Earth and Space Science #6

Primary waves

- (P waves) Primary waves are "push-pull" waves - they momentarily push (squeeze) and pull (stretch) rock in the direction the wave is traveling. This wave motion is similar to that of the sound waves created by human vocal cords.

Strike-slip faults

- Faults in which the dominant displacement is horizontal and parallel to the trend or strike of the faults surface is called a strike-slip fault.

Lower Mantle

- From 660 kilometers deep to the top of the core, at a depth of 2900 kilometers, is the lower mantle. Because of an increase in pressure (caused by the weight of the rock above), the mantle gradually strengthens with depth. Despite their strength, however, the rocks within the lower mantle are very hot and capable of very gradual flow.

Secondary waves

- (S waves) Secondary waves "shake" the particles at right angles to their direction of travel. Unlike P waves, which temporarily change the volume of intervening material by alternately squeezing and stretching it, S waves change the shape of the material that transmits them. Because fluid (gases and liquids) do not resist stresses that cause changes in shape - meaning fluids will not return to their original shape once the stress is removed- they will not retain S waves.

Brittle deformation

- (brittle failure) the factors that influence the strength of a rock and how it will deform include temperature, confining pressure, rock type, and time. Rocks near the surface, where temperatures and confining pressures are low, tending to behave like a brittle solid and fracture one their strength is exceeded. This type of deformation is called brittle failure or brittle deformation.

Deformation

- (de = out, forma = forms) Every body of rock, no matter how strong, has a point at which it will fracture or flow. Deformation is a general term that refers to all changes in the shape, position, or orientation of a rock mass. Significant crustal deformation occurs along plate margins. Plate motions and the interactions along plate boundaries generate the tectonic forces that cause rock to deform. When rock are subjected to forces (stresses) grater than their own strength, they begin to deform, usually by folding, flowing, or fracturing.

Locating an earthquake

- (determining its epicenter) The process of locating an earthquake relies on the fact the P waves travel faster than S waves. The greater length of the "race" the greater in difference in their "arrival" time. So, the larger the differences in arrival time, the farther away the epicenter.

Focus

- (foci = a point) The origin of an earthquake occurs at depths between 5 and 700 kilometers, at the focus.

Magnitude

- (measures that describe the size of an earthquake) The development of seismographs made it possible to measure ground motion using instruments. This quantitative measurement, called magnitude, relies on data gleaned from seismic records to estimate the amount of energy released at an earthquake's source.

Intensity

- (measures that describe the size of an earthquake) a measure of the degree of earthquake shaking at a given locale based on observed damage.

Circum-Pacific belt

- About 95% of the energy released by earthquakes originates in the few relatively narrow zones of greatest seismic activity, called the circum-Pacific belt, encompasses the coastal regions of Chile, Central America, Indonesia, Japan, and Alaska, including the Aleutian Islands. Most earthquakes in the circum-Pacific belt occur along convergent plate boundaries where one plate slides at a low angle beneath another.The zone of contact between the subducting and overlying plates forms a huge thrust fault called a megathrust, along which Earth's largest earthquakes are generated. Because subduction zone earthquakes usually happen beneath the ocean they may also generate destructive waves called tsunami.

Synclines

- Almost always found in association with anticlines are downfolds, or troughs, called synclines. Depending on their orientation, these basic folds are described as symmetrical when the limbs are mirror images of each other as asymmetrical when they are not. An asymmetrical fold is said to be overturned if one, or both, limbs are tilted beyond the vertical. An overturned fold can also "lie on its side" so a plane extending through he axis of the fold would be horizontal. These recumbent folds are common in highly deformed mountainous regions such as the Alps.

Folds

- Along convergent plate boundaries, flat-lying sedimentary and volcanic rocks are often bent into a series of wavelike undulations called folds.

Amplification of Seismic Waves

- Although the region near the epicenter will experience about the same intensity of ground shaking, destruction may vary considerably within this area. Such differences are usually attributable to the nature of the ground on which the structures are built. Soft sediment, for example, generally amplify the vibrations more than solid bedrock.

