Stress and Strain

Key Concepts

• This chapter helps one differentiate between stress and strain
• This chapter helps one identify the three major types of stress
• This chapter helps one differentiate between brittle, ductile, and elastic deformation
• This chapter helps one describe the geological map symbol used for strike and dip of strata
• This chapter helps one name and describe different fold types
• This chapter helps one differentiate the three major fault types and describe their associated movements
• This chapter helps one explain how elastic rebound relates to earthquakes
• This chapter helps one describe different seismic wave types and how they are measured
• This chapter helps one explain how humans can induce seismicity
• This chapter helps one describe how seismographs work to record earthquake waves
• This chapter helps one locate the epicenter of an earthquake from seismograph records
• This chapter helps one explain the difference between earthquake magnitude and intensity
• This chapter helps one list earthquake factors that determine ground shaking and destruction
• This chapter helps one identify secondary earthquake hazards
• This chapter helps one describe notable historical earthquakes

Crustal Deformation and Earthquakes

• Crustal deformation happens when applied forces are greater than the internal strength of rocks, leading to physical changes in their shapes
• Forces involved in tectonic processes, gravity, and igneous pluton emplacement, result in strains in rocks, producing folds, fractures, and faults
• Rocks under large shear stress, break with rapid, brittle deformation, releasing energy as seismic waves, which creates an earthquake

Stress and Strain

• Stress is a force exerted per unit area
• Strain is the physical change in response to stress
• When stress is greater than a rock's internal strength, the rock deforms
• Strain changes rock volume and shape and may fracture it

Types of Stress

• Tensional stress involves forces pulling in opposite directions, stretching and thinning rock
• Compressional stress involves forces pushing together, causing rock folding and thickening
• Shear stress involves transverse forces, causing opposing blocks to move past each other

Stress and Strain Table

• Tensional stress occurs at divergent plate boundaries, resulting in stretching and thinning, associated with normal faults
• Compressional stress occurs at convergent plate boundaries, resulting in shortening and thickening, associated with reverse faults
• Shear stress occurs at transform plate boundaries, resulting in tearing, associated with strike-slip faults

Deformation

• Strain from stress may be elastic, ductile, or brittle is generally called deformation
• Elastic deformation is reversible when stress is released
• Ductile deformation results in permanent changes when stress surpasses the yield point
• Brittle deformation happens when rock fractures under increasing stress
• Deformation depends on pore pressure, strain rate, rock strength, temperature, stress intensity, time, and confining pressure

Factors Affecting Deformation

• Pore pressure is exerted on rock through fluids in open spaces
• Strain rate is how quickly a material is deformed; rocks bend before breaking when stress is slowly applied
• Rock strength is how easily a rock deforms under stress

Strain Response Table

• Increased temperature causes more ductile strain
• Increased strain rate causes more brittle strain
• Increased rock strength causes more brittle strain

Geological Maps

• Geologic maps are 2D representations of Earth's surface geologic formations and structures
• Geologic maps show formations, faults, folds, inclined strata, and rock types
• Geologic formations are recognizable, mappable rock units indicated by color and label

Formation Labels

• Formation labels include symbols that follow a specific protocol beginning with uppercase letters that represent the geologic time period of the formation
• One or more uppercase letters indicate a formation associated with multiple time periods
• Lowercase letters represent the formation name, rock description, or both

Cross Sections

• Cross sections are subsurface interpretations based on surface and subsurface measurements
• Maps display geology in the horizontal plane
• Cross sections show subsurface geology in the vertical plane

Strike and Dip

• Strike and dip describe rock layer orientation relative to North/South and Horizontal
• Strike and dip symbols show inclined bed orientation, resembling a T with a short trunk and wide top
• Dip is the angle a bed plunges into the Earth from horizontal, represented numerically
• Strike is a horizontal line's compass direction angle relative to true north or south
• Attitude is strike and dip considered together

Folds

• Geologic folds are curved or bent rock layers caused by ductile deformation
• Folds are formed by compressional forces at depth with high temperatures and pressures

Describing folds

• Folds are described by axes, axial planes, and limbs
• The axial plane splits the fold into two halves
• The fold axis is the line along which bending occurs
• The hinge line follows the line of greatest bend
• Fold limbs are the two sides of the fold

