15 things you may not know about earthquakes

Earthquakes can be so small they are never felt, or so large that the entire earth shakes from one.

They have happened in every U.S. state and even occur on the moon.

While there are many things we do not know about earthquakes, here, from the U.S. Geology Survey, are some facts about the powerful and unpredictable natural occurrences.

  1. The world's deadliest recorded earthquake occurred in 1556 in central China. It struck a region where most people lived in caves carved from soft rock. These dwellings collapsed during the earthquake, killing an estimated 830,000 people. In 1976 another deadly earthquake struck in Tangshan, China, where more than 250,000 people were killed.
  2. The largest recorded earthquake in the United States was a magnitude 9.2 that struck Prince William Sound, Alaska on Good Friday, March 28, 1964. 
  3. The earliest reported earthquake in California was felt in 1769 by the exploring expedition of Gaspar de Portola while the group was camping about 30 miles southeast of Los Angeles.
  4. The average rate of motion across the San Andreas Fault Zone during the past 3 million years is estimated to be about 2 inches a year. This is about the same rate at which your fingernails grow. Assuming this rate continues, scientists project that Los Angeles and San Francisco will be adjacent to one another in approximately 15 million years.
  5. The first "pendulum seismoscope" to measure the shaking of the ground during an earthquake was developed in 1751, and it wasn't until 1855 that faults were recognized as the source of earthquakes.
  6. Moonquakes (earthquakes on the moon) do occur, but they happen less frequently and have smaller magnitudes than earthquakes on the Earth. It appears they are related to the tidal stresses associated with the varying distance between the Earth and the moon. They also occur at great depth, about halfway between the surface and the center of the moon.
  7. Although both are sea waves, a tsunami and a tidal wave are two different unrelated phenomena. A tidal wave is a shallow water wave caused by the gravitational interactions between the sun, moon and Earth. A tsunami is a sea wave caused by an underwater earthquake or landslide, usually triggered by an earthquake, displacing the ocean water.
  8. The "Ring of Fire," also called the Circum-Pacific belt, is the zone of earthquakes surrounding the Pacific Ocean. About 90 percent of the world's earthquakes occur there. The next most seismic region, where 5-6 percent of the world's earthquakes happen, is the Alpide belt. It extends from the Mediterranean region, eastward through Turkey, Iran and northern India.
  9. It is estimated that 500,000 detectable earthquakes occur in the world each year. Around 100,000 of those can be felt. Around 100 of them causing damage.
  10. Each year, the southern California area has about 10,000 earthquakes. Most of them are so small that they are not felt. Only several hundred are greater than magnitude 3.0, and only about 15-20 are greater than magnitude 4.0.
  11. As far as we know, there is no such thing as "earthquake weather." Statistically, there is an equal distribution of earthquakes in all types of weather.
  12. From 1975-1995 there were only four states that did not have any earthquakes: Florida, Iowa, North Dakota and Wisconsin.
  13. Alaska is the most earthquake-prone state and one of the most seismically active regions in the world. Alaska experiences a magnitude 7 earthquake almost every year, and a magnitude 8 or greater earthquake on average every 14 years.
  14. The largest recorded earthquake in the world was a magnitude 9.5 (Mw) in Chile on May 22, 1960. When the earthquake hit, seismographs recorded seismic waves that traveled all around the Earth. These seismic waves shook the entire earth for many days. This phenomenon is called the free oscillation of the Earth.
  15. Human beings can detect sounds in the frequency range of 20-20,000 Hertz. Most earthquake waves have a frequency of less than 20 Hz, so the waves themselves are usually not heard. Most of the rumbling noise people hear during an earthquake comes from the movement of the building they are in or the building's contents.

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