Active continental margin

- At some point, the continental margin becomes active. A subduction zone forms and the deformation process begins. A good place to examine an active continental margin is the wast coast of South America, where the Nazca plate is being suducted beneath the South American plate along the Peru-Chile trench. The subduction zone probably formed prior to the breakup of the supercontient of Pangaea.

Domes and basins

- Broad upwarps in basement rock may deform the overlying cover of sedimentary strata and generate large folds. When this upwarping produces a circular or elongated structure, the feature is called a dome. Downwarped structures having a similar shape are termed basins.

Ductile defomration

- By contrast, at depth, where temperatures and confining pressures are high, rock exhibit ductile behavior. Ductile deformation is a type of solid-state flow that produces a change in the size and shape of an object without fracturing. Ordinary objects that display ductile behavior include modeling clay, bee's wax, caramel candy, and most metals.

Transform fault

- Some strike-slip faults cut through the lithosphere and accommodate motion between two large tectonic plates. Recall this special kind of strike-slip fault is called a transform fault.

Aftershocks

- Strong earthquakes are followed by numerous smaller tremors, called aftershocks, that gradually diminish in frequency and intensity over a period of several months. Within 24 hours of the massive 1964 Alaskan earthquake, 28 aftershocks were recorded, 10 of which had magnitudes that exceed 6. More than 10,000 aftershocks with magnitudes of 3.5 or above occurred in the following 69 days and thousands of minor tremors were recorded over a span of 18 months.

Causes of Earthquakes

- Tectonic stresses acting over tens to hundreds of years slowly deform the crustal rocks on both sides of a fault. When deformed by differential stress, rocks bend and store elastic energy, much like a wooden stick does if bent. Eventually, the frictional resistance holding the rocks in place is overcome. Slippage allows the deformed (strained) rocks to "snap back" to its original, stress-free, shape. The spring back was termed ELASTIC REBOUND (elastic rebound).

Earth's core

- The composition of the core is thought to be an iron-nickel alloy with minor amounts of oxygen, silicon, and sulfur elements that readily form compounds with iron. At the extreme pressure found in the core, this iron-rich material has an average density of nearly 11 grams per cubic centimeter and approaches 14 times the density of water at Earth's center. The core is divided into two regions that exhibit very different mechanical strengths. The outer core is a liquid layer 2270 kilometers thick. It is the movement of metallic iron within this zone that generates Earth's magnetic field. The inner core is a shere with a radius of 1216 kilometers. Despite its higher temperature, the iron in the inner core is solid due to the immense pressures that exist in the center of the planet.

Earth's crust

- The crust, Earth's relatively thin, rocky outer skin, is of two types - continental crust and oceanic crust. The continental crust averages 35 to 40 kilometers (22 - 25 miles) thick but mat exceed 70 kilometers (40 miles) in some mountain regions. Unlike oceanic crust, which has relatively homogeneous chemical composition, the upper crust has an average composition of granitic rock. Continental rocks have an average density of about 2.7 grams per cubic centimeter, and some are 4 billion years old.

Passive continental margin

- The first stage in the development of an idealized Andean-type mountain belt occurs prior to the formation of the subduction zone. During this period, the continental margin is a passive continental margin; that is, it is not a plate boundary but a part of the same plate as the adjoining oceanic crust.

Landslides and Ground Subsidence

- The greatest damage to structures is often caused by landslides and ground subsidence triggered by earthquake vibrations.

Epicenter

- The point at the surface directly above the focus is called the epicenter.

Seismograms

- The records obtained by seismographs, called seismograms, provide useful information about the nature of seismic waves.

Seismology

- The study of earthquake waves.

Orogenesis

- The term for the processes that collectively produce a mountain belt is orogenesis (oros = mountain, genesis = to come into being). Most major mountain belts display striking visual evidence of great horizontal forces that have shortened and thickened the crust. These compressional mountains contain large guantities of preexisting sedimentary and crystalline rocks that have been faulted and contorted into a series of folds.

Normal faults

- Dip-slip faults are classified as normal faults when the hanging wall block moves down relative to the footwall block. Because of the downward motion of the hanging wall block, normal faults accommodate lengthening, or extension, of the crust. Normal faults are found in a variety of sizes, some are small, having displacements of only a meter or so,. Other extend for tens of kilometers where they may sinuously trace the boundary of a mountain front. Most large, normal faults have relatively steep dips that tend to flatten out with depth.

