![]() From all the slipping and grinding, the gouge is composed of very fine-grained material that resembles clay. The region can be quite small, about as wide as a pencil is long, and it is identified by the finely ground rocks called cataclasite (we call the ground up material found closer to the surface, gouge). The center of the fault is the most deformed and is where most of the offset or slip between the surrounding rock occurs. Structure of an exposed section of a vertical strike-slip fault zone (after Chester et al., Journal of Geophysical Research, 1993). A fault "zone" consists of several smaller regions defined by the style and amount of deformation within them. ![]() These numbers should give you a rough idea of the size of structure that we are talking about when we discuss earthquakes.Īlthough the number of observations of deep fault structure is small, the available exposed faults provide some information on the deep structure of a fault. Some estimates of rupture area are presented in the table below (The original data are from Wells and Coppersmith, Bulletin of the Seismological Society of America, 1994).ġ40 x 15, 100 x 20, 72 x 30, 50 x 40, 45 x 45 Generally, the area of the fault that ruptures increases with magnitude. When we plot earthquake locations on a map, we usually center the symbol representing an event at the epicenter. The point on Earth's surface directly above the hypocenter is called the epicenter. We call the "point" (or region) where an earthquake rupture initiates the hypocenter or focus. That area is usually outlined by the distribution of aftershocks in the sequence. When an earthquake occurs only a part of a fault is involved in the rupture. Geological Survey)įault scarp formed during the DecemeDixie-Valley-Fairview Peaks, Nevada earthquakes (Photo from the Steinbrugge Collection, Earthquake Engineering Research Center, U.C. We call a rupture of the surface by a fault a fault scarp and identifying scarps is an important task for assessing the seismic hazards in any region.įence offset about 11 feet during the 1906 San Francisco California Earthquake (Photo from the U.S. When a fault does intersect the surface, objects may be offset or the ground may cracked, or raised, or lowered. Not all faults intersect Earth's surface, and most earthquakes do no rupture the surface. In addition to variation in size and orientation, different faults can accommodate different styles of rock deformation, such as compression and extension. They range in size from micrometers to thousands of kilometers in length and tens of kilometers in depth, but they are generally much thinner than they are long or deep. ![]() Earthquakes are the agents of brittle rock failure.Ī fault is a crack across which the rocks have been offset. In the cooler parts of Earth, rocks are colder and brittle and respond to large stresses by fracturing. Under high temperature and pressure conditions common deep within Earth, rocks can bend and flow. This video explains faults, where they form and why they form in a easy to understand fashion.Rocks are very slowly, but continuously moving and changing shape. Reverse Fault:Reverse faults occur when the crust of the earth is compressed, causing the two rock walls to move closer together to accommodate for the pressure.Normal Fault: Normal faults occur when the crust of the earth is extended, causing the two rock walls to move farther apart from one another to accommodate for extension.With normal faults forming at ~60° from horizontal and reverse faults forming at ~30° from horizontal. ![]() There are two types of dip-slip faults, normal and reverse. Right-lateral Strike Slip Fault: when you stand on one side of the fault and look towards the other, the other side has appeared to move to right.ĭip-slip Faults:This type of fault is associated with either extension of earth’s crust or compression of it.Left-lateral Strike Slip Faults: when you stand on one side of the fault and look towards the other, the other side has appeared to move to left.There are two types of these faults, left-lateral and right-lateral. Strike Slip Faults: this is where the fault plane within the rock is near vertical at around 90° from the horizontal surface. There are two types of faults, strike-slip and dip-slip. Movement along these faults is the cause of most earth quakes. Faults are fractures in a rock with movement along them due to tectonic pressures. This allows Geologists to study the stresses involved in the formation of these faults and thus infer the processes that formed the fault. Faults are an important part of geology, as they form along planes of weakness in rocks.
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