Earthquakes and Seismology

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Questions and Answers

Define an earthquake.

An earthquake is the shaking or trembling of the ground caused by the sudden release of energy, usually as a result of faulting, which involves the displacement of rocks along fractures.

Explain the elastic rebound theory.

The elastic rebound theory explains how rocks undergoing deformation bend and store elastic energy. When the stress exceeds the rock's strength, the rock snaps back to a less deformed state, releasing the stored energy as seismic waves.

What characteristic do geologists indicate regarding movement along a fault?

The direction of movement along the fault.

Define seismology.

Seismology is the scientific study of earthquakes and the propagation of seismic waves through the Earth. Signup and view all the answers

What is a seismograph and what does it record?

A seismograph is an instrument that detects, records, and measures the ground motion caused by earthquakes (seismic waves). It produces a record called a seismogram. Signup and view all the answers

Define an earthquake's focus.

An earthquake's focus (or hypocenter) is the point within the Earth where seismic energy is first released, usually along a fault. Signup and view all the answers

Define an earthquake's epicenter.

An earthquake's epicenter is the point on the Earth's surface directly above the focus (hypocenter). Signup and view all the answers

Explain the relationship between an earthquake's focus depth and different types of plate boundaries.

Earthquakes generated along divergent (spreading centers) or transform plate boundaries are invariably shallow-focus. Along convergent margins (subduction zones), a range of earthquake depths occurs, including shallow-focus, intermediate-focus, and deep-focus earthquakes. Signup and view all the answers

What is the significance of the deepening pattern of earthquakes observed along a Wadati-Benioff zone?

Wadati-Benioff zones are dipping seismic zones common to convergent plate boundaries where one plate is subducted beneath another. The pattern of earthquakes getting progressively deeper indicates the angle of plate descent along the subduction zone. Signup and view all the answers

Name the two major seismic belts where approximately 95% of all earthquakes take place.

The Circum-Pacific belt and the Mediterranean-Asiatic belt. Signup and view all the answers

Explain why most earthquakes occur in the Circum-Pacific and Mediterranean-Asiatic seismic belts.

These belts correspond to major plate boundaries where tectonic plates converge, diverge, and slide past each other, resulting in significant stress accumulation and release through earthquakes. Signup and view all the answers

Where do the remaining approximately 5% of earthquakes occur?

The remaining 5% of earthquakes occur mostly within plate interiors (intraplate earthquakes) and along oceanic spreading ridges. Signup and view all the answers

What is the suspected cause of earthquakes that occur within plate interiors?

Geologists hypothesize that intraplate earthquakes arise from localized stresses caused by the compressional forces that most plates experience along their margins, potentially reactivating ancient fault zones within the plate. Signup and view all the answers

Name and briefly describe the two main types of seismic body waves.

<ol> <li>P-waves (Primary waves): These are compressional waves that travel fastest through the Earth, causing particles to vibrate parallel to the direction of wave propagation. 2. S-waves (Secondary waves): These are shear waves that travel slower than P-waves, causing particles to vibrate perpendicular to the direction of wave propagation.</li> </ol> Signup and view all the answers

Name and describe the two main types of seismic surface waves.

<ol> <li>R-waves (Rayleigh waves): These waves cause particles to move in an elliptical path, similar to water waves, in the vertical plane along the direction of travel. 2. L-waves (Love waves): These waves cause particles to move back and forth in a horizontal plane, perpendicular to the direction of wave travel.</li> </ol> Signup and view all the answers

Explain the process used to locate an earthquake's epicenter.

Data from at least three seismograph stations are needed. For each station, the time difference between the arrival of the first P-wave and the first S-wave (P-S time interval) is measured. This interval is used with a time-distance graph to determine the distance from that station to the epicenter. A circle with that radius is drawn around each station on a map; the point where the three circles intersect is the epicenter. Signup and view all the answers

Define the magnitude of an earthquake.

Earthquake magnitude is a measure of the total amount of energy released by an earthquake at its source. Signup and view all the answers

What does the Richter Magnitude Scale measure?

The Richter Magnitude Scale measures the magnitude of an earthquake based on the maximum amplitude of seismic waves recorded by a specific type of seismograph. Signup and view all the answers

What are the limitations of the Richter Magnitude Scale, especially concerning large earthquakes?

The Richter Scale tends to underestimate the energy of very large earthquakes (magnitude > 7). This is because it measures the highest peak amplitude on a seismogram, which represents only an instant and doesn't fully capture the total energy released over a larger fault rupture area and longer duration. Signup and view all the answers

What factors determine the destructiveness of an earthquake?

The destructiveness of an earthquake is determined by its magnitude (energy released), the duration of shaking, the distance from the epicenter, the geology of the affected region (foundation materials), and the types and construction quality of structures. Signup and view all the answers

List the four major types of destructive effects directly caused by earthquakes.