Seismic waves

- During large earthquakes, a massive amount of energy is released as heat and seismic waves - a form of elastic energy that causes vibrations in the material that transmits them.

Accretionary wedge

- During the development of this continental volcanic arc, sediment derived from the land and scraped from the subducting plate is plastered against the landward side of the trench like piles of dirt in front of a bulldozer. This chaotic acculmulation of sedimentary and metamorphic rocks with occasional scraps of ocean crust is called an accretionay wedges. Prolonged subduction can build an accretionary wedge that is large enough to stand above sea level.

Dip-slip faults

- Faults in which movement is primarily parallel to the dip (or inclination) of the fault surface are called dip-slip faults.

Elastic deformation

- Geologist discoveredthat when stress is gradually applied, rocks first respond by deforming elastically. Changes that result from elastic deformation are recoverable; that is, like rubber band, the rock will return to nearly its original size and shape when the force is removed. During elastic deformation the chemical bonds of the minerals within the rock are stretched, but do not break. Once the elastic limit of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).

Liquefaction

- In areas where unconsolidated materials are saturated with water, earthquake vibrations can turn stable soil into a mobile fluid, a phenomenon known as liquefaction. As a result, the ground is not capable of supporting buildings and underground storage tanks and sewer lines may literally float toward the surface.

Foreshocks

- In contrast to aftershocks, small earthquakes called foreshocks often precede a major earthquake by days, or in some cases, by several years. Monitoring of foreshocks to predict forthcoming earthquakes has been attempted with limited success.

Moment Magnitude

- In recent years, seismologist have come to favor a newer measure called moment magnitude (MvW), which determines the strain energy released from the entire fault surface. Because moment magnitude establishes the total energy released, it is better for measuring or describing very large earthquakes. In light of this, seismologists have recalculated the magnitudes of older, strong earthquakes using the moment magnitude scale. Moment magnitude can be calculated from geologic fieldwork by measuring the average amount of slip on the fault, the area of the fault surface that slipped, and the strength of the fault rock. The area of the fault plane can be roughly calculated by multiplying the surface-rupture length by the depth of the aftershocks. Moment magnitude can also be calculated using data from seismograms.

Fault-block mountains

- In the western United States, large normal faults are associated with structures called fault-block mountains. Excellent examples of fault-block mountains are found in the Basin and Range Province, a region that encompasses Nevada and portions of the surrounding states. Here the crust has been elongated and broken to create more than 200 relatively small mountain ranges. Averaging about 80 kilometers in length, the ranges rise 900 - 1500 meters above the adjacent down-faulted basins.

Hanging wall blocks

- It has become common practice to call the rock surface that is immediately above the fault the hanging wall block.

Tsunami

- Large undersea earthquakes occasionally set in motion massive waves that scientists call seismic sea waves. The Japanese game it the name Tsunami. Because of Japan's location along the circum-Pacific belt and and its expansive coastline, it is especially vulnerable to tsunami destruction. Most tsunamis are caused by the displacement of a slab of seafloor, or a large submarine landslide triggered by an earthquake. Once generated, a tsunami resembles ripples. Tsunamis advance across the ocean at amazing speeds. With a height of 1 meter, tsunamis usually can pass undetected in open ocean. However, upon entering shallow coastal waters, these destructive waves "feel bottom" and slow, causing the water to pile up.

Mantle

- More than 82% of Earth's volume is contained in the mantle, a solid, rocky shell that extends to a depth of about 2900 kilometers. The boundary between the crust and mantle represents a marked change in chemical composition. The dominant rock type in the uppermost mantle is peridotite, which is richer i the metals magnesium and iron than the minerals found in wither the continental or oceanic crust. The Upper mantle extends from the crust-mantle boundary down to a depth of about 660 kilometers. The upper mantle can be divided into two different parts. The top portion of the upper mantle is part of the stiff lithosphere, and beneath that is the weaker asthenosphere. The lithosphere consists of the entire crust and uppermost mantle and forms Earth's relatively cool, rigid outer shell. Averaging about 100 kilometers in thickness, the lithosphere is more than 250 kilometers thick below the oldest portions of the continents. Beneath this stiff layer to a depth of about 350 kilometers lies a soft, comparatively weak later known as asthenosphere. The top portion of the asthenosphere has a temperature/pressure regime that results in a small amount of melting. Within this very weak zone, the lithosphere is mechanically detached from the layer below. The result is that the listhosphere is able to move independently of the asthenosphere.

