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Asperity Activities
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Introduction
What is an asperity?
An asperity is an area on a fault that is stuck or locked. In the Earth, tectonic earthquakes are caused by slip along a fault plane, where two rock bodies are in rigid contact. The friction along the fault plane is not uniform in strength, so overall movement involves slip on one or more asperities, or "stuck patches" where the friction is highest. Most of the energy that is released by earthquakes comes from the patches that become "unstuck."
Multiple Asperities on a Strike-Slip Fault Plane
Oblique view of a right-lateral strike-slip fault with multiple asperities. When one asperity slips, there is an added load on the adjoining asperities. In a large earthquake there is a cascading effect as each zone that slips loads the next zone, which then slips, and so forth, sometime for hundreds of miles, in a process that can continue for 5 or more minutes. Narration by John C. Lahr taken from the "Spaghetti Vice" video lecture below.
Direct Link to Asperities Along Fault Zone (Med 656 Kb)
Direct Link to Asperity Along Fault zone (Lg 1.4 Mb)
Direct Link to Asperities with No Text (Med 788 Kb)
Direct Link to Asperities with No Text (Lg 1.2 Mb)
Quick Demo
Asperity Quakes Compared to Chopstick Breaks
Most earthquakes happen on faults. The process that causes them is similar to what happens if you bend a chopstick until it breaks.
Comparing the multiple asperities along a fault zone with the multiple failures of a bamboo chopstick:
- Regional compression and extension are acting on the "fault zone" (plate tectonics is acting on fault zones; hands are acting on the chopstick)
- There is a build-up of stress (hold the tips of the chopstick with the tips of your fingers and feel the stress build up as elastic energy is stored in the chopstick ). If you release the energy before the chopstick breaks, it will return to its pre-stressed shape.
- Knowledge of the rate of strain buildup allows one to "forecast" that it (the chopstick or the fault) will break. But it is difficult to predict the time and place where it will breaks next.
- The weakest zones will break first.
- One may hear some precursors (weakest asperities breaking).
- There is elastic deformation and brittle failure.
- There is elastic rebound as the stored energy in the deformed material is released, the material rebounds to its previous shape.
- Sound waves generated by the breaking chopstick can be compared to the compressive seismic waves (P waves) of an earthquake.
Simple Models of Fault Movement with Single Asperity, High Friction, and Little or No Friction
Single Asperity Along Fault Zone
View looking into a fault zone with a single asperity. Regional right lateral strain puts stress on the fault zone. A single asperity resists movement of the green line which deforms before finally rupturing.
Direct Link to Asperity Along Fault Zone (Med 216 kb)
Direct Link to Asperity Along Fault Zone (Lg 420 Kb)
Low-friction Fault Zones
View looking into right- and left-lateral fault with low friction along fault contact. There is no deformation of the rock adjacent to contact.
Direct Link to Left Lateral (Lg 260 Kb)
Direct Link to Right Lateral (Lg 260 Kb)
John Lahr Demonstrates Asperities Along a Strike-Slip Fault
Direct Link to Demonstration (Med 1.2Mb)
Direct Link to Demonstration (Lg 3.7Mb)
More About Asperities
Total fault offset accumulates through time in an uneven fashion, primarily by movement on first one, and then another section of the fault. The portions of the fault that produce great earthquakes can remain "locked" and quiet for one hundred or more years, while the strain is building up; then, in great lurches, the strain is released, producing a great earthquake.
Asperities, which may be caused by protrusions on the fault, act like welded contacts between the sides of the fault. Younger faults have rougher surfaces with more asperities. As a fault repeatedly ruptures, the asperities can be worn down, creating fault gouge and smoothing the fault. The gouge material often decomposes to a fine clay and forms a thin layer which "greases" the fault for easier sliding. Fluids can also facilitate slip by reducing the normal stress on the fault.
The San Andreas Fault is actually a fault system that is more than 800 miles long and the seismically active portion extends to depths of at least 10 miles within the Earth and ranges from a few hundred feet to a mile or more wide. It doesn't slip all at once, but rather, earthquakes jump around on it as local asperities break. On some stretches of some faults, however, such as around Hollister on the Calaveras fault, date movement occurs primarily by constant repeated creep events rather than by sudden earthquake offsets. In historical times, these creeping sections have not generated earthquakes of the magnitude seen on "locked" sections.
"The dynamics of fault rupture are complex, but general fault behavior can be explained with a simple model in which slip promotes fault weakening. Fault slip occurs in three stages: 1) initiation of sliding on a small portion of the fault, 2) growth of the slip surface, and 3) termination of slip and fault healing. Earthquakes occur on preexisting faults operating in a "stick- slip" mode. Earthquakes are "slip" episodes; they are followed by periods of no slip ("stick"), during which elastic strain increases away from the fault. Although some growth of the fault may occur with each earthquake, we can generally assume that for large earthquakes (M>6) the faulting process primarily involves repeated breaking of the same fault segment rather than creation of a new fault surface."
From "Can Earthquakes be Predicted?" by Michelle Kathleen Hall-Wallace
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Animations By Jenda Johnson
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