Post by 1dave on Dec 27, 2021 10:49:22 GMT -5
www.usgs.gov/faqs/what-fault-and-what-are-different-types
What is a fault and what are the different types?
A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. 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.
Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse (thrust), depending on their motion. Faults which move horizontally are known as strike-slip faults and 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.
The following definitions are adapted from The Earth by Press and Siever.
normal fault - a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.
www.usgs.gov/media/videos/normal-fault
Normal Fault Animation
www.usgs.gov/media/videos/thrust-fault
thrust fault - a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under another as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.
Thrust Fault Animation
www.usgs.gov/media/videos/blind-thrust-fault
Blind Thrust Fault Animation
strike-slip fault - a fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.
www.usgs.gov/media/videos/strike-slip-fault
Strike-slip Fault Animation
A left-lateral strike-slip fault is one on which the displacement of the far block is to the left when viewed from either side.
A right-lateral strike-slip fault is one on which the displacement of the far block is to the right when viewed from either side.
www.usgs.gov/faqs/why-are-there-so-many-earthquakes-and-faults-western-united-states
Why are there so many earthquakes and faults in the Western United States?
This region of the United States has been tectonically active since the supercontinent Pangea broke up roughly 200 million years ago, and in large part because it is close to the western boundary of the North American plate. Since the formation of the San Andreas Fault system 25-30 million years ago, the juxtaposition of the Pacific and North American plates has formed many faults in California that accommodate lateral motion between the plates. North and east of California, the Basin and Range province between the Wasatch Mountains in Utah and the Sierra Nevada Mountains in eastern California is actively spreading and stretching westward.
In New Mexico and west Texas, similar spreading has opened a north-south rift that starts in central Colorado and extends into northern Mexico. The geologic conditions and plate tectonic setting in much of the Western U.S. has resulted in the region being underlain by relatively thin crust and having high heat flow, both of which can favor relatively high deformation rates and active faulting.
In contrast, in the Central and Eastern U.S. (CEUS) the crust is thicker, colder, older, and more stable. Furthermore, the CEUS is thousands of miles from active plate boundaries, so the rates of deformation are low in this region. Nevertheless, the CEUS has had some rather large earthquakes in historical times, including a series of major earthquakes near New Madrid, Missouri in 1811-1812, a large earthquake near Charleston, S.C. in 1886, and the Cape Ann earthquake northeast of Boston in 1755.