Terrane

- Refers to any crustal fragment that has a geologic history distinct from that of the adjoining terranes.

Surface waves

- Seismograms reveal that two main groups of seismic waves are generated by the slippage of a rock mass: 1) Surface wave, which travel along the outer part of Earth.

Body waves

- Seismograms reveal that two main groups of seismic waves are generated by the slippage of a rock mass: 2) Body waves, which travel through Earth's interior. Body waves are divided into two groups: 1) Primary waves and 2) Secondary waves. Body waves are identified by their mode of travel.

Horsts/Grabens

- The topography of the Basin and Range province evolved in association with a system of roughly north-south trending normal faults. Movement along these faults produced alternating uplifted fault blocks called horsts and down-dropped blocks called gradens. Horsts generate elevated topography, whereas grabens form basins. Structures called half-grabend, which are tilted fault blocks, also contribute to the alternating topographic highs and lows in the Basin and Range Province. The horsts and higher ends of the tilted fault blocks are the source of sediments that have accumulated in the basins created by the grabens and lower ends of the tilted blocks.

Anticlines

- The two most common types of folds are anticlines and synclines. Anticlines usually arise by upfolding, or arching, or sedimentary layers and are sometimes spectacularly displayed along highways that have been cut through deformed strata.

Fault scarps

- Vertical displacements along dip-slip faults may produce long, low cliffs called fault scarps (scrape = a slope). Fault scarps are produced by displacement that generate earthquakes.

Reverse and thrust faults

- are dip-slip faults in which the hanging wall block moves up relative to the footwall block. Thrust faults are reverse faults having dips less than 45 degrees, so the overlying block moves nearly horizontally over the underlying block.

Faults

- are fractures in the crust along which appreciable displacement has taken place. Occasionally, small faults can be recognized in road cuts where sedimentary beds have been offset a few meters. Faults of this scale usually occur as single discrete breaks. By contrast, large faults, such as the San Andreas Fault in California, have displacements of hundreds of kilometers and consistof many interconnecting faults sufaces. These fault zones can be several kilometers wide and are often easier to identify from high-altitude photographs than at ground level.

Earthquakes

- are natural geologic phenomena caused by the sudden and rapid movement of a large volume of rock. The violent shaking and destruction associated with earthquakes are the result of rupture and slippage along fractures in Earth's crust called faults.

Richter scale

- based on the amplitude of the largest seismic wave (P, S or surface wave) recorded on a seismorgram. Because seismic waves weaken as the distance between the focus and the seismograph increases, Richter developed a method that accounts for the decrease in wave amplitude with increasing distance. Theoretically, as long as equivalent instruments are used, monitoring stations at various locations will often obtain slightly different Richter magnitudes for the same earthquake - a consequence of the variations in rock types through which the waves travel.

Faults

- cracks in Earth's crust.

Seismographs

- instruments that record earthquake waves, are similar to the instruments used by the Chinese. Seismographs have a weight freely suspended from a support that is securely attached to bedrock. When vibrations from an earthquake reach the instrument, the inertia of the weight keeps it relatively stationary while Earth and the support move. (Inertia is the tendency of objects at rest to stay at rest and objects in motion to remain in motion.)

Triangulation

- the method for determining seismic wave direction.

Footwall blocks

- the rock surface below the fault.

The Modified Mercalli Intensity Scale

- was developed using California buildings as its standard. For example, if some well-built wood structures and most masonry building were destroyed by an earthquake, the affected area would be assigned an intensity of X (ten) on the Mercalli scale. Despite their usefulness in providing seismologists with a tool to compare earthquake severity, intensity scales have significant drawbacks. Such intensity scales are based on effects (largely destruction) that depend not only on the severity of ground shaking but also on factors such as building design and the nature of surface materials.