Blind thrust earthquake

A blind thrust earthquake occurs along a thrust fault that does not show signs on the Earth's surface, hence the designation "blind".^[1] 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.

Diagram of blind-thrust faulting

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^[2] 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, 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.^[3]

Examples of occurrence

Some known faults

• Los Angeles, 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.^[4][5] A NASA study which combined satellite radar images and Global Positioning System (GPS) observations found that "tectonic squeezing across Los Angeles" "will likely produce earthquakes on either the blind Elysian Park or Puente Hills thrust fault systems".^[6]
• Bajo Segura Fault Zone, Spain
• Fukaya Fault System, Japan (near Tokyo)
• Uemachi Fault System, Osaka Basin, Japan

Specific events

• 1987 Whittier Narrows earthquake
• 1994 Northridge earthquake
• 2010 Haiti earthquake

References

1. "Earthquake Glossary - blind thrust fault". USGS. 2012-07-24. Retrieved 2016-02-01.
2. Pratt, Thomas L.; Shaw, John H.; Dolan, James F.; Christofferson, Shari A.; Williams, Robert A.; Odum, Jack K.; Plesch, Andreas (2002). "Shallow seismic imaging of folds above the Puente Hills blind-thrust fault, Los Angeles, California" (PDF). Geophysical Research Letters 29 (9). doi:10.1029/2001GL014313. ISSN 0094-8276. Archived from the original (PDF) on March 5, 2005. Retrieved 2016-02-01.
3. Washington Post, "7.5 quake on California fault could be disastrous", 30 March 2014: accessed 30 March 2014.
4. Shaw, John H.; Suppe, John (1996). "Earthquake hazards of active blind-thrust faults under the central Los Angeles basin, California". Journal of Geophysical Research 101 (B4): 8623. doi:10.1029/95JB03453. ISSN 0148-0227.
5. Shaw, John H. (1999-03-05). "An Elusive Blind-Thrust Fault Beneath Metropolitan Los Angeles". Science. Retrieved 2016-02-01.
6. Perlman, D. "L.A. moves with water table / Changing water table moves L.A. / City rises and falls with annual pumping from ground storage". SFGate. Retrieved 2 February 2016.