Blind thrust earthquake

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.

Blind thrust faults

Blind thrust faults generally exist near tectonic plate 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 moreFact|date=June 2008.

Los Angeles

Los Angeles, 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.shtml] [http://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 faults 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.