Blind thrust earthquake
Encyclopedia
A blind thrust earthquake is an earthquake
along a thrust fault
that does not show signs on the Earth's surface, hence the designation "blind" http://earthquake.usgs.gov/learning/glossary.php?term=blind%20thrust%20fault. 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
; 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
.
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.
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
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
profiles http://faculty.washington.edu/tpratt/GRL02.pdf show the disturbed rock that hides a blind thrust fault
.
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, it 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.
, California
, USA has had many earthquakes, and the area is studied intensively. In addition to surface faults, a number of blind-thrust faults have been found under the basin and metropolitan area http://www.agu.org/pubs/crossref/1996/95JB03453.shtmlhttp://www.sciencemag.org/cgi/content/abstract/283/5407/1516?ck=nck. A NASA
study which combined satellite radar images and Global Positioning System
(GPS) observations http://quake.usgs.gov/research/deformation/modeling/socal/index_gerald.html measured the tectonic squeezing of the region. The study found that "the tectonic squeezing across Los Angeles will likely produce earthquakes on either the blind Elysian Park or Puente Hills
thrust fault systems". Large regions of metropolitan Los Angeles were rising and falling by up to 11 cm per year, but this was largely due to pumping and recharging of groundwater.
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
along a thrust fault
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
that does not show signs on the Earth's surface, hence the designation "blind" http://earthquake.usgs.gov/learning/glossary.php?term=blind%20thrust%20fault. 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
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
exploration seismology
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
; 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
Urban seismic risk
Urban seismic risk is a subset of the general term seismic risk which describes the problems specific to centers of population when they are subjected to earthquakes. Many risks can be minimised with good earthquake construction, and seismic analysis...
.
Blind thrust faults
Blind thrust faults generally exist near tectonic platePlate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
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.
 |
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
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
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
Reflection seismology 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 seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
profiles http://faculty.washington.edu/tpratt/GRL02.pdf show the disturbed rock that hides a blind thrust fault
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
.
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
Richter magnitude scale
The expression Richter magnitude scale refers to a number of ways to assign a single number to quantify the energy contained in an earthquake....
6 to 7 compared to the largest magnitude 9 earthquakes of recent times, it 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.
Los Angeles
Los AngelesLos Ángeles
Los Ángeles is the capital of the province of Biobío, in the commune of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123,445 inhabitants...
, California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, USA has had many earthquakes, and the area is studied intensively. In addition to surface faults, a number of blind-thrust faults have been found under the basin and metropolitan area http://www.agu.org/pubs/crossref/1996/95JB03453.shtmlhttp://www.sciencemag.org/cgi/content/abstract/283/5407/1516?ck=nck. A NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
study which combined satellite radar images and Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
(GPS) observations http://quake.usgs.gov/research/deformation/modeling/socal/index_gerald.html measured the tectonic squeezing of the region. The study found that "the tectonic squeezing across Los Angeles will likely produce earthquakes on either the blind Elysian Park or Puente Hills
Puente Hills
The Puente Hills is a chain of hills, one of the lower Transverse Ranges, in an unincorporated area in eastern Los Angeles County, California.-Geography:...
thrust fault systems". Large regions of metropolitan Los Angeles were rising and falling by up to 11 cm per year, but this was largely due to pumping and recharging of groundwater.