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. The epicentre was about 30 km to the southwest of the city of Kobe Figure 6. 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: The severity of an earthquake The severity of an earthquake can be expressed in terms of both intensity and magnitude. Geological Survey. 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. Year Published: Our changing continent Where were the land areas and oceans of the North American Continent one million years ago, compared to our present geography?
Filter Total Items: 3. Date published: March 1, Attribution: Science Application for Risk Reduction. Date published: April 8, Date published: February 10, Filter Total Items: List Grid. January 23, October 3, Building in Mexico City after Sept. December 31, Building damanged by the February earthquake in Christchurch, New Zealand.
Attribution: Natural Hazards. February 10, Listen to hear the answer. April 28, February 24, November 30, Block diagram of a subduction zone. S Waves cause the crustal material to move back and forth perpendicular to the direction they are travelling. By looking at the amount of time between the P and S wave on a seismogram recorded on a seismograph, scientists can tell how far away the earthquake was from that location. If they draw a circle on a map around the station where the radius of the circle is the determined distance to the earthquake, they know the earthquake lies somewhere on the circle.
But where? Scientists then use a method called triangulation to determine exactly where the earthquake was see image below. It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter!
No, and it is unlikely they will ever be able to predict them. Scientists have tried many different ways of predicting earthquakes, but none have been successful. On any particular fault, scientists know there will be another earthquake sometime in the future, but they have no way of telling when it will happen.
These are two questions that do not yet have definite answers. If weather does affect earthquake occurrence, or if some animals or people can tell when an earthquake is coming, we do not yet understand how it works. Triangulation can be used to locate an earthquake. The seismometers are shown as green dots. The calculated distance from each seismometer to the earthquake is shown as a circle. The location where all the circles intersect is the location of the earthquake epicenter. Skip to main content.
Search Search. 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.
0コメント