What Makes Inactive Fault Lines Become Active Again
 SLU EAS-A193 Course Notes | Faults and Faulting |
Contents
- Earthquakes and Faulting
- Fault Construction
- Fault Classifications
- Agile, Inactive, Reactivated
- Faulting Geometry
- Faults and Forces
- Focal Mechanisms
- Stress and Strain
- Elastic Rebound
Introduction
Rocks are very slowly, but continuously moving and changing shape. Under high temperature and pressure atmospheric condition common deep within Earth, rocks tin can bend and flow. In the libation parts of Earth, rocks are colder and breakable and answer to large stresses by fracturing. Earthquakes are the agents of brittle stone failure.
A fault is a crack across which the rocks accept 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 adapt different styles of rock deformation, such as compression and extension.
Non all faults intersect Globe's surface, and about earthquakes do no rupture the surface. When a error does intersect the surface, objects may be offset or the footing may cracked, or raised, or lowered. We call a rupture of the surface past a fault a fault scarp and identifying scarps is an important chore for assessing the seismic hazards in any region.
 Fence beginning most 11 feet during the 1906 San Francisco California Earthquake (Photo from the U.Southward. Geological Survey) |  Fault scarp formed during the Decemeber 16, 1954 Dixie-Valley-Fairview Peaks, Nevada earthquakes (Photo from the Steinbrugge Collection, Earthquake Engineering Research Center, U.C. Berkley). |
Earthquakes and Faults
When an convulsion occurs simply a part of a fault is involved in the rupture. That expanse is commonly outlined past the distribution of aftershocks in the sequence.
We call the "point" (or region) where an convulsion rupture initiates the hypocenter or focus. The bespeak on Earth's surface directly above the hypocenter is chosen the epicenter. When nosotros plot convulsion locations on a map, we usually center the symbol representing an consequence at the epicenter.
Generally, the surface area of the mistake that ruptures increases with magnitude. Some estimates of rupture area are presented in the table beneath (The original data are from Wells and Coppersmith, Bulletin of the Seismological Guild of America, 1994).
| Appointment | Location | Length (km) | Depth (km) | (Mw) |
|---|---|---|---|---|
| 04/xviii/06 | San Francisco, CA | | | |
| 07/21/52 | Kern County, CA | | | |
| 12/16/54 | Fairview Peak, NV | | | |
| 12/xvi/54 | Dixie Peak, NV | | | |
| 06/28/66 | Parkfield, CA | | | |
| 02/09/71 | San Fernando Valley, CA | | | |
| 10/28/83 | Borah Peak, ID | | | |
| 10/18/89 | Loma Prieta, CA | | | |
| 06/28/92 | Landers, CA | | | |
Although the verbal expanse associated with a given size earthquake varies from place to place and issue to event, nosotros can brand predictions for "typical" earthquakes based on the available observations.
| Magnitude | Mistake Dimensions (Length x Depth, in km) |
|---|---|
| | ane.2 x ane.2 |
| | 3.3 ten 3.3 |
| | 10 10 10 |
| | 16 ten 16, 25 ten 10 |
| | 40 x 20, 50 x 15 |
| | 140 ten 15, 100 10 20, 72 x thirty, 50 x forty, 45 x 45 |
| | 300 ten twenty, 200 ten 30, 150 x 40, 125 x l |
These numbers should give you a rough idea of the size of structure that we are talking virtually when we discuss earthquakes.
Error Structure
Although the number of observations of deep fault structure is small, the available exposed faults provide some information on the deep construction of a error. A mistake "zone" consists of several smaller regions defined by the way and amount of deformation within them.
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The heart of the fault is the most deformed and is where nigh of the offset or slip betwixt the surrounding rock occurs. The region can be quite small, most as wide as a pencil is long, and information technology is identified by the finely ground rocks called cataclasite (nosotros telephone call the basis up cloth found closer to the surface, gouge). From all the slipping and grinding, the gouge is composed of very fine-grained textile that resembles dirt. Surrounding the key zone is a region several meters across that contains abundant fractures. Outside that region is some other that contains distinguishable fractures, only much less dumbo than the preceding region. Last is the competent "host" stone that marks the end of the error zone.
Error Classifications
Active, Inactive, and Reactivated Faults
Agile faults are structure along which nosotros await displacement to occur. By definition, since a shallow convulsion is a process that produces displacement across a fault, all shallow earthquakes occur on active faults.
Inactive faults are structures that we tin can identify, but which do no take earthquakes. Equally you tin can imagine, because of the complication of earthquake activity, judging a fault to exist inactive can be tricky, but often we tin can measure out the last time substantial offset occurred across a mistake. If a mistake has been inactive for millions of years, information technology's certainly safe to call it inactive. Nonetheless, some faults merely accept large earthquakes once in thousands of years, and we need to evaluate carefully their hazard potential.