The four major destructive effects are: 1. Ground Shaking, 2. Fire, 3. Landslides (ground failure), and 4. Tsunami. Signup and view all the answers

How does the underlying geology affect seismic wave amplitude and potential earthquake damage?

The nature of the ground material significantly influences seismic wave amplitude. Soft, unconsolidated sediments tend to amplify ground shaking much more than solid bedrock. Areas with thicker, softer sediments generally experience stronger and longer shaking. Signup and view all the answers

Describe the process of soil liquefaction.

Soil liquefaction occurs when water-saturated, unconsolidated sediments (like sand or silt) are subjected to prolonged ground shaking. The shaking increases water pressure between the grains, causing the sediment to temporarily lose its strength and behave like a fluid. Signup and view all the answers

What factors are considered when assessing the likelihood of future earthquakes?

Factors include the historical seismicity and estimated recurrence intervals (time frame), the identification of active faults and seismic gaps (location), the potential magnitude based on fault characteristics (strength), seismic risk maps incorporating geological and historical data, and paleoseismology (study of prehistoric earthquakes). Signup and view all the answers

Discuss the current understanding regarding the possibility of controlling earthquakes.

Preventing earthquakes entirely is considered unlikely. Some research explores whether it might be possible to gradually release built-up stress along faults, perhaps by injecting liquids into locked segments or seismic gaps to trigger smaller, less harmful quakes. However, this carries the significant risk of inadvertently triggering a large, destructive earthquake. Signup and view all the answers

Define wave refraction and wave reflection in the context of seismic waves.

Wave refraction is the bending of seismic waves as they pass from one material into another with different density or elasticity, causing a change in wave speed. Wave reflection occurs when seismic waves encounter a boundary between materials of different density or elasticity and bounce off that boundary. Signup and view all the answers

How are the reflection and refraction of seismic waves used to investigate the Earth's interior structure?

By analyzing the travel times, paths, and changes in seismic waves (both P-waves and S-waves) recorded at seismograph stations around the world, scientists can deduce the properties of the materials they travel through. Reflected and refracted waves indicate boundaries between different layers (like the crust-mantle boundary or the core-mantle boundary). Changes in wave speed (refraction) reveal variations in density and composition. Signup and view all the answers

Define rock deformation.

Rock deformation refers to any changes in the original shape, size (volume), or orientation of a rock body, primarily caused by tectonic stresses. Signup and view all the answers

Define stress and strain in a geological context.

Stress is the force applied per unit area on a rock. Strain is the resulting deformation or change in shape or volume of the rock caused by that stress. Signup and view all the answers

Define the three primary types of stress that affect rocks.

<ol> <li>Compression: Stress resulting from forces directed toward one another along the same line, squeezing the material (associated with folding and reverse faulting). 2. Tension: Stress resulting from forces acting along the same line but in opposite directions, pulling the material apart (associated with normal faulting). 3. Shear stress: Stress resulting from forces acting parallel to one another but in opposite directions across a surface, causing shearing or tearing (associated with strike-slip faulting).</li> </ol> Signup and view all the answers

Describe the three types of strain (deformation) that rocks can exhibit.

<ol> <li>Elastic strain: Temporary deformation where rocks return to their original shape when the stress is removed. 2. Plastic (or ductile) strain: Permanent deformation involving flowing or bending (folding) without fracturing. 3. Fracture (or brittle strain): Permanent deformation where rocks break or rupture.</li> </ol> Signup and view all the answers

Define strike and dip.

Strike is the compass direction (bearing) of a horizontal line formed by the intersection of an inclined geological plane (like a rock layer or fault) and a horizontal plane. Dip is the angle of inclination of the geological plane, measured downward from the horizontal plane, perpendicular to the strike. Signup and view all the answers

How are strike and dip used to describe the orientation of rock layers or faults?

Geologists measure and record the strike and dip of rock layers or fault surfaces in the field. These measurements, plotted on geological maps using specific symbols, allow them to understand and represent the three-dimensional geometry of subsurface structures. Signup and view all the answers

Distinguish between ductile and brittle behavior in rocks and relate this to temperature.

Ductile behavior involves plastic strain (folding or flowing) before potential fracture. Brittle behavior involves little or no plastic strain before the rock fractures. Generally, higher temperatures promote ductile behavior, while lower temperatures favor brittle behavior. Signup and view all the answers

Define a geologic structure.

A geologic structure is any feature within rocks that results from deformation, such as folds, faults, joints, and foliations. Signup and view all the answers

List the three basic types of folds.

The three basic types of folds are Monoclines, Anticlines, and Synclines. Signup and view all the answers

Define faults in geology.

Faults are fractures or zones of fractures in rock along which there has been significant displacement or movement of the two sides relative to each other. Signup and view all the answers

Define the terms hanging wall block and footwall block in relation to a fault.

For an inclined fault, the hanging wall block is the block of rock that lies above the fault plane. The footwall block is the block of rock that lies beneath the fault plane. Signup and view all the answers

Describe the main types of dip-slip faults based on relative block movement.

