GEOL 344 Test #2
Four Functions that affect Fault Planes
1) Asperities 2) Bends 3) Water Pressure 4) Dip of Fault Planes
Blind Faults
A blind thrust earthquake occurs along a thrust fault that does not show signs on the Earth's surface, hence the designation "blind". Such faults, being invisible at the surface, have not been mapped by standard surface geological mapping. Some faults do not break through to the surface anywhere along their length. Naturally, these faults are not easy to locate or study and often go unnoticed for many years. Sometimes faults are "buried" by deposition of material atop the surface trace during the period between surface ruptures, but other times, faults simply do not reach the surface and/or diffuse into a fold or folds beneath the surface. When a fault does not reach the surface, and consequently has no surface trace, it is known as a blind fault.
Fault Scarp
A fault scarp is a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other. It is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults. M7 or higher
Wasatch Fault Zone
A large group or system of normal faults that stretches north to south across Utah. Separates the higher Colorado plateau to the East from the lower deserts of Utah, which lie to the West. Vertical Stress
Thrust Fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger.
Asperity
An asperity is an area on a fault that is stuck. The earthquake rupture usually begins at an asperity. A rough spot on a fault surface. 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." Asperities, which may be caused by roughness, or 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.
3 B's
Batholith: A very large igneous intrusion extending deep in the earth's crust. Belt: Orogens or orogenic belts develop when a continental plate is crumpled and is pushed upwards to form mountain ranges, and involve a great range of geological processes collectively called orogenesis. Basalts: An aphanitic igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and where at least 65% of the feldspar is in the form of plagioclase. Basalt features a glassy matrix interspersed with minerals.
Hertz
Cycles/Seconds
Dip-Slip
Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse. A fault's sense of slip is defined as the relative motion of the rock on each side of the fault with respect to the other side. Fault slip can be classified in a basic way by its relation to the horizontal. If slip occurs primarily in a vertical sense, it is known as dip slip, since it roughly parallels the dip of the fault. If slip occurs primarily in a horizontal sense, it is known as strike slip, since it roughly parallels the strike of the fault. When slip occurs exactly parallel to the dip, or to the strike, it is known as pure dip, or strike, slip. Fault slip which occurs at a sizable angle with respect to both the dip and the strike of the fault is known as oblique slip, and can be thought of as a combination of both dip slip and strike slip.
Velocity
Distance/Time Wavelength/Period V= Wavelength * frequency V = Wavelength/ (1/period)
Steps
Fault steps are features similar, in many ways, to fault bends. Just as fault bends are either left bends or right bends, fault steps are either left steps or right steps. They generally create local compression or extension much like fault bends. However (as shown in the image to the right), in an area with a fault step, the main trace of the fault does not change trend. It merely "steps" over, sideways, and continues along a similar trend. For this reason, fault steps are typically small-scale features, and their effects are limited to the immediate area near the step-over. No fault step even comes close to causing the sort of effects the "Big Bend" of the San Andreas does.
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: Dry friction resists relative lateral motion of two solid surfaces in contact.
Frequency
How many cycles per second 1/period
Period
How many seconds per cycle 1/frequency
Left Lateral
If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the left block moves toward you and the right block moves away
Right Lateral
If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the right block moves toward you and the left block moves away
Parkfield
Parkfield earthquake is a name given to various large earthquakes that occurred in the vicinity of the town of Parkfield, California, United States. The San Andreas fault runs through this town, and six successive magnitude 6 earthquakes occurred on the fault at unusually regular intervals, between 12 and 32 years apart (with an average of every 22 years), between 1857 and 1966.[1] The most recent significant earthquake to occur here happened on September 28, 2004. M6. Earthquakes may occur regularly here because the location is about midway on a fault segment between a locked segment to the south (last major earthquake 1857) and a creeping segment to the north where two tectonic plates are continuously moving without major earthquakes.
Amplitude
Resting position to peak
Wavelength
Same as Hertz Cycles/Second
Fault
Shear fracture
Creep
Slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. Faults that are creeping do not tend to have large earthquakes.
Hollister
Stable sliding or creep occurs here. Does not have many large earthquakes and does not exhibit the normal stick slip sliding that most of the San Andres exhibits.
Geodetic Modeling
Statisticians analyzing spatial data often need to detect and model associations based upon distances on the Earth's surface. Accurate computation of distances are sought for exploratory and interpretation purposes, as well as for developing numerically stable estimation algorithms. When the data come from locations on the spherical Earth, application of Euclidean or planar metrics for computing distances is not straightforward. Yet, planar metrics are desirable because of their easier interpretability, easy availability in software packages, and well-established theoretical properties. While distance computations are indispensable in spatial modeling, their importance and impact upon statistical estimation and prediction have gone largely unaddressed. This article explores the different options in using planar metrics and investigates their impact upon spatial modeling.
