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Seismic Waves Help (page 3)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Earthquake Predictions

Since millions of earthquakes (most very minor) hit somewhere on the planet every year, there is no way they can all be predicted or even recorded. Most people just want to know when to expect the “Big One.”

In fact, some people even think there is an earthquake season. There is no such thing. Statistically, earthquakes take place all the time in all kinds of weather. Season has no affect on rock stresses deep in the earth. Barometric pressure changes are nothing compared to plate tectonic pressures. Table 12-3 lists the number of earthquakes and their numbers over the past hundred years in the United States.

Table 12-3 Earthquake totals of the past 100 years.

Time frame

Number of earthquakes

1900–1910

  232

1911–1920

  198

1921–1930

  166

1931–1940

  212

1941–1950

  314

1951–1960

  192

1961–1970

  211

1971–1980

  193

1981–1990

  106

1991–2000

  173

2001–2003

    44

Total

2041

 

But like real estate, location is everything. Alaska has the most earthquakes and is one of the most seismically active areas in the world. Alaska has a magnitude 7 earthquake almost every year and a magnitude 8 or greater earthquake about every 14 years.

In the continental United States, only Florida, Iowa, North Dakota, and Wisconsin had no recorded earthquakes between the years of 1975–1995. So, if you really don’t like earthquakes, these states might be on your “move to” list.

Though geologists are beginning to get a handle on tectonics, faulting, and earthquake processes, earthquake predictions are still fairly uncertain.

On February 4, 1975, the Chinese government issued an immediate earthquake warning to the area around the city of Haicheng and began a huge evacuation effort. Haicheng experienced an earthquake of magnitude 7.3, about nine hours later. Fortunately most of the population was outside when 90% of the city’s buildings were severely damaged or destroyed. Thankfully, injuries were few.

Although there have been other prediction successes in this region of China, the misses are frequent. On July 28, 1976, a 7.8 magnitude earthquake hit the Chinese city of Tangshan, 150km east of Beijing, and home to over one million people. The hypocenter was located directly under the city, at a depth of 11 kilometers. About 93% of all buildings in the city were destroyed, and approximately 240,000 people were killed. The earthquake came as a complete surprise to Chinese seismologists, even though the area was monitored constantly.

Earthquake Swarms

Earthquakes sometimes occur in small clusters or swarms with no really big jolt. The larger vibrations in a swarm are all about the same magnitude. Swarms take place in a small area and can last a day or months.

In some ways, swarms resemble aftershocks. Aftershocks from a main earthquake are thought to come from the sudden stress loads placed upon the rocks near a fault rupture.

Swarm patterns are different. Geologists believe geothermal energy in the crust and areas of high heat flow play a role in swarm mechanisms. After a main shock, heat allows rocks to release stress more easily. Areas of high heat flow are subject to long swarms that die down slowly.

Other ideas about earthquake swarms include:

  • the formation of new faults
  • fault creep
  • increase in the pressure of fluids at depth, and
  • sub-surface movements of magma.

Earthquake prediction could become a reality. Just as the National Weather Service predicts hurricanes, tornadoes, and other bad storms, the National Earthquake Information Center may one day release earthquake forecasts. The United States Geological Survey and other federal and state agencies, as well as universities and private institutions are also working on the problem.

Tsunami

A tsunami is a series of waves caused by large earthquakes or landslides at or beneath the sea floor. This seismic movement with the displacement of huge volumes of the sea water above it creates large, fast moving waves. A tsunami results in a coastal earthquake or from an earthquake in a far distant part of the ocean. Coastal areas may suffer little damage from an oceanic earthquake, but can be ruined by the resulting tsunami.

Although both are sea waves, a tsunami and a tidal wave are two different unrelated processes. A tidal wave is produced by high winds, while a tsunami is caused by an underwater earthquake or landslide (usually triggered by an earthquake) displacing ocean water.

Geologists have learned a lot about the wild tectonic mechanisms that happen within the Earth. As better instrumentation and understanding develops, earthquakes may lose their element of surprise!

Practice problems of this concept can be found at: Earthquake Practice Test

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