Why and Where Earthquakes Occur

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If two pieces of Earth, all of the sudden slip past each other then an Earthquake will follow. Faults, fault lines or fault planes are surfaces that slip past each other. An earthquake starts in the hypocenter, which is located beneath Earth’s surface. The epicenter is located right above Earth’s. Sometimes large earthquakes have a foreshock. Earthquakes that are smaller than the mainshock that happen after a bigger earthquake in the same place are called foreshocks. Mainshocks are what the larger earthquakes are called. Aftershocks always happen after a mainshock. Foreshocks and aftershocks are the same thing. Aftershock can last for week, months, or maybe even years depending on the mainshock.

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The San Andreas fault line is one of many fault lines. The San Andreas Fault is a sliding boundary between the Pacific Plate and Northern America Plate. California is sliced in two from Cape Mendocino to the Mexican border by the San Andreas Fault line. San Diego, Los Angeles and Big Sur are on the Pacific Plate. San Francisco, Sacramento and the Sierra Nevada are all on the North American Plate. Besides the San Francisco legendary earthquake in 1906, the San Andreas Fault line does not run through the city. Desert Hot Springs, San Bernardino, Wrightwood, Palmdale, Gorman, Frazier Park, Daly City, Point Reyes Station and Bodega Bay all lie on the fault line.

The San Andreas Fault is a transform fault. A transform fault is a strike-slip fault that is a boundary between two plates of earth’s crust.Causes of an Earthquake- Earthquakes happen because of faulting, which is a sudden rock movement that happens along a rupture or break surface. Earth’s surface is constantly moving slowly. The plates continue to move causing a strain and rock to be distorted. As the movement continues the strain build up. This continues until the rock cannot take it anymore, so the rock breaks and the two sides move. An earthquake is the shaking that happens when the rock breaks. People used to think that it was the opposite way around, and that the earthquake cause the rock to break.

How Often Earthquakes Occur

Since Earth is an active place, earthquakes are constantly happening. Studies show that there are about 12,000 to 14,000 earthquakes a year. Several hundreds of magnitude 2 and smaller earthquakes on average happen per day. Magnitude 7 and greater earthquakes happen more than once a month. Magnitude 8 and higher earthquake happen about once a year.


An earthquake with a magnitude of 2.5 or less is normally not felt, and occurs about 900,000 times per year. Earthquakes with a magnitude between 2.5 and 5.4 only create minor damage, and are often felt. They also happen about 30,000 times a year. Magnitude of 5.5 between 6.0 earthquakes slightly creates damage to buildings and other structures, and they occur about 500 times a year. An earthquake with magnitude between 6.1 and 6.9 can cause a lot of damage to populated cities, and happen about 100 time every year. Magnitudes of 7.0 through 7.9 earthquakes are major and create serious damage. These earthquakes happen about 20 times each year. Finally earthquakes with a magnitude of 8.0 or higher (called great earthquakes) can destroy communities near the epicenter, and they occur once every 5 to 10 years.

Where Earthquakes Happen the Most

The edges of oceanic and continental plates are where earthquakes happen the most. Plates are what the Earth’s crust is made of, and the Earth’s crust is the outer layer of planet Earth. Plates under the ocean are oceanic plates and all the other plates are continental plates. When plates are running into each other or sliding past each other that is earthquakes normally occur. Earthquakes do not always happen near plates. In fact, some of them happen some earthquakes happen close to faults. When sections of two or one plate(s) are moving in different directions on a crack in Earth it is a fault. All the bumping and sliding those plates do cause faults to happen. There are normal faults, reverse faults, and strike-slip faults. When a block of rock is sliding downward, and away from the other block of rock it is a normal fault. When a crack is formed by a plate pushing another plate it is a reverse fault. Finally a strike-slip fault is when two plates have cracks that slide past one and other.

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