Dip-slip faults involve movement primarily parallel to the dip of the fault plane. The main types are: 1. Normal fault: The hanging wall block moves down relative to the footwall block, typically caused by tensional stress. 2. Reverse fault: The hanging wall block moves up relative to the footwall block, typically caused by compressional stress. (A low-angle reverse fault is called a thrust fault). Signup and view all the answers

Define a strike-slip fault and its key characteristics.

A strike-slip fault is a fault where the movement (slip) is primarily horizontal and parallel to the strike of the fault plane. It results from shear stresses. Blocks on either side slide past each other. They can be classified as right-lateral or left-lateral depending on the apparent direction of offset from an observer's perspective. Signup and view all the answers

Define an orogeny.

An orogeny is an episode of intense rock deformation, commonly accompanied by metamorphism and igneous activity, resulting in the formation of a mountain range. Signup and view all the answers

Discuss the fundamental relationship between plate tectonics and mountain building (orogeny).

Mountain building primarily occurs at convergent plate boundaries where tectonic plates collide. The type of collision (oceanic-oceanic, oceanic-continental, or continental-continental) dictates the specific processes (like subduction, volcanic activity, folding, thrust faulting, accretion) and the resulting type of mountain range. Signup and view all the answers

What geological features are typically produced by orogenic activity at oceanic-oceanic plate boundaries?

At oceanic-oceanic convergent boundaries, subduction of one oceanic plate beneath another leads to the formation of a deep-sea trench, seismic activity (including deep earthquakes tracing the Wadati-Benioff zone), and magma generation that forms a chain of volcanic islands known as a volcanic island arc on the overriding plate. An accretionary wedge may also form. Signup and view all the answers

What geological features are typically produced by orogenic activity at oceanic-continental plate boundaries?

When an oceanic plate subducts beneath a continental plate, features include a deep-sea trench offshore, significant seismic activity, the formation of an accretionary wedge (sediments scraped off the subducting plate), volcanic eruptions forming a continental volcanic arc (mountain range) on the edge of the continent, and associated folding, thrust faulting, metamorphism, and emplacement of plutons (like granite batholiths) within the continental crust. Signup and view all the answers

What geological features are typically produced by orogenic activity at continental-continental plate boundaries?

Collision between two continental plates results in intense compression because neither plate subducts significantly. This leads to widespread crustal thickening through extensive folding, numerous thrust faults, high-grade metamorphism, and the formation of towering, non-volcanic mountain ranges. Suture zones mark where the continents joined. Signup and view all the answers

Explain the relationship between terranes and mountain systems.

Terranes are crustal fragments (like island arcs, microcontinents, or oceanic plateaus) with distinct geologic histories that have been transported by plate movements and accreted (added) onto the margin of a larger continent during orogenic events. Many complex mountain systems, especially those formed along convergent margins over long periods, are mosaics composed of numerous accreted terranes sutured together. Signup and view all the answers

Define the principle of isostasy.

Isostasy is the concept that Earth's crust is in gravitational equilibrium, essentially 'floating' at an elevation dependent on its thickness and density, within the denser, more fluid-like asthenosphere (upper mantle). Signup and view all the answers

Define isostatic rebound.

Isostatic rebound is the gradual rising of the Earth's crust after the removal of a large surface load (like melting ice sheets or significant erosion), as the crust adjusts to regain isostatic equilibrium. Signup and view all the answers

Define mass wasting.

Mass wasting is the downslope movement of rock, regolith (soil and loose sediment), and snow/ice under the direct influence of gravity. Signup and view all the answers

What determines the shear strength of a slope, and how does it relate to slope stability?

A slope's shear strength is its internal resistance to movement. It depends on factors like the inherent strength and cohesion of the slope material, the internal friction between grains, and any external support. Slope stability represents a balance between shear strength (resisting force) and the component of gravity acting parallel to the slope (driving force). Slope failure (mass wasting) occurs when the driving force exceeds the shear strength. Signup and view all the answers

What does it mean for a slope to be in a state of dynamic equilibrium?

A slope in dynamic equilibrium is one that is constantly adjusting its form and processes in response to changing conditions (such as climate, vegetation cover, or erosion rates) to maintain a relatively stable state over time. Signup and view all the answers

Discuss how various factors contribute to mass wasting events.

Several factors influence slope stability and can trigger mass wasting: <ul> <li>Slope Angle: Steeper slopes increase the downslope component of gravity.</li> <li>Weathering & Climate: Weathering weakens rock/soil strength; climate affects weathering rates and water content.</li> <li>Water Content: Water reduces friction between particles, adds weight, and can increase pore pressure, significantly reducing shear strength.</li> <li>Vegetation: Roots bind soil and absorb water, generally increasing stability; removal of vegetation often destabilizes slopes.</li> <li>Overloading: Adding weight (e.g., buildings, fill) increases the downslope force.</li> <li>Geology: Weak rock layers, fractures, or layers dipping parallel to the slope create failure surfaces.</li> <li>Triggering Mechanisms: Events like heavy rainfall, earthquakes, volcanic eruptions, or human activities (undercutting slopes) can initiate failure on already unstable slopes.</li> </ul> Signup and view all the answers

How do geologists identify areas potentially susceptible to slope failure?

