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Earthquake Questions and Answers

Shake, rattle, and roll! In this short article, we answer common earthquake questions and answers to give you a basic understanding of this disruptive event.

What is an earthquake? 

An earthquake is the movement of the Earth's crust caused by pieces of the crust suddenly shifting at faults, or cracks in the Earth.

What determines that an earthquake is really an aftershock?

Aftershocks are earthquakes. Aftershocks do not have a precise definition, but are less intense than a mainshock (generally, the event with the largest magnitude) and occur in the same area. Any subsequent earthquake in the same area, but with less magnitude, is considered an aftershock, until activity in that area returns to normal. Aftershocks are generally caused by the "readjustment" of Earth's crust in response to a larger earthquake and occur even after very small earthquakes.

Where was the largest-magnitude earthquake in the 50 states, and when was it?

The strongest recorded earthquake in the United States was near Prince William Sound, Alaska, on March 27, 1964. It measured a 8.4 on the Richter scale and killed 131 people. It also caused a 50-foot tsunami that traveled 8,445 miles at 450 miles per hour. This earthquake's tremors were felt in California, Hawaii, and Japan.

However, the New Madrid (Missouri) Earthquake, which was not recorded, is considered by many to have been the most severe in U.S. history. This series of earthquakes started in December 1811 and lasted until March 1812. It shook more than two-thirds of the United States and was felt in Canada. It changed the level of the land by as much as 20 feet, altered the course of the Mississippi River, and created new lakes west of Mississippi and Tennessee. Because the area was so sparsely populated, there was no known loss of life.

How often do severe earthquakes occur on the West Coast?

The states of California and Nevada experience the most earthquakes. More than 300,000 earthquakes have been recorded in these two states since 1836, including 10 of the 15 largest earthquakes in the contiguous United States.

The largest earthquake in California, and the second largest in the United States, registered 7.9 on the Richter scale and occurred along the San Andreas Fault in Fort Tejon in 1857. One person was killed, and the earthquake caused significant property damage.

Here are some more recent examples of severe earthquakes in that area:

  • A 1933 quake in Long Beach, California, registered 6.3, killed 115 people, and caused $40 million in damage.
  • A 1952 quake in Kern County registered 6.1, killed 12 people, and caused $60 million in damage.
  • The famous Loma Prieta quake of 1989—watched by many during the World Series—registered 7.1, killed 63 people, injured more than 3,700, and caused $6 billion in damage.
  • The Northridge quake of 1994, which happened in a densely populated area of Los Angeles, registered 6.8, killed 57 people, seriously injured more than 1,500, and resulted in $20 billion in damage, including several important Los Angeles freeways. For many days after the earthquake, thousands of homes were without gas and electricity, and 49,000 homes had no water, making this one of the biggest earthquakes in terms of disruption of life.

What have been the most recent earthquakes and where were they?

In April 2009, one earthquake struck central Italy, registering a 6.3. The next day, a 4.9–magnitude aftershock hit the same area.

So far, in 2010, there have been six recent earthquakes.
Four were in January: a 6.5–magnitude quake off the shore of Northern California, 4.3–magnitude quake in Southern California, a 7.0–magnitude quake approximately 16 miles from Haiti's capital city of Port-au-Prince, and a 6.1–magnitude aftershock again in Haiti.
One was in February in central Chile; it registered an 8.8.
One other quake was in April; it registered a 7.2 and struck just south of the U.S. border near Mexicali.

Where does the seismograph come from?

The American scientist John Winthrop (1714-1779) was one of the first to make scientific studies of earthquakes and is known as the founder of seismology. However, we cannot say that he was the inventor of the seismograph, as it is quite likely that various versions of this "machine" had already been constructed by his time.
We do know that Zhang Heng invented an earthquake detector in China around A.D. 132. It consisted of a copper-domed urn with dragons' heads—each containing a bronze ball—around the outside. Inside the dome was a pendulum that would swing when the earth shook and knock a ball from the mouth of a dragon into the mouth of a bronze toad waiting below. The ball made a loud noise and signaled the occurrence of an earthquake.

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Comments

What causes the Boom of an

By watani

What causes the Boom of an earthquake, the sudden noise? Is it the actual ledges way underground shifting? Out in the country where there are not other bldgs, it cannot be the noise of bldgs moving. So please explain the noise(s) that occur as a quake is happening or about to happen. The rattleing can continue without more noise. How come?

Good question, watani, and

By Almanac Staff

Good question, watani, and not an easy answer. This is a topic on which there is not a lot known. One of the best studies—in fact, the first to record earthquake sounds—dates to 1986! Many factors contribute to the “sound” that an earthquake makes. In brief, yes, the boom emanates from the ground shifting. (Again, simply put), quakes generate P waves first that move back-and-forth, and followed by S waves that move with more of a sideways motion. The P waves, which travel fastest, create the sound. The stronger S waves, follow within seconds and vibrate more slowly.
BTW, in very strong earthquakes, both waves may be felt; in small quakes, usually only the S wave/s may be felt—but the sound of the P wave may be heard. Note, too, that P and S waves refer underground effects; these are not waves that travel on Earth’s surface.

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