Blind thrust earthquakeA particularly deadly type of earthquake takes place on what is known as blind thrusts. It is not that these are the largest type of earthquakes known, but that conditions combine to make this a classic urban earthquake, affecting urban seismic risk.
A blind thrust earthquake is an earthquake along a thrust fault, that has not been mapped by standard surface geological mapping, hence the designation 'blind'. These types of faults generally exist near plate margins (plate tectonics), 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. The valley rock is very weak and usually highly weathered, presenting deep, fertile soil. Naturally, this is the area that becomes populated.
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 only a Magnitude 6 or 7 earthquake, it is especially deadly because the seismic waves are highly directed, and the soft basin soil of the valley can amplify the ground motions by over a factor of 10.
It is said that these types of 'urban earthquakes' contribute more to urban seismic risk than the 'big ones', of Magnitude 8 or more. All the more reason to better prepare for earthquakes wherever people live under this threat.