Geologists identify potentially hazardous areas by recognizing signs of past or current instability. This includes looking for features such as scarps (steep cliffs formed by slides), open fissures or cracks, tilted trees or structures, hummocky (irregular bumpy) ground surfaces, evidence of past landslides, and sudden changes in vegetation patterns. Signup and view all the answers

What methods are used to minimize the danger and damage from mass wasting?

Various engineering and preventative measures are used: building retaining walls or structures to support slopes, installing drainage systems to remove excess water and reduce pore pressure, regrading slopes to reduce their steepness, and planting vegetation to bind soil and stabilize surfaces. Signup and view all the answers

How can the direction of movement along a fault be indicated?

Direction can be indicated using arrows on geologic maps, classifying the fault type (normal, reverse, strike-slip based on relative block movement), or observing offset geological features. Signup and view all the answers

Explain how a seismograph works and what it records.

A seismograph works on the principle of inertia. It has a heavy weight suspended such that it remains relatively stationary while the ground moves beneath it. A pen attached to the weight or a sensor records the ground motion relative to the weight onto a rotating drum (seismogram) or digitally. It records the arrival time and amplitude of seismic waves. Signup and view all the answers

Define an earthquake's focus (or hypocenter).

The point within the Earth where energy is first released during an earthquake. Signup and view all the answers

Explain the relationship between an earthquake's focus depth and plate boundaries.

Earthquakes generated along divergent (spreading centers) or transform plate boundaries are invariably shallow-focus. Along convergent margins (subduction zones), earthquakes can be shallow-, intermediate-, or deep-focus, generally getting deeper in the direction of subduction. Signup and view all the answers

Explain the pattern of earthquake depths observed along the Wadati-Benioff zone.

Wadati-Benioff zones are dipping planes of earthquakes common to convergent plate boundaries where one plate is subducted beneath another. Earthquakes occur within the downgoing slab, and their foci become progressively deeper along the dip of the slab. Signup and view all the answers

Explain why most earthquakes occur in the Circum-Pacific and Mediterranean-Asiatic belts.

These belts represent major plate boundaries where tectonic plates converge (collide or subduct), diverge (spread apart), or slide past each other (transform faults), leading to the stress buildup and release that causes earthquakes. Signup and view all the answers

Identify where the remaining 5% of earthquakes typically occur.

They occur mostly in plate interiors (intraplate earthquakes) and along oceanic spreading ridges. Signup and view all the answers

Discuss what geologists think is the possible cause of intraplate earthquakes.

Geologists hypothesize that intraplate earthquakes may arise from localized stresses caused by the transmission of compressional forces from plate margins into the plate interior, often reactivating ancient fault zones. Signup and view all the answers

Define the two main types of seismic body waves.

P-waves (Primary waves): Compressional waves that travel fastest and can pass through solids, liquids, and gases. S-waves (Secondary waves): Shear waves that are slower than P-waves and can only travel through solids. Signup and view all the answers

Define the two main types of seismic surface waves.

R-waves (Rayleigh waves): Particles move in an elliptical path, similar to ocean waves. L-waves (Love waves): Particles move back and forth in a horizontal plane perpendicular to the direction of wave travel. Signup and view all the answers

Explain how an earthquake's epicenter is located.

At least three seismograph stations are needed. The time difference between the arrival of the first P-wave and the first S-wave (P-S time interval) is measured at each station. This time interval corresponds to a specific distance from the epicenter, determined using a time-distance graph. A circle with that distance as its radius is drawn around each station on a map. The point where the three circles intersect is the epicenter. Signup and view all the answers

Discuss what the Richter Magnitude Scale measures.

The Richter Magnitude Scale measures the magnitude of an earthquake based on the maximum amplitude of seismic waves recorded by a specific type of seismograph (Wood-Anderson), adjusted for distance from the epicenter. Signup and view all the answers

Discuss the limitations of using the Richter Magnitude Scale.

The Richter Scale underestimates the energy of very large earthquakes (typically above magnitude 7) because it measures only the highest peak amplitude on a seismogram, which doesn't fully capture the total energy released over the duration and rupture area of a major quake. It also becomes less accurate for distant earthquakes. Signup and view all the answers

Discuss the factors that determine the destructiveness of an earthquake.

Key factors include: magnitude (energy released), duration of shaking, distance from the epicenter, the geology of the affected region (soil and rock types can amplify shaking), and the design and construction quality of buildings and infrastructure. Signup and view all the answers

List the four major destructive effects commonly caused by earthquakes.

