Hi All. The problem with thrust faults is that they almost never show themselves in the form of a rupture at the surface. There are 4 such quakes that come to mind with the possibility of a 5th to be added to the list.

“The first of these shocks took place in 1980 in El Asnam, Algeria. It measured 7.3 on the moment-magnitude scale (a successor to the Richter scale, designated M), killed 3,500 people in three North African cities and dammed a major river.”

“The second and third earthquakes, the 1983 M = 6.5 Coalinga shock and the adjacent 1985 M = 6.1 Kettleman Hills event, shook areas of California. Because these sites are remote, the earthquakes caused only modest damage and one death, but the areas did include several major toxic-waste storage sites, and in the small town of Coalinga, 75 percent of the unreinforced buildings were demolished.”

“The next shock struck within California's populous Los Angeles basin on October 1,1987. Although this M = 6.0 Whittier Narrows quake was only one tenth the size of the Coalinga event, it caused 10 times the damage-$350 million - and took eight lives. The fifth, candidate event was the tragic 1988 Armenian earthquake, which claimed at least 25,000 lives.”

“The most obvious feature of the three California earthquakes was that none of the faults that slipped cut the surface of the earth. In the Algerian event the fault did cut the earth's surface, but the amount of fault slip at ground level was far less than the amount of slip at the depth of the earthquake's focus, about 10 kilometers down. These observations suggest that in fold earthquakes the slip diminishes from earthquake level toward ground level, and so there is little, if any, rupture of the surface.”

There are a number of ways to determine if there is a fault of any kind in a given area. One is by the occurrence of many small quakes. These can be used to determine the approximate location of a fault. The second is by a large quake that ruptures the surface, but this only tells you there is a fault where the rupture can be seen. It doesn’t tell you if the fault ends at the ends of the rupture. Then there is trenching, but the problem with this is is knowing where to trench. You could dig a hundred trenches and never find a fault because you didn’t dig in the right place.

There is a new quake you can add to your vocabulary. It’s not really a new quake, just a new name for a quake. This is the “fold quake” and may be far more dangerous then any of the others we know about. It occurs along a blind thrust fault and is thought to be the cause of the rolling foothills. It was thought that these foothills were the results of gradually uplifting, but now it is thought they grow one quake at a time in a violent way. Look around you. See any foothills? They don’t have to be big ones. They could be the results of a hidden fault with a quake looking for a place to happen. Don in creepy town

http://quake.wr.usgs.gov/research/deformation/modeling/reviews/times_magazine/time_review.html

• Fold Quakes