, significant global volcanic warming is known only from the ancient geological record, while volcanic cooling has been witnessed during modern times.