Ground Shaking, Fire (often from broken gas lines or electrical wires), Landslides (triggered by ground shaking), and Tsunamis (generated by undersea earthquakes or landslides). Signup and view all the answers

What is the general relationship between seismic wave amplitude and the underlying geology?

Seismic wave amplitude tends to increase (amplify) when waves pass from solid bedrock into softer, less consolidated materials like sand, gravel, or artificial fill. This amplification leads to stronger shaking and potentially greater damage in areas built on such materials. Signup and view all the answers

Discuss the factors involved in forecasting the likelihood of possible future earthquakes.

Forecasting considers factors such as: the historical earthquake record (time frame, location, strength), the identification of active faults and seismic gaps (segments of active faults that haven't ruptured recently), seismic risk maps based on geological and seismological data, and paleoseismology (the study of prehistoric earthquakes). Signup and view all the answers

Discuss the viability and possibility of earthquake control.

Preventing large earthquakes entirely is considered unlikely with current technology. Some ideas involve attempting to gradually release stored energy in rocks, potentially by injecting liquids into locked fault segments or seismic gaps to trigger smaller, less harmful quakes. However, this carries the significant risk of inadvertently triggering a large, damaging earthquake. Signup and view all the answers

Define seismic wave refraction and reflection.

Refraction is the bending of seismic waves as they pass from one material into another with different density or elasticity, causing the wave's speed and direction to change. Reflection is the bouncing back of seismic waves when they encounter a boundary separating materials of different properties. Signup and view all the answers

How are seismic wave reflection and refraction used to determine the Earth's interior structure?

By analyzing the travel times, paths, and locations where seismic waves (P-waves and S-waves) from earthquakes or artificial sources are reflected and refracted as they travel through the Earth, scientists can identify boundaries between layers with different physical properties (density, rigidity, state - solid/liquid). This allows mapping of the crust, mantle, outer core (liquid), and inner core (solid). Signup and view all the answers

Define stress and strain in the context of geology.

Stress is the force applied per unit area to a rock. Strain is the resulting deformation (change in shape or volume) caused by that stress. Signup and view all the answers

Define the three main varieties of stress.

Compression: Stress resulting from forces pushing toward one another along the same line, squeezing or compressing the material (associated with folding and reverse faulting). Tension: Stress resulting from forces pulling away from one another along the same line, stretching the material (associated with normal faulting). Shear stress: Stress resulting from forces acting parallel to each other but in opposite directions, causing deformation along parallel planes (associated with strike-slip faulting). Signup and view all the answers

Define the three types of strain (rock behavior under stress).

Elastic strain: Temporary deformation where rocks return to their original shape when the stress is removed. Plastic (or ductile) strain: Permanent deformation involving folding or flowing of rock without fracturing. Fracture (or brittle strain): Permanent deformation where rocks break or rupture. Signup and view all the answers

Explain how strike and dip are used to describe the orientation of rock layers.

Strike and dip provide a standardized way to quantify and record the three-dimensional orientation of planar geologic features like sedimentary beds, faults, or metamorphic foliations. Strike gives the compass direction of the plane's horizontal trace, and dip gives the angle and direction of its maximum inclination from horizontal. Signup and view all the answers

Define ductile versus brittle behavior of rocks and its relation to temperature.

Ductile behavior (plastic strain) occurs when rocks deform by flowing or folding without breaking. Brittle behavior (fracture) occurs when rocks break or shatter under stress. Generally, higher temperatures promote ductile behavior, while lower temperatures favor brittle behavior. Increased confining pressure also promotes ductility. Signup and view all the answers

Define faults.

Fractures or breaks in rock along which there has been significant movement or displacement of the two sides relative to each other. Signup and view all the answers

Define the terms hanging wall block and footwall block as they relate to an inclined fault.

The hanging wall block is the block of rock that lies above an inclined fault plane. The footwall block is the block of rock that lies beneath the inclined fault plane. Signup and view all the answers

Discuss the main types of dip-slip faults.

Normal fault: The hanging wall block moves down relative to the footwall block, caused by tensional stress (pulling apart). Reverse fault: The hanging wall block moves up relative to the footwall block, caused by compressional stress (pushing together). A low-angle reverse fault (dip < 45 degrees) is called a thrust fault. Signup and view all the answers

Define a strike-slip fault.

A fault where the movement is primarily horizontal and parallel to the strike of the fault plane. They result from shear stresses. Depending on the relative motion observed across the fault, they are classified as right-lateral or left-lateral. Oblique-slip faults exhibit both dip-slip and strike-slip movement. Signup and view all the answers

What is the general relationship between plate tectonics and mountain building (orogeny)?

