When does earthquakes occur

Click on the map circles for more information. Earthquakes are among the most deadly natural hazards. They strike without warning and many earthquake zones coincide with areas of high population density. Seismometers are used to record the seismic waves produced by earthquakes.

Relative arrival times of these waves is used to determine earthquake location. The extent of damage an earthquake causes depends not only on the magnitude of the earthquake, but also on local geology and on building techniques.

The causes of earthquakes and the different types of tectonic boundaries and faults on which they usually occur. Where do earthquakes occur? Discovering Geology — Earthquakes. You may also be interested in:. Earthquakes Earthquakes are among the most deadly natural hazards. Is there any place in the world that doesn't have earthquakes? Florida and North Dakota are the states with the fewest earthquakes.

Antarctica has the least earthquakes of any continent, but small earthquakes can occur anywhere in the World. What is the "Ring of Fire"? Most earthquakes and volcanic eruptions do not strike randomly but occur in specific areas, such as along plate boundaries. One such area is the circum-Pacific Ring of Fire , where the Pacific Plate meets many surrounding tectonic plates.

The Ring of Fire is the most seismically and volcanically active zone in the world. Learn more: U. Filter Total Items: 8. Hayes, Gavin P. View Citation. Hayes, G. M, , Seismicity of the Earth — U. Lamb, Rynn M. Year Published: This dynamic planet: World map of volcanoes, earthquakes, impact craters and plate tectonics Our Earth is a dynamic planet, as clearly illustrated on the main map by its topography, over volcanoes, 44, earthquakes, and impact craters.

Simkin, Tom; Tilling, Robert I. Year Published: Earthquake hazards: a national threat Earthquakes are one of the most costly natural hazards faced by the Nation, posing a significant risk to 75 million Americans in 39 States. Year Published: Earthquakes-Rattling the Earth's Plumbing System Hydrogeologic responses to earthquakes have been known for decades, and have occurred both close to, and thousands of miles from earthquake epicenters.

Sneed, Michelle; Galloway, Devin L. Frankel, A. Year Published: This dynamic earth: the story of plate tectonics In the early s, the emergence of the theory of plate tectonics started a revolution in the earth sciences.

Kious, W. Jacquelyne; Tilling, Robert I. Filter Total Items: 4. Date published: March 14, Date published: January 8, At least deaths worldwide resulted from earthquake activity in Date published: February 10, Date published: October 6, Filter Total Items: 9. List Grid. When the force of movement finally overcomes the friction, sections of the crust suddenly break or become displaced, releasing the pent-up pressure in the form of seismic waves.

This is a naturally occurring earthquake, sometimes referred to as a tectonic earthquake. While tectonic earthquakes can occur at any location around the world, the majority of large earthquakes—about 80 percent—occur at the circum-Pacific seismic belt found along the rim of the Pacific Ocean.

Two other regions regularly exhibiting earthquakes include the Alpide belt, extending along the southern margin of Eurasia through the Himalayan Mountains, Sumatra, and Java; and the Mid-Atlantic Ridge running along the floor of the Atlantic Ocean.

It involves the injection of large volumes of water, sand, and chemicals under high pressure into a bedrock formation to create new fractures in the rock or increase the size, extent, and connectivity of existing fractures, leading to more permeability.

On rare occasions, fracking can lead directly to earthquakes. More frequently, earthquakes are induced by the disposal of waste water associated with the oil production into underground wells. What is the real place of origin of an earthquake? As the initial break propagates, it releases energy along the fault, so in a sense the earthquake originates from the entire fault — which may be 1 km long. However, it is useful to refer to one part of the fault as the place of origin of an earthquake, and this is taken as the initial break.

It is called the focus plural foci of the earthquake. This is usually below not at the Earth's surface. The point on the Earth's surface directly above the focus is called the epicentre Figure 4. A large earthquake is frequently followed by a series of smaller earthquakes on the same fault, called aftershocks.

These can continue for months after the main earthquake. They are caused by readjustment in the positions of the rocks following the main earthquake, releasing smaller, localised buildups of energy on the fault.

Sometimes the main earthquake is preceded by one or more smaller foreshocks, although these cannot be identified as foreshocks until after the main earthquake has occurred. A earthquake in California had a sequence of aftershocks that revealed the size and orientation of the fault. This earthquake occurred on 17 October with the epicentre about km southeast of San Francisco, and is usually referred to as 'the Loma Prieta earthquake' after the mountain just to the east of the epicentre.

The earthquake involved movement on a fault about 40 km in length. It was large enough to cause destruction not only in the nearby town of Santa Cruz but also in San Francisco and its surroundings, where some buildings, bridges and raised roads collapsed.

Figure 5a is an epicentral map a map showing the location of the epicentres for the Loma Prieta earthquake and its aftershocks. The earthquake and aftershocks occurred along a well-known fault in California — the San Andreas Fault shown in Figure 3. This shows the positions and depths of the earthquake and aftershocks, i.

The San Andreas Fault is the largest and possibly most active of many faults in California. As well as the Loma Prieta earthquake, movement along the fault has caused other large earthquakes, including a major San Francisco earthquake in This resulted in the almost total destruction of the city by fire, following the rupture of gas mains; rupturing of the water mains made it impossible to put out the fires.

The land to the west of the fault moved as much as 6 m north relative to the other side during this earthquake. Now we know a bit more about what causes earthquakes, we can return to the first earthquake we considered Kobe, to look at it in terms of movement along a fault.