On normal faults , the hanging wall has moved down relative to the foot wall. How these faults are able to form and slip at such a low angle is a mechanical mystery. A horst is an upthrown block between oppositely dipping normal faults. A graben is a downthrown block between oppositely dipping normal faults. Large faults are often associated with smaller, secondary faults.
When these faults dip in the same direction as the main fault, they are synthetic faults , and when oppositely-dipping, they are called antithetic faults. The amount of extension is the length gained in a region. Extension strain , also known as elongation strain , is the change in length of a region divided by the original length:. Reconstruction of an originally planar, horizontal marker is a straightforward way to measure extension and extension strain.
Simple reconstructions like these, however, are not always possible geologically. Balancing a graben, for example, is challenging without producing gaps or overlaps. In practice, faults bounding grabens may change angles or become damage zones instead of single simple planes, and rocks may undergo significant breakup to fill in spaces.
Listric normal faults have decreasing dip with depth. Slip causes tilting in the hanging wall, resulting in a rollover fold. Listric fault surfaces often have irregular, strongly curved fault corrugations , also called mega-mullions , which indicate the slip down-dip direction. In the cooler parts of Earth, rocks are colder and brittle and respond to large stresses by fracturing.
Earthquakes are the agents of brittle rock failure. A fault is a crack across which the rocks have been offset. 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.
In addition to variation in size and orientation, different faults can accommodate different styles of rock deformation, such as compression and extension. Not all faults intersect Earth's surface, and most earthquakes do no rupture the surface.
When a fault does intersect the surface, objects may be offset or the ground may cracked, or raised, or lowered. 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.
When an earthquake occurs only a part of a fault is involved in the rupture. That area is usually outlined by the distribution of aftershocks in the sequence. We call the "point" or region where an earthquake rupture initiates the hypocenter or focus. The point on Earth's surface directly above the hypocenter is called the epicenter. When we plot earthquake locations on a map, we usually center the symbol representing an event at the epicenter.
Generally, the area of the fault that ruptures increases with magnitude. 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, Magnitude Fault Dimensions Length x Depth, in km 4. Although the number of observations of deep fault structure is small, the available exposed faults provide some information on the deep structure of a fault.
A fault "zone" consists of several smaller regions defined by the style and amount of deformation within them. Structure of an exposed section of a vertical strike-slip fault zone after Chester et al. The center of the fault is the most deformed and is where most of the offset or slip between the surrounding rock occurs. 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.
From all the slipping and grinding, the gouge is composed of very fine-grained material that resembles clay. Surrounding the central zone is a region several meters across that contains abundant fractures.
Outside that region is another that contains distinguishable fractures, but much less dense than the preceding region. Last is the competent "host" rock that marks the end of the fault zone. Active faults are structure along which we expect displacement to occur. By definition, since a shallow earthquake is a process that produces displacement across a fault, all shallow earthquakes occur on active faults. Inactive faults are structures that we can identify, but which do no have earthquakes.
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. If a fault has been inactive for millions of years, it's certainly safe to call it inactive. However, some faults only have large earthquakes once in thousands of years, and we need to evaluate carefully their hazard potential.
Reactivated faults form when movement along formerly inactive faults can help to alleviate strain within the crust or upper mantle.
Deformation in the New Madrid seismic zone in the central United States is a good example of fault reactivation. Structure formed about Ma ago are responding to a new forces and relieving strain in the mid-continent. Faulting is a complex process and the variety of faults that exists is large. We will consider a simplified but general fault classification based on the geometry of faulting, which we describe by specifying three angular measurements: dip, strike, and slip.
Dip The fault illustrated in the previous section was oriented vertically. In Earth, faults take on a range of orientations from vertical to horizontal. Dip is the angle that describes the steepness of the fault surface.
See Rocks deform above for an example of having students create boudins. Analogs, however, are difficult to scale appropriately both in time and space to the gigantic scale on which geologic structures form. Students may still have difficulty understanding the tremendous scale of forces needed to bend or break rock and the long time scales involved to generate structures.
Make sure that you make it clear to your students that these pitfalls exist. More detailed ideas for analogs are available at Teaching Structural Geology analog materials web page. Once students have mastered the connections among stress, strain and structure, I develop a 3 x 2 table of different structures that form under differing stress and strain conditions.
I then proceed to fill out the table with students' help. Let's look at what features are found under different stress conditions and with different styles of strain. We'll do this by making a table. What are the three types of stress? Compression, tension, and shearing. Now, what are the 2 types of permanent deformation? Ductile and brittle. Let's make a table that is three columns by two rows and fill it in with appropriate structures!
When we are finished, we should have 6 kinds of deformation features. There are many factors that contribute to the style of the deformation in a rock, including pressure, temperature, rock composition, presence or absence of fluids, type of stress, rate of stress, and others. However, the type of stress, the rate of stress and the temperature may be the most critical factors for most introductory students. What controls how it will deform? Your Account. Show Stress, strain, structure - What's the difference?
Stress is a force acting on a rock per unit area. It has the same units as pressure, but also has a direction i. There are three types of stress: compression, tension, and shear. Stress can cause strain, if it is sufficient to overcome the strength of the object that is under stress. Strain is a change in shape or size resulting from applied forces deformation.
Rocks only strain when placed under stress. Any rock can be strained.
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