Most major mountain ranges form at or near convergent plate boundaries. Subduction at oceanic-oceanic and oceanic-continental boundaries leads to volcanic arcs and associated mountains. Continental-continental collisions produce extensive folded and faulted mountain belts. Less commonly, mountains can form due to continental rifting. Signup and view all the answers

Discuss the typical features produced by orogenic activity occurring at oceanic-oceanic plate boundaries.

Convergence of two oceanic plates leads to subduction of one plate beneath the other, forming a deep ocean trench and generating magma that rises to form a chain of volcanic islands known as a volcanic island arc. Associated features include an accretionary wedge (sediments scraped off the subducting plate) and significant earthquake activity. Signup and view all the answers

Discuss the features produced by orogenic activity occurring at oceanic-continental plate boundaries.

When an oceanic plate converges with a continental plate, the denser oceanic plate subducts beneath the continent. This forms a deep ocean trench offshore, generates magma that rises to form a volcanic mountain range on the continent (continental volcanic arc), and creates an accretionary wedge composed of sediments scraped off the subducting plate and material eroded from the overriding plate. Folding and thrust faulting also occur in the continental crust. Signup and view all the answers

Discuss the features produced by orogenic activity occurring at continental-continental plate boundaries.

When two continental plates collide, neither subducts completely due to their low density. Instead, the collision results in intense compression, causing extensive folding, thrust faulting, and thickening of the continental crust, forming high, complex mountain ranges. Igneous activity and metamorphism are also common. Signup and view all the answers

What is the relationship between terranes and mountain systems?

Many mountain systems, particularly those formed at convergent margins over long periods, are composed of multiple accreted terranes. Terranes are crustal blocks (ranging from island arcs to microcontinents) with distinct geologic histories that originated elsewhere and were transported by plate movements until they collided with and became attached (accreted) to the edge of a continent. Signup and view all the answers

Define isostatic rebound (or isostatic adjustment).

The vertical movement of the Earth's crust in response to changes in load (mass). When a load (like a large ice sheet or mountain range) is removed through melting or erosion, the crust slowly rises (rebounds) to regain isostatic equilibrium. Conversely, adding a load causes the crust to subside. Signup and view all the answers

Discuss the factors determining the shear strength of a slope.

A slope's shear strength, which resists downslope movement, depends on factors including the inherent strength and cohesion of the slope material, the amount of internal friction between grains within the material, and any external support (like vegetation roots or retaining walls). Water content significantly affects shear strength; small amounts can increase cohesion, but saturation usually reduces friction and increases weight, decreasing shear strength. Signup and view all the answers

Discuss what is meant by a slope being in a state of dynamic equilibrium.

Slopes are considered to be in dynamic equilibrium because they are constantly adjusting to changing conditions (such as weathering, erosion, changes in water content, vegetation cover, or seismic activity). While they may appear stable for periods, they are continually evolving and responding to these factors, potentially leading to future instability. Signup and view all the answers

How do factors like slope angle, weathering, climate, water content, vegetation, overloading, geology, and triggering mechanisms contribute to mass wasting events?

Steeper slope angles increase the component of gravity acting parallel to the slope, promoting failure. Weathering weakens rock and soil. Climate influences weathering rates and water content. Water reduces friction between particles, adds weight, and can cause liquefaction. Vegetation roots bind soil (stabilizing), but removal increases erosion and instability. Overloading (e.g., adding buildings) increases the downslope force. Underlying geology (rock type, orientation of layers, presence of faults) dictates material strength and potential failure planes. Triggering mechanisms like earthquakes, heavy rainfall, or human activity can initiate failure on an already unstable slope. Signup and view all the answers

Identify some major types of mass wasting, briefly noting their characteristics.

Common types include: Rockfall (very rapid fall of detached rock fragments), Slump (rotational sliding of material along a curved surface, typically slow to moderate), Rock slide (rapid sliding of rock mass along a planar surface), Mudflow/Debris flow (rapid flow of water-saturated soil/regolith, often channelized), Debris Avalanche (very rapid flow of soil and rock), Earthflow (slower flow of finer-grained material), Creep (very slow, gradual downslope movement of soil/regolith), Solifluction (slow flow of saturated soil over permafrost). Signup and view all the answers

Explain how geologists identify areas with high potential for slope failure.

Geologists identify potentially hazardous areas by looking for evidence of past landslides (such as scarps, hummocky terrain, tilted trees or structures, disrupted drainage patterns) and identifying conditions conducive to failure (steep slopes, weak rock/soil types, unfavorable geologic structures, undercutting of slopes, high water tables, lack of vegetation). They use field mapping, aerial imagery, LiDAR, and geotechnical analysis. Signup and view all the answers

Discuss various methods used to minimize the danger and damage from mass wasting.

Methods include: building retaining walls or other support structures, installing drainage systems to reduce water pressure in slopes, regrading slopes to reduce their angle, planting vegetation to bind soil and reduce erosion, avoiding construction or overloading in high-risk areas, and using rock bolts or netting to stabilize rock faces. Signup and view all the answers

What is an earthquake?