Reactivated faults form when motility along formerly inactive faults can aid to alleviate strain within the chaff or upper mantle. Deformation in the New Madrid seismic zone in the central U.s.a. is a skillful case of fault reactivation. Structure formed nigh 500 Ma ago are responding to a new forces and relieving strain in the mid-continent.
Faulting Geometry
Faulting is a complex procedure and the multifariousness of faults that exists is large. Nosotros will consider a simplified merely general mistake nomenclature based on the geometry of faulting, which we describe by specifying 3 athwart measurements: dip, strike, and slip.
DipThe error illustrated in the previous department 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 mistake surface. This angle is measured from Earth's surface, or a plane parallel to Earth'southward surface. The dip of a horizontal fault is zero (unremarkably specified in degrees: 0°), and the dip of a vertical fault is 90°. We utilise some quondam mining terms to characterization the rock "blocks" above and below a fault. If you were tunneling through a error, the textile beneath the mistake would be past your feet, the other textile would be hanging to a higher place yous head. The material resting on the fault is called the hanging wall, the material below the mistake is called the foot wall. |  |
 | StrikeThe strike is an angle used to specify the orientation of the fault and measured clockwise from north. For example, a strike of 0° or 180° indicates a fault that is oriented in a north-south direction, 90° or 270° indicates east-due west oriented structure. To remove the ambivalence, we always specify the strike such that when you "expect" in the strike direction, the fault dips to you lot correct. Of course if the error is perfectly vertical you have to describe the situation as a special instance. If a fault curves, the strike varies along the error, simply this is seldom causes a communication problem if yous are careful to specify the location (such as latitude and longitude) of the measurement. |
SlipDip and strike describe the orientation of the fault, we also take to describe the direction of motility across the fault. That is, which way did one side of the fault movement with respect to the other. The parameter that describes this motion is chosen the slip. The slip has ii components, a "magnitude" which tells united states of america how far the rocks moved, and a management (it's a vector). We normally specify the magnitude and direction separately. The magnitude of sideslip is simply how far the two sides of the fault moved relative to 1 some other; it's a distance usually a few centimeters for small-scale earthquakes and meters for big events. The management of slip is measured on the mistake surface, and like the strike and dip, it is specified as an bending. Specifically the slip direction is the direction that the hanging wall moved relative to the footwall. If the hanging wall moves to the right, the sideslip direction is 0°; if it moves upwards, the slip angle is 90°, if it moves to the left, the slip angle is 180°, and if information technology moves downwardly, the slip angle is 270° or -ninety°. |  |
| Hanging wall movement determines the geometric classification of faulting. We distinguish betwixt "dip-slip" and "strike-slip" hanging-wall movements. Dip-slip movement occurs when the hanging wall moved predominantly up or downwardly relative to the footwall. If the motion was down, the fault is called a normal error, if the movement was upwards, the fault is called a opposite error. Downward movement is "normal" considering we commonly would look the hanging wall to slide downwards along the foot wall considering of the pull of gravity. Moving the hanging wall up an inclined error requires work to overcome friction on the fault and the downward pull of gravity. When the hanging wall moves horizontally, information technology's a strike-skid earthquake. If the hanging wall moves to the left, the earthquake is called correct-lateral, if it moves to the right, it's called a left-lateral fault. The fashion to keep these terms directly is to imagine that y'all are standing on ane side of the fault and an earthquake occurs. If objects on the other side of the fault movement to your left, it's a left-lateral fault, if they move to your right, information technology's a right-lateral fault. When the hanging wall motility is neither dominantly vertical nor horizontal, the movement is called oblique-slip. Although oblique faulting isn't unusual, it is less common than the normal, reverse, and strike-slip movement. |
| Mistake Styles |
Faults and Forces
The fashion of faulting is an indicator of rock deformation and reflects the type of forces pushing or pulling on the region.
About Earth's surface, the orientation of these forces are unremarkably oriented such that one is vertical and the other ii are horizontal. The precise direction of the horizontal forces varies from place to place equally does the size of each force.
The style of faulting that is a reflection of the relative size of the different forces - in particular is the relative size of the vertical to the horizontal forces. At that place are 3 cases to consider, the vertical force can be the smallest, the largest, or the intermediate (neither smallest or largest). If the vertical force is the largest, nosotros go normal faulting, if it is the smallest, we get reverse faulting. When the vertical force is the intermediate force, we get strike-slip faulting.
Normal faulting is indicative of a region that is stretching, and on the continents, normal faulting ordinarily occurs in regions with relatively high summit such as plateaus.
Reverse faulting reflects compressive forces squeezing a region and they are common in uplifting mountain ranges and along the declension of many regions bordering the Pacific Bounding main. The largest earthquakes are generally depression-bending (shallow dipping) contrary faults associated with "subduction" plate boundaries.