Stick - Slip - MOVEMENT
Stick slip refers to the fast movement that occurs between two sides of a fault when the two sides of the fault become unstuck. The rock becomes distorted, or bent, but holds its position until the earthquake occurs. Unstable frictional sliding "Elastic Rebound"
Strike - Slip - FAULT
Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral.
Borah Peak
The Borah Peak earthquake is the largest ever recorded in Idaho - both in terms of magnitude and in amount of property damage. It caused two deaths in Challis, about 200 kilometers northeast of Boise, and an estimated $12.5 million in damage in the Challis-Mackay area. A maximum MM intensity IX was assigned to this earthquake on the basis of surface faulting. Vibrational damage to structure was assigned intensities in the VI to VII range.
Cascadia
The Cascadia Subduction Zone (CSZ) "megathrust" fault is a 1,000 Km long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino California. It separates the Juan de Fuca and North America plates. New Juan de Fuca plate is created offshore along the Juan de Fuca ridge. The Juan de Fuca plate moves toward, and eventually is shoved beneath, the continent (North American plate). Most powerful earthquakes A new comparison of earthquakes that have taken place along the West Coast during the past 10,000 years suggests that seismic activity in the Cascadia Subduction Zone off Oregon and Washington may actually have triggered earthquake events in the San Andreas Fault in the San Francisco Bay area.
Fort Tejon
The Fort Tejon earthquake occurred at about 8:20 AM (Pacific time) on January 9, 1857. It ruptured the San Andreas Fault for a length of about 350 kilometers (220 mi), between Parkfield and San Bernardino. Displacement along the fault was as much as 9 meters (30 feet) in the Carrizo Plain but less along the Palmdale section of the fault, closest to Los Angeles. The amount of fault slip gives this earthquake a moment magnitude of 7.9, comparable to that of the 1906 San Francisco earthquake. Based on the (uncertain) distribution of foreshocks for this earthquake, it is assumed that the beginning of the fault rupture (the epicenter) was in the area between Parkfield and Cholame, about 60 miles northwest. Nevertheless, it is usually called the "Fort Tejon" earthquake because this was the location of the greatest damage, most of the area being unpopulated at the time
Lewis Thrust
The Lewis Overthrust is a geologic thrust fault structure of the Rocky Mountains within Glacier National Park in Montana, USA and Waterton Lakes National Park in Alberta, Canada, as well as into Lewis and Clark National Forest. It provides scientific insight into geologic processes happening in other parts of the world, like the Andes and the Himalaya Mountains. Scientific study of this region is practical because the original rock characteristics were well-preserved and recently sculpted by glaciers. The Lewis and Livingston mountain ranges are the visible reminders of this fault. The Lewis and Livingston mountain ranges are the visible reminders of this fault. The Rocky Mountain Front is the easternmost portion of the Lewis Range which rises abruptly 4,000 to 5,000 feet (1,200 to 1,500 m) above the Great Plains. The Lewis Overthrust started during the formation of the Rocky Mountains 170 million years ago as a result of colliding tectonic plates. Stresses on the continental plates pushed a huge rock wedge eastward more than 50 miles (80 km). The rock wedge, which was several miles thick and several hundred miles long, consisted of Proterozoic rock formations. The underlying layer consisted of softer, Cretaceous age rocks that were over 1,400 million years younger than the overthrust layer.
San Andreas
The San Andreas Fault is a continental transform fault that extends roughly 1300 km (810 miles) through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into three segments, each with different characteristics and a different degree of earthquake risk, the most significant being the southern segment, which passes within about 35 miles of Los Angeles.
Fault Bends
Were the bend oriented the other way (in a north-south direction rather than an east-west orientation) with the existing right-lateral plate motion, as shown at upper right, this bend would become a divergent bend, and a basin would form around the fault, as in the example above. If we took the hypothetical new bend and reversed the plate motion so that things moved left-laterally, this bend would become a convergent bend (bottom right). The situation would then resemble what is currently going on in southern California -- compressional forces causing the uplift of mountains. Any smaller bend works in the same way, but of course, on a scale to match that of the bend and the fault involved. Convergent bends will always cause compression, as divergent bends will always result in extension. Fault bends can be referred to as "left bends" or "right bends" depending on their configuration. The Big Bend of the San Andreas is a "left bend" -- if you were to walk along the fault, starting on a "straight" section, you would have to veer left when you came to the bend, regardless of the direction of your approach. The hypothetical bend shown in the figures above is a "right bend". By comparing the slip of a fault to a bend along its length, you can quickly tell if it is a convergent or divergent bend. If the sense of strike-slip and the bend have the opposite "handedness" -- that is, left-lateral strike-slip with a right bend, or right-lateral strike-slip with a left bend -- the bend will be convergent. If the handedness is the same, the bend will be divergent.
Normal Fault
normal fault. A geologic fault in which the hanging wall has moved downward relative to the footwall. Normal faults occur where two blocks of rock are pulled apart, as by tension. Compare reverse fault.