#RIGHT LATERAL STRIKE SLIP FAULT CRACK#

Structure formed about 500 Ma ago are responding to a new forces and relieving strain in the mid-continent. Deformation in the New Madrid seismic zone in the central United States is a good example of fault reactivation. Reactivated faults form when movement along formerly inactive faults can help to alleviate strain within the crust or upper mantle. However, some faults only have large earthquakes once in thousands of years, and we need to evaluate carefully their hazard potential. If a fault has been inactive for millions of years, it's certainly safe to call it inactive. As you can imagine, because of the complexity of earthquake activity, judging a fault to be inactive can be tricky, but often we can measure the last time substantial offset occurred across a fault. Inactive faults are structures that we can identify, but which do no have earthquakes. By definition, since a shallow earthquake is a process that produces displacement across a fault, all shallow earthquakes occur on active faults. Last is the competent 'host' rock that marks the end of the fault zone.įault Classifications Active, Inactive, and Reactivated FaultsĪctive faults are structure along which we expect displacement to occur. Outside that region is another that contains distinguishable fractures, but much less dense than the preceding region.

Surrounding the central zone is a region several meters across that contains abundant fractures. 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.

#RIGHT LATERAL STRIKE SLIP FAULT CRACK#

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. Rocks are very slowly, but continuously moving and changing shape.

Right lateral strike slip fault