Why Earthquakes Occur

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Why and where earthquakes occur and can man cause them.

Usually when we think of earthquakes here in the United States we think of California. You might also think of earthquakes in remote areas of China, India, Iran or Turkey. Europe isn’t usually on the top of the list even though Italy recently had and has had major earthquakes.

Earthquakes can happen anywhere on the planet, some places seem to have a much higher occurrence and risk for them. An area called the “Ring of fire” is one area that has more earthquakes than anywhere else. This is a large area that rings the pacific ocean and includes places like California up to Alaska over to Japan and down through Indonesia and around to the western side of South America and Mexico. The largest earthquake ever recorded on the planet occurred in this ring of fire on May 22, 1960 in Chile and measured 9.5.

Large North American earthquakes

There are areas other than the west coast of major concern in the United States. There is a large fault that runs through the mid-western part of the US called the New Madrid fault. In December 1811 one of the largest earthquakes in North America occurred along this fault. The Richter scale had not yet been invented yet but from the damage accounts of this quake it has been estimated at 8.0. The earthquake was centered in the state of Missouri and was felt as far away as Chicago, Charlotte, Quebec City and Boston where the shaking woke up residents and caused church bells to ring. This earthquake was so large that it actually changed the course of the Mississippi River.

In 1906, San Francisco was devastated by an earthquake that was estimated at 7.7 and on March 27, 1964 (Good Friday) the second largest earthquake ever measured on the planet hit Alaska and measured an almost unbelievable 9.2 on the Richter scale.

Faults and tectonic plates

There are several types of faults, dip-slip, strike-slip and lateral. The most well known fault is called the San Andreas fault that runs through most of California and has many branches of faults off of the main one. This fault separates two tectonic plates. The North American plate is what most of North America sits on out into the Atlantic Ocean and to the west of the plate in California is the Pacific plate. The San Andreas fault is where these two plates meet. These plates move and grind against each other. The Pacific Plate is moving northwest past the North American Plate at about 2 inches per year. Usually these plates grind against each other without much trouble, once in a while they get stuck and then pull apart and there are minor quakes. In other areas the two plates get stuck and in some areas have been stuck for a long time. The longer two plates are stuck and cannot move past each other the more pressure it builds up. When something finally gives and the plates get past each other, this releases all of this built up pressure and causes the large earthquakes along the fault lines.

Press your palms together as if in a praying motion rather tightly and move your right hand slowly upward and past your left hand. This is like two tectonic plates moving past each other. Once in a while there is a little jerk of a motion and that would represent the plates slightly stuck together and a minor quake. Now press your palms together even more tightly and try and move your hand upward, all of a sudden your hand slips and goes right past your left hand. This represents a great deal of pressure and a release while the plate suddenly slips past the other resulting in a strong earthquake.

Failed rift

There are major earthquakes in areas where there aren’t two tectonic plates sliding by each other. The area where the major New Madrid earthquake occurred is not on a tectonic plate as it is right in the middle of the North American Plate. It is on what is called a failed rift known as the Reelfoot Rift.

A failed rift is where the Earths continents were trying to break apart and failed to completely break apart. The area remains weak and sometimes volcanic. A failed rift turns into an aulacogen. Though there are no volcanoes in the area of the New Madrid rift, earthquakes do occur in these rifts.

Another well-known rift is the Great African Rift in Central Africa where Mt. Kilimanjaro in Tanzania is located. Mt. Kilimanjaro is an inactive volcano. Other rifts are the Gulf of Suez, the Red Sea, the Rio Grande and the Rhine Rift in Germany.

Earthquakes can cause tsunamis

Earthquakes can cause a tsunami but not always. After the Alaskan quake, the resulting tsunami that hit Alaska killed more people than the earthquake itself did. The more recent Indian Ocean tsunami the day after Christmas 2004 was the most devastating in recent memory, killing an estimated 225,000 people.

Can humans cause earthquakes?

The largest and most widely known instance where humans have actually caused earthquakes occurred in the mid 1960’s when the US Army was pumping wastewater into the ground near Denver, Colorado. This resulted in numerous earthquakes with the largest being 5.5. When the Army stopped pumping the water into the ground, the earthquakes stopped. 1

Factoid about the Richter scale

When you hear of an earthquake you always hear of the Richter scale magnitude of the quake, but the Richter scale is not an instrument, it is a mathematical formula, which uses the recordings of a seismograph to get the magnitude. It is a logarithmic scale, meaning that a 5.0 earthquake is 10 times larger than a 4.0 earthquake.

© 2009 Sam Montana

Sources

U.S. Geological Survey

Original CBS news from the 1964 Alaskan quake

1 Nicholson, Craig and Wesson, R.L., 1990, Earthquake Hazard Associated with Deep Well Injection--A Report to the U.S. Environmental Protection Agency: U.S. Geological Survey Bulletin 1951, 74 p.

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Sam Montana

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