Shaking or trembling of the ground caused by the sudden release of energy (usually as a result of faulting), which involves the displacement of rocks along fractures. Signup and view all the answers

How is the direction of movement along a fault indicated?

Movement along a fault is described by the relative motion of the hanging wall and footwall for dip-slip faults, or by the direction of horizontal offset for strike-slip faults. Signup and view all the answers

What is seismology?

The study of earthquakes and seismic waves. Signup and view all the answers

How does a seismograph work and what does it record?

Seismographs are instruments that detect, record, and measure the ground motion caused by earthquakes. They typically work on the principle of inertia: a heavy weight suspended in a frame remains relatively stationary while the ground and frame move around it. This relative motion is recorded. Signup and view all the answers

What is an earthquake's focus (or hypocenter)?

The point within the Earth where energy is first released during an earthquake. Signup and view all the answers

What is an earthquake's epicenter?

The point on the Earth's surface directly above the focus. Signup and view all the answers

What is the significance of the deepening pattern of earthquakes along a Wadati-Benioff zone?

Wadati-Benioff zones are dipping seismic zones common to convergent plate boundaries where one plate is subducted beneath another. The pattern of progressively deeper earthquakes along this zone indicates the angle of plate descent (subduction) beneath the overriding plate. Signup and view all the answers

What are the two major seismic belts where 95% of earthquakes occur?

The Circum-Pacific belt and the Mediterranean-Asiatic (or Alpine-Himalayan) belt. Signup and view all the answers

Why do most earthquakes occur in the Circum-Pacific and Mediterranean-Asiatic belts?

These belts correspond to major plate boundaries where plates converge, diverge, and slide past each other, resulting in significant stress accumulation and release. Signup and view all the answers

Where do the remaining 5% of earthquakes primarily occur?

Within plate interiors (intraplate earthquakes) and along oceanic spreading ridges. Signup and view all the answers

What is the suspected cause of intraplate earthquakes?

Geologists think that they arise from localized stresses caused by the compression that most plates experience along their margins, often reactivating ancient fault zones. Signup and view all the answers

What are the two main types of seismic body waves and their characteristics?

P-waves (Primary waves) are compressional waves that travel fastest through materials. S-waves (Secondary waves) are shear waves that are slower than P-waves and cannot travel through liquids. Signup and view all the answers

What are the two main types of seismic surface waves and their characteristics?

R-waves (Rayleigh waves) cause particles to move in an elliptical path similar to water waves. L-waves (Love waves) cause particles to move back and forth in a horizontal plane perpendicular to the direction of wave travel. Signup and view all the answers

How is an earthquake's epicenter located?

Three seismograph stations are needed. The time difference between the arrival of the first P-wave and the first S-wave (P-S time interval) is measured at each station. This time interval corresponds to a specific distance from the epicenter, which can be found using a time-distance graph. A circle with that radius is drawn around each station on a map; the point where the three circles intersect is the epicenter. Signup and view all the answers

What does the magnitude of an earthquake represent?

The total amount of energy released by an earthquake at its source. Signup and view all the answers

What are the limitations of the Richter Magnitude Scale?

It underestimates the energy of very large earthquakes because it measures the highest peak amplitude on a seismogram, which represents only an instant during an earthquake and doesn't fully capture the total energy released over the entire rupture duration and area. Signup and view all the answers

What are four major destructive effects caused by earthquakes?

Ground Shaking, Fire, Landslides, and Tsunami. Signup and view all the answers

What is the relationship between seismic wave amplitude and the underlying geology?

Seismic wave amplitude generally increases in softer, less consolidated materials (like sediments or fill) compared to solid bedrock. This amplification leads to greater shaking intensity and potential damage in areas built on such materials. Signup and view all the answers

What factors are considered when predicting the likelihood of future earthquakes?

Factors include the historical seismicity of a region, the identification of active faults, estimation of recurrence intervals (time frame), potential earthquake strength (magnitude), seismic risk maps based on past data and geology, and studies of past earthquakes (paleoseismology). Signup and view all the answers

How are the reflection and refraction of seismic waves used to determine the Earth's interior structure?

By analyzing the travel times, paths, changes in speed (refraction), and bounced signals (reflection) of seismic waves from earthquakes or artificial sources as they pass through different materials, scientists can map out the layers (crust, mantle, core) and boundaries within the Earth's interior. Signup and view all the answers

What is rock deformation?

Changes in the shape, size, or orientation (volume) of rock bodies. Signup and view all the answers

What are the three main varieties of stress that affect rocks?

Compression (squeezing forces directed toward one another), Tension (pulling forces acting in opposite directions), and Shear stress (parallel forces acting in opposite directions across a surface). Signup and view all the answers

What are the three types of strain observed in rocks?

Elastic strain (temporary deformation; rock returns to original shape when stress is removed), Plastic (or ductile) strain (permanent deformation by flowing or folding), and Fracture (breaking or faulting). Signup and view all the answers

How are strike and dip used to determine the orientation of rock layers?

