An earthquake rupture occurs and relieves some of the stresses but generally not all. There are three basic types of fault: normal, reverse and strike-slip. Certain types of fault are characteristic of the different plate boundaries, although often more than one type of fault occurs there. This can help us understand the relative movement of the plates and the type of deformation.
In a normal fault, the block above the fault moves down relative to the block below the fault. In a reverse fault, the block above the fault moves up relative to the block below the fault. In a strike-slip fault, the movement of blocks along a fault is horizontal. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.
Faults are classified using the angle of the fault with respect to the surface known as the dip and the direction of slip along the fault.
Faults that move along the direction of the dip plane are called dip-slip faults while strike-slip faults are classified as either right-lateral or left-lateral. Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults. Boundaries between tectonic plates are made up from a system of faults. Discovering Geology introduces a range of geoscience topics to school-age students and learners of all ages.
The Earth beneath our feet is constantly shifting and moving, and violently with catastrophic and immediate results. Find out more about earth hazards. 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. What causes earthquakes? Discovering Geology — Earthquakes.
The structure of the Earth Seismic waves from large earthquakes pass throughout the Earth. The crust This brittle, outermost layer varies in thickness from about 25 to 70 km under continents and from about 5 to 10 km under the oceans. The mantle Below the crust lies the dense mantle, extending to a depth of km.
Plate tectonic map of the world showing direction of movement. Divergent boundary Plates can move apart at a boundary. This idea of drifting continents intrigued some scientists. Many others, particularly geologists, were unimpressed, hostile, even horrified. The principle was thought to demand that the continents be fixed in place. In , English geologist Arthur Holmes came up with a potential explanation for that movement.
He proposed that the continents might be floating like rafts atop a layer of viscous, partially molten rocks deep inside Earth. Heat from the decay of radioactive materials, he suggested, sets this layer to a slow boil , creating large circulating currents within the molten rock that in turn slowly shift the continents about.
Holmes admitted he had no data to back up the idea, and the geology community remained largely unconvinced of continental drift. Geologists turned to other matters, such as developing a magnitude scale for earthquake strength and devising a method to precisely date organic materials using the radioactive form of carbon, carbon Rekindled interest in continental drift came in the s from evidence from an unexpected source — the bottom of the oceans.
World War II had brought the rapid development of submarines and sonar, and scientists soon put the new technologies to work studying the seafloor. Using sonar, which pings the seafloor with sound waves and listens for a return pulse, researchers mapped out the extent of a continuous and branching underwater mountain chain with a long crack running right down its center.
This finding suggests that each of the bands formed at different times. Meanwhile, growing support for the detection and banning of underground nuclear testing also created an opportunity for seismologists: the chance to create a global, standardized network of seismograph stations. Thanks to the resulting flood of high-quality seismic data, scientists discovered and mapped rumbles along the mid-ocean rift system, now called mid-ocean ridges, and beneath the trenches.
The quakes near very deep ocean trenches were particularly curious: They originated much deeper underground than scientists had thought possible. And the ridges were very hot compared with the surrounding seafloor, scientists learned by using thin steel probes inserted into cores drilled from shipboard into the seafloor.
In the early s, two researchers working independently, geologist Harry Hess and geophysicist Robert S. The mid-ocean ridges, each asserted, might be where circulation pushes hot rock toward the surface. Into the gap, lava burbles up — and new seafloor is born. The momentum culminated in a two-day gathering of perhaps just earth scientists in , held at the Goddard Institute for Space Studies in New York. Sykes, then a newly minted Ph. This pattern showed that the seafloor on either side of the ridges was pulling apart, a pivotal piece of evidence for plate tectonics.
It soon became clear that these findings were building toward one unified narrative: Mid-ocean ridges were the birthplaces of new seafloor, and deep ocean trenches were graves where old lithosphere was reabsorbed into the interior.
This cycle of birth and death had opened and closed the oceans over and over again, bringing the continents together and then splitting them apart. Wilson laid out the various lines of evidence for this new view of the world to a much larger audience in Washington, D. But how were these blocks moving, all in concert, around the planet? To plot out the choreography of this complex dance, two separate groups seized upon a theorem devised by mathematician Leonhard Euler way back in the 18th century.
The theorem showed that a rigid body moves around a sphere as though it is rotating around an axis. McKenzie and geophysicist Robert Parker used this theorem to calculate the dance of the lithospheric blocks — the plates. Unbeknownst to them, geophysicist W. Jason Morgan independently came up with a similar solution. With this last piece, the unifying theory of plate tectonics was born.
Plate tectonics has also shaped new research across the sciences, offering crucial information about how the climate changes and about the evolution of life on Earth. New technologies have also joined the toolbox, including satellite positioning systems such as GPS that can help track ground movements over time and ever-more-powerful computers that can interpret and analyze large amounts of data.
In the s, for example, researchers had demonstrated that underwater mountain chains called mid-ocean ridges were places where two tectonic plates pulled away from each other, and where new seafloor was forming. But in the early s, scientists for the first time saw the consequences close-up , with the first manned submersible explorations of a mid-ocean ridge in the Atlantic Ocean. To the amazement of the team members, the vents were teeming with giant tube worms and clams and other forms of life.
Once thought to originate from magma pooling just under the surface, seismic images of the mantle suggested that the volcanoes are instead fueled by giant, buoyant plumes of hot, molten material originating hundreds to thousands of kilometers deep inside the planet, some nearly to its core. But for every new discovery or question answered, dozens more arise about the dynamic nature of the planet. Here are a few of the big ones:.
To try to answer that, scientists are looking to understand the physics of how faults move. These fault zones can include both microscopic cracks and vast fissures. Some faults may suddenly slip, causing an earthquake; others may inch along more slowly , possibly heralding a much larger quake in the near future. The presence — or removal of — groundwater adds yet another wrinkle. The violent shaking from earthquakes can:. If your home was built before , you may have structural risks that could affect your safety.
Follow the Seven Steps to Earthquake Safety. Decrease your risk of damage and injury from a major earthquake by identifying possible home hazards:. Do you know about the primary geologic hazards where you live? Other factors include:. If your home was built before , it may also be vulnerable to serious structural damage. With safety planning, reinforcing the structure of your home, securing your personal property, and buying earthquake insurance , you stand a better chance of riding out the next California earthquake.
Learn how to prepare your home. Phone: Translate Share. Also called lithospheric plate. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society. National Geographic Society.
For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource.
If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media. Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.
0コメント