Strike-slip faulting indicates neither extension nor compression, just identifies regions where rocks are sliding past each other. The San Andreas error system is a famous case of strike-skid deformation - part of coastal California is sliding to the northwest relative to the residuum of Northward America - Los Angeles is slowly moving towards San Francisco.
As you might await, the distribution of faulting styles is not random, simply varies systematically beyond Earth and was ane of the most important observations in constructing the plate tectonic model which explains so much of what nosotros observe happening in the shallow office of Earth.
Fault Type: Deformation Fashion: Force Orientation:
Earthquake Focal Mechanisms (Beach Assurance)
Nosotros use a specific set of symbols to place faulting geometry on maps. The symbols are called earthquake focal mechanisms or sometimes "seismic embankment balls". A focal mechanism is a graphical summary the strike, dip, and slip directions.
An convulsion focal mechanism is a project of the intersection of the fault surface and an imaginary lower hemisphere (we'll employ the lower hemisphere, only we could also employ the upper hemisphere), surrounding the center of the rupture.
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| The intersection between the error "plane" and the sphere is a curve. The focal mechanism shows the view of the hemisphere from straight above. We can evidence the orientation of a plane (i.e. the strike and dip) using just one bend, to include data on the skid, we use two planes and shade opposite quadrants of the hemisphere. |
The price we pay for the ability to represent slip is that you cannot place which of the two planes on the focal mechanism is the fault without additional information (such as the location and tendency of aftershocks).
Some example focal mechanisms are shown beneath.
You should memorize the top three, which represent to dip-slip contrary and normal faulting on a fault dipping 45°, and strike-sideslip faulting on a vertical error. The lower two mechanisms correspond to a low-angle reverse earthquake (the dip is low) and the last example is an oblique event with components of both strike-slip and dip-slip move. The strike of any plane can exist read from a focal mechanism by identifying the intersection of the mistake (shown as the boundary between shaded and unshaded regions) with the circumvolve surrounding the machinery (and using the dip-to-the-right dominion).
Stress and Strain
Stress is a forcefulness per unit area or a forcefulness that acts on a surface.
When I described the types of forces associated with the different styles of faulting (in the section "Faults and Faulting"), I was describing stresses (the force per unit area on the fault).
Friction is a stress which resists movement and acts in all natural systems.
For convulsion studies, friction on faults and the orientation and relative magnitudes of the "regional" stresses that determine the style of faulting are of master involvement and importance.
Strain is a measure of material deformation such as the amount of compression when you squeeze or the amount of elongation when yous stretch something.
In rubberband deformation the amount of elongation is linearly proportional to the applied stress, and an elastic cloth returns to its original shape after the stress is relieved. Additionally, a strained, elastic material stores the energy used to deform it, and that energy is recoverable.
Elastic Rebound
Equally you know, some regions repeatedly experience earthquakes and this suggests that peradventure earthquakes are function of a cycle. The effects of repeated earthquakes were first noted tardily in the nineteenth century by American geologist Chiliad. K. Gilbert. Gilbert observed a fresh mistake scarp following the 1872 Owens Valley, California convulsion and correlated the scarp and uplift from a single convulsion with the uplift of the Sierra Nevada mountains. Decades subsequently, following the 1906 San Francisco, California earthquake, H. F. Reid presented a like hypothesis to explain improve-documented movements along coastal California measured both before and after a large earthquake. Reid'due south model of the earthquake wheel has become known as the "Rubberband Rebound Model".
| The key to Reid's success was the availability of "before" and "after" observations for the earthquake which allowed him to see strain build in the chaff before the upshot, so see that strain release during the convulsion. In the diagram, we take two blocks of rock separated past a fault. Equally the 2 blocks motion in reverse directions, friction acting on the fault resists motion and keeps the two sides from sliding. The stone strains as elastic free energy is added, eventually, the strain loads the fault too much and overcomes the frictional "strength" of the fault. The rocks on either side of the mistake jerk past each other in an earthquake. The earthquake releases the stored elastic strain free energy equally rut forth the fault and as seismic vibrations. |  |
For an ideal rubberband-rebound fault, the stress on the fault periodically cycles betwixt a minimum and maximum value and if the two blocks continue to move at a constant rate, the recurrence fourth dimension (the fourth dimension between earthquakes) is also uniform.
Unfortunately, actual faults are more circuitous, and the recurrence fourth dimension is not periodic (which is 1 reason why earthquake prediction is then difficult). We have few observations of complete earthquake cycles because earthquakes take so long to recur.
The Figure higher up shows the observations from the Nankaido region of Nihon (the gray region, the older values are estimated from earthquake histories), one of the few regions where observations on strain throughout several earthquake cycles exist. Y'all can see that neither the time nor the slip is uniform from earthquake-to-earthquake.
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