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How do Earthquakes Happen?

By Edited Jan 14, 2015 1 0

Anyone that has ever looked at a map of the world has noticed that parts of the continents seem to fit together like a jigsaw puzzle. Scientists in the 1800s also begin to document various fossils that existed in different parts of the world separated by thousands of miles, with no apparent way to have migrated across oceans.

In an attempt to explain the geographic similarities in the continents and the fossilized remains spread across ocean, Alfred Wegener published The Origins of Continents and Oceans in 1912. In his research, he suggested that the continents had once been connected into a giant land mass he referred to as Pangaea and that over the course of many thousands of years, they had “drifted” apart to their current positions. However, Wegener did not have any scientific proof as to how this could occur so his theories were dismissed by the scientific community for decades.

In fact, during this era the only scientific proof of anything strange going on besides some matching fossils on different continents was some data gathered by the HMS Challenger on an expedition in 1872 for the British Navy. During that voyage, they attempted to map the depths of the Pacific Ocean by dropping an anchor overboard at various intervals and measuring how much rope was used before it hit bottom. As they sailed further into the western Pacific Ocean, they noticed the readings starting to get deeper and deeper until they hit a point near the Marianas Islands on March 25, 1875 which measured an astonishing 26,850 feet deep. The Marianas Trench as it became known is the deepest point on Earth. However, no one knew why this part of the world was so deep and what could be causing it.

In 1929, Arthur Holmes took another look at some of Wegener’s ideas and suggested that as the Earth’s mantle is heated, its density decreases and rises to the surface where it is cooled and then sinks again. Holmes theorized that this repeated cycle of heating and cooling created a “current” which moved continents like a conveyor belt.

It wasn’t until after World War II that the scientific proof would begin to emerge to validate Wegener and Holmes’ theories. In the late 1940’s, the United States Navy begin to map the bottom of the ocean using sonar to aid the submarine fleet to navigate the depths. Immediately geologist noticed huge mid-ocean ridges, essentially mountain ranges as large as the Himalayas. The sonar maps also revealed that there were deep trenches which bisected the length of each of the underwater mountain ranges. In fact, the deepest parts of the ocean can be found in these trenches.

After discovery of the massive underwater ridges and trenches, scientist begin to use magnometers which discovered the magnetic field alternated around the ridges which implied that the ridges were constantly forming new rock. As the new rock is “born” from the Earth, older rock is pushed further away from the ridge line which produces symmetrical magnetic stripes to either side of the ridge.

Therefore, as the ocean floor spreads outward in each direction from the mid-ocean ridges, the oldest sea floor can be found the further you get from the ridge. In addition, the trenches that run near or parallel to the underwater mountain ranges are often the deepest portions of ocean on Earth. Not only that, the oldest sea floor is most often found in these vast deep sea trenches.

By the 1960s, using the sonar and other data gathered from underwater mapping, scientists were finally able to piece together a theory based on Holmes’ convection theory 30 years before. Essentially, Wegener and Holmes were right, they just did not have the technology available to them during their lifetimes to prove their theories.

What Causes Earthquakes?

How do Earthquakes Happen?
Credit: Opensource

The development of the scientific evidence for plate tectonics proved that the Earth’s surface is composed of a series of plates that float on the mantel.  

Plate tectonics describes the process of how the ocean floor moves because of the constant magma erupting from these underwater ridges producing currents of magma flow in the opposite directions outward from the ridge. These currents produce cracks in the ridges which allow molten magma to rise and form the newest ocean floor.

Over millions of years, that new sea floor travels further away from the ridges in both directions and eventually runs into a continental plate  where subduction occurs. Subduction simply describes the process of the now older ocean floor diving under the continental shelf, whether it be the North American plate or the various other ones around the Ring of Fire in the Pacific Ocean and all over the Earth. The same process occurs across a mid-Atlantic ridge, although that region of the Earth is much less seismically active. Nevertheless, the North American plate continues to move about an inch away from the European plate each year.

It is this constant pressure in the subduction zones that cause Earthquakes. The tension of the plates grinding together, one diving beneath the other, which builds up over decades or hundreds of years until it snaps causing a jolt of several inches to several feet along the fault lines in the Earth’s crust.

Worst Earthquakes in History

How do Earthquakes Happen?
Credit: Opensource

The following is a list of the strongest earthquakes in recorded human history based on the Richter magnitude scale. To understand the scale that earthquakes are measured, think of it in terms of a base 10 rating. Therefore an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times greater a 4.0 earthquake and roughly 1/3 more release of energy.

It is certain that there have been many more destructive earthquakes in the history of the planet, however, either no one was there to document it, or the Richter scale of measuring it did not exist.

Magnitude: 8.5

  • Kamchatka, Russia February  3, 1923
  • Southern Sumatra, Indonesia 2007

Magnitude: 8.6

  •  Assam – Tibet Earthquake August 15, 1950
  • Andreanof Islands, Alaska 1957

Magnitude: 8.7

  • Sumatra, Indonesia March 28, 2005
  • Aleutian Islands February 4, 1965

Magnitude: 8.8

  • Ecuador Jan. 31, 1906
  • Chile Earthquake February 27, 2010

Magnitude: 9.0

  • Sumatra, Indonesia December 26, 2004
  • Honshu, Japan March 11, 2011
  • Kamchatka Peninsula, Russia November 4, 1952

Magnitude: 9.1

  • Aleutian Islands March 9, 1957
  • West Coast of Northern Sumatra 2000

Magnitude: 9.2

  • Prince William Sound, Alaska March 28, 1964

Magnitude: 9.5

  • Chile May 22, 1960

The 1964 Alaskan Earthquake



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