Types of Folds

• Symmetrical folds have a vertical axial plane and equal but opposite dips
• Asymmetrical folds have dipping, non-vertical axial planes with unequal limb dips
• Overturned folds have steeply dipping axial planes, limbs dip in the same direction at different angles
• Recumbent folds have horizontal or nearly horizontal axial planes
• Plunging folds have axes that plunge into the ground

Categories of folds

• Folds are classified into anticlines, synclines, monoclines, domes, and basins

Anticline

• Anticlines are arch-like folds, convex-upward
• Limbs curve downward, beds dip down and away from the axis
• Oldest rock strata are central, younger beds are outside
• Geologic maps show strata attitude and rock age
• Oil accumulates in anticlines apex, forming oil traps
• An antiform has an anticline shape with undetermined bed ages

Syncline

• Synclines are trough-like folds, concave-upward
• Beds dip down and in toward the central axis
• Older rock is outside, youngest rock is inside the fold axis
• A synform has a syncline shape but lacks distinguishable age zones

Monocline

• Monoclines are step-like folds where flat rocks are upwarped or downwarped, then continue flat
• Monoclines are common on the Colorado Plateau
• Monoclines form "reefs," topographic barriers not to be confused with ocean reefs
• Monoclines are caused by bending of shallower strata as faults grow below
• The faults commonly called “blind faults” because they end before reaching the surface

Dome

• A dome is a symmetrical to semi-symmetrical upwarping of rock beds with a bowl shape
• Domes are formed by compressional forces, igneous intrusions, salt diapirs, or meteorite impacts

Basin

• A basin is the inverse of a dome, a bowl-shaped depression in a rock bed
• Structural basins are sedimentary basins that collect sediment over time
• Sedimentary basins form via folding, mountain building, or faulting
• As a basin sinks or subsides, it accumulates more sediment because the weight of the sediment causes more subsidence

Faults

• Faults are where brittle deformation occurs
• Rock blocks move relative to one another, most common at plate boundaries
• Normal and reverse faults display vertical (dip-slip) motion
• Footwall is below the fault plane
• Hanging wall is above the fault plane
• Faulting refers to rock rupture caused by plate boundaries

Additional Fault Details

• Fault plane is where faults slip
• Fault scarp is the offset seen where the fault breaks the surface
• Slickensides are polished, grooved surfaces along the fault plane

Joints and Fractures

• Joint or fracture is a plane of brittle deformation in rock
• Fractures are created by movement that is not offset or sheared
• Joints result from cooling, depressurizing, or folding

Normal Faults

• The hanging-wall moves downward relative to the footwall, caused by tensional forces
• Normal faults commonly occur at divergent plate boundaries
• Grabens drop down relative to adjacent blocks and create valleys
• Horsts rise up relative to down-dropped blocks, creating higher topography
• Half-grabens tilt blocks with a normal fault on one side, creating an asymmetrical valley-mountain arrangement

Detachment Faults

• Normal faults dips decrease with depth
• Large extension along normal faults can create detachment faults
• Detachment faults are low-angle normal fault known as listric faults
• Detachment faults have large offsets, unrelated rock origins

Reverse Faults

• Reverse faults develop when hanging wall moves up relative to footwall in compressional forces
• Thrust faults are reverse faults with low dip angle (less than 45°)
• Thrust faults carry older rocks over younger rocks
• Thrust faults can repeat rock units

Convergent Boundaries and Faults

• Convergent plate boundaries with subduction zones create megathrust faults
• Denser oceanic crust drives below less dense crust causing the largest earthquakes
• Megathrust faults cause massive destruction and tsunamis

Strike-Slip Faults

• Strike-slip faults have side-to-side motion, commonly at transform plate boundaries
• Fault blocks move laterally, not up or down
• Sinistral motion moves the opposing block left
• Dextral motion moves the opposing block right

Additional Strike-Slip Fault facts

• Bends along strike-slip faults cause compression or tension
• Tensional stresses create transtensional features with normal faults and basins
• Compressional stresses create transpressional features with reverse faults, and mountain building
• San Andreas Fault is a dextral strike-slip fault
• Dead Sea Fault is a sinistral strike-slip fault

Earthquake Essentials

• Earthquakes occur when energy is quickly released by blocks of rock sliding
• Seismic energy travels through the Earth as seismic waves
• Most earthