What impact do earthquakes have on landforms?

3 comments

1. 

   Mark V on January 1, 2010 at 6:32 pm

   Earthquakes cannot have impacts on climate or weather. Earthquakes are quick, infinitesimally small blippy events over geologic time. There may be an area of regular earthquakes that over millions of years can possibly impact those things, but in general, they do not.

   Faulting may be caused either tensional or compressional forces exerted on rocks either laterally or vertically. The throw on a fault is the amount of offset across the fault. To describe the orientation of the fault, we geologists use the angles strike and dip. Strike is the orientation of the fault’s intersection with Earth’s surface with reference to north. Dip is the angle that the fault plane makes with Earth’s surface measured perpendicular to strike. That’s information for you, but don’t really concern yourself with it. It takes about half of a semester for college students to begin to understand, because they’re counter-intuitive, and require some good compass skills.

   There are four basic types of faults.

   The normal fault doesn’t mean "normal" as in "most common." What is normal about them is that their movement tends to follow the gravitational pull on the fault blocks involved. The fault plane on the normal fault is generally very steep. In a normal fault the two involved blocks are (by gravity) pulling away from one another causing, one of the fault blocks to slip upward (the hanging wall0 and the other downward (the footwall) with respect to the fault plane (it is hard to determine whether both or just one block has moved without good analysis of local strata). The exposed upward block forms a cliff-like feature known as a fault scarp. A scarp may range from a few to hundreds of meters in height and their length may continue for 300 or more kilometers (around 200 miles).

   The reverse fault is a normal fault, except the general movement of the fault blocks is toward
   each other, not away from each other. This forms a thrust fault type expression/landform on the surface with material overlaying other material. This can be seen because older layers may be sitting on top of younger layers, which by the Law of Superposition and Original Horizontality, can’t happen. The most famous reverse fault known in the United States may be the Hebgen Lake, or Montana-Yellowstone Earthquake of 1959.

   The most well known and well studied fault is the transform/transcurrent (strike-slip) fault known as the San Andreas fault of California. This fault marks the margin line between the Pacific and North American tectonic plates. Movement on a strike-strip fault is generally horizontal. On the surface, scarps form as hills crossing the fault zone are torn apart by movement over time. Actually anything crossing this fault zone is either slowly torn apart, or offset. Rivers crossing the fault line are called offset streams and are classic signatures of fault activity along the San Andreas and other transform fault zones. These faults can be very long – the San Andreas is nearly 600 miles long, from just north of the Sea of Cortez to north of San Francisco.

   Finally, we have thrust faults. In the 1994 Northridge, California event, a deep thrust fault located about 18 km under the city of Los Angeles produced an earthquake that registered a magnitude of 6.7. When thrust faults are exposed on the surface, overburden material lies over the main block. They are normally associated with areas of folded surfaces and or mountaineous regions. The dip angles of thrust faults are normally not as steep as a normal fault/reverse fault. Chief Mountain, in Montana is a good example of a thrust mountain studied by us geologists.

   Grabens (sometimes called rift valleys) are the subsidence of the land between two normal faults. These can be hundreds of miles in length. Some well known grabben valleys are the Red Sea Basin, the Alamosa Valley of Colorado, The Rift Valley of East Africa, the the mid-region of Arizona. Horsts, are the relatively rare opposites of grabbens. A horst (sometimes called block mountains) is an uprisings of a fault block with scarps on either side.

   I will add the caveat of a fifth type of fault. Often the slip on a fault is not perfectly dip slip or strike slip. We call such faults oblique slip, when they’re too far off from being described adequately by the other terms.

   Obviously, all of these things take place of long periods of time. You won’t have 100m of horizontal or vertical offset at one time (if you did, it would mean that there was something seriously wrong with the Earth). As soon as any surface is available, the forces of erosion will begin to work on them, and given enough time, can turn horsts, or reverse faults, and so on into regular rounded features that no one outside of geologists like myself would probably ever take any notice of. A study of outcrop bedrock geology will often show not just that faulting occurred, but you can actually walk up and lay your cheek right against the fault plane – that’

2. 

   Lina on January 1, 2010 at 6:32 pm

   We have just studied an earthquake unit in science class and I know a lot about earthquakes. Earthquakes cause: anticlines, synclines, fault block mountains, folded mountains, tsunamis, liquefaction in soil and can create mountains along a fault line. Earthquakes can tear the crust, squeeze it together and the crust can slip past each other on both sides of a fault line. This depends on the type of fault. Earthquakes can lead to fires because of broken gas pipes and this can cause a cloudy overcast sky.

3. 

   grace on October 5, 2010 at 10:21 am

   ah