A blind thrust earthquake occurs along a

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

that does not show signs on the Earth's surface, hence the designation "blind". Such faults, being invisible at the surface, have not been mapped by standard surface geological mapping. Sometimes they are discovered as a by-product of oil exploration

seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

; in other cases their existence is not suspected. Although such earthquakes are not amongst the most energetic, they are sometimes the most destructive, as conditions combine to form an urban earthquake which greatly affects urban seismic risk. A blind thrust earthquake is quite close, in meaning, to a

buried rupture earthquake In seismology, a buried rupture earthquake, or blind earthquake, is an earthquake which does not produce a visible offset in the ground along the fault (as opposed to a surface rupture earthquake, which does). When the fault in question is a thr ...

, if a buried rupture earthquake is not specifically about the fault, but signs the earthquake leaves, on the Earth's surface.

Blind thrust faults

Blind thrust faults generally exist near

tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

margins, in the broad disturbance zone. They form when a section of the Earth's crust is under high compressive stresses, due to plate margin collision, or the general geometry of how the plates are sliding past each other. Blindthrust2

As shown in the diagram, a weak plate under compression generally forms thrusting sheets, or overlapping sliding sections. This can form a hill and valley landform, with the hills being the strong sections, and the valleys being the highly disturbed thrust faulted and folded sections. After a long period of

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

the visible landscape may be flattened, with material eroded from the hills filling up the valleys and hiding the underlying hill-and-valley geology. The valley rock is very weak and usually highly weathered, presenting deep, fertile soil; naturally, this is the area that becomes populated.

Reflection seismology Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seism ...

profiles show the disturbed rock that hides a blind

. If the region is under active compression these faults are constantly rupturing, but any given valley might only experience a large earthquake every few hundred years. Although usually of magnitude 6 to 7 compared to the largest magnitude 9 earthquakes of recent times, such a temblor is especially destructive because the seismic waves are highly directed, and the soft basin soil of the valley can amplify the ground motions tenfold or more. It is said that blind thrust earthquakes contribute more to urban seismic risk than the 'big ones' of magnitude 8 or more.

Examples of occurrence

Some known faults

Los Angeles Los Angeles ( ; es, Los Ángeles, link=no , ), often referred to by its initials L.A., is the largest city in the state of California and the second most populous city in the United States after New York City, as well as one of the world ...

, California, USA, has many earthquakes and is well-studied. In addition to surface faults, a number of blind-thrust faults have been found under the basin and metropolitan area. A

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...

study which combined satellite radar images and

Global Positioning System The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite ...

(GPS) observations found that "tectonic squeezing across Los Angeles" "will likely produce earthquakes on either the blind

Elysian Park Elysian Park is one of the largest parks in Los Angeles at 600 acres (240 ha). Most of Elysian Park falls in the neighborhood of the same name, but a small portion of the park falls in Echo Park. The park was created by city ordinance on April 5, ...

or Puente Hills thrust fault systems". * Bajo Segura Fault Zone, Spain * Fukaya Fault System, Japan (near Tokyo) * Uemachi Fault System, Osaka Basin, Japan *

Main Frontal Thrust The Main Frontal Thrust (MFT), also known as the Himalayan Frontal Thrust (HFT) is a geological fault in the Himalayas that defines the boundary between the Himalayan foothills and Indo-Gangetic Plain. The fault is well expressed on the surfac ...

, Himalaya

Specific events

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References

External links