Geologists measure the strike (compass bearing of the horizontal intersection line) and the dip (angle and direction of steepest incline) of rock layers or planar features like faults to describe and map their three-dimensional orientation in space. Signup and view all the answers

What is a geologic structure?

Any feature within rocks that results from deformation, such as folds, faults, and joints. Signup and view all the answers

What are the three basic types of folds?

Monoclines (a simple bend or flexure in otherwise horizontal layers), Anticlines (upward-arching folds where older rocks are typically in the core), and Synclines (downward-arching folds where younger rocks are typically in the core). Signup and view all the answers

What is a fault?

A fracture or zone of fractures in rock along which there has been significant displacement or movement. Signup and view all the answers

Define the terms hanging wall block and footwall block as they relate to a fault.

For an inclined fault, the hanging wall block is the block of rock overlying the fault plane. The footwall block is the block of rock underneath the fault plane. Signup and view all the answers

What are the main types of dip-slip faults?

Normal faults, where the hanging wall block moves down relative to the footwall block (caused by tensional stress). Reverse faults, where the hanging wall block moves up relative to the footwall block (caused by compressional stress). Thrust faults are low-angle reverse faults. Signup and view all the answers

What is a strike-slip fault?

A fault where the movement is predominantly horizontal, parallel to the strike of the fault plane. They result from shear stresses. Movement is described as either right-lateral or left-lateral depending on the apparent direction of offset from an observer's perspective. Signup and view all the answers

What is an orogeny?

An episode of mountain building, typically involving significant rock deformation (folding and faulting), metamorphism, and igneous activity. Signup and view all the answers

What is the relationship between plate tectonics and mountain building?

Mountain ranges primarily form at convergent plate boundaries through processes driven by plate collisions, including volcanic activity (in subduction zones), folding, thrust faulting, metamorphism, and the accretion of terranes. Signup and view all the answers

What features are produced by orogeny at oceanic-oceanic plate boundaries?

Orogeny at oceanic-oceanic convergent boundaries typically produces deep ocean trenches, volcanic island arcs (chains of volcanic islands parallel to the trench), and associated seismic activity. Signup and view all the answers

What features are produced by orogeny at oceanic-continental plate boundaries?

Features include a deep ocean trench, an accretionary wedge (sediments scraped off the subducting plate), a volcanic mountain range (continental volcanic arc) on the edge of the continent, and associated folding, faulting, and seismic activity. Signup and view all the answers

What features are produced by orogeny at continental-continental plate boundaries?

Continental-continental collisions result in extensive folding and thrust faulting, creating high, complex mountain ranges with thickened continental crust. There is significant seismic activity but little to no volcanism because continental crust is too buoyant to subduct easily. Signup and view all the answers

What is the principle of isostasy?

The concept that the Earth's lithosphere (crust and upper mantle) floats in gravitational balance (equilibrium) on the denser, fluid-like asthenosphere below. Thicker or less dense parts of the lithosphere float higher, while thinner or denser parts float lower. Signup and view all the answers

What is isostatic rebound?

The gradual rising of the Earth's crust after the removal of a large weight (such as melting ice sheets or erosion of mountains). This unloading allows the crust to rise slowly until it reaches a new state of isostatic equilibrium. Signup and view all the answers

What is mass wasting?

The downslope movement of rock, soil, and regolith (surface debris) under the direct influence of gravity. Signup and view all the answers

What determines the shear strength of a slope?

A slope's shear strength (resistance to downslope movement) depends on factors like the intrinsic strength and cohesion of the slope material, the amount of internal friction between grains, and any external support. Gravity acting parallel to the slope promotes instability; when this force exceeds the shear strength, failure occurs. Signup and view all the answers

How do factors like slope angle, weathering, water content, vegetation, overloading, geology, and triggering mechanisms contribute to mass wasting?

Steeper slopes increase the component of gravity acting parallel to the slope. Weathering weakens materials. Water content reduces friction and adds weight. Vegetation roots bind soil, increasing stability (removal decreases it). Overloading (e.g., adding buildings) increases downslope force. Weak underlying rock layers or unfavorable orientation (geology) reduce stability. Triggering mechanisms (earthquakes, heavy rain, undercutting) provide the final push to initiate failure. Signup and view all the answers

How do geologists identify areas with high potential for slope failure?

Geologists look for signs of past landslides (scarps, hummocky terrain, tilted trees/structures) and conditions indicating instability, such as open fissures, sudden changes in vegetation, undercut slopes, areas with weak rock layers dipping downslope, and regions prone to triggering events like earthquakes or heavy rainfall. Signup and view all the answers

Flashcards

Earthquake

Shaking or trembling of the ground caused by the sudden release of energy, usually from faulting.

Elastic Rebound Theory

Rocks deform, bend, and store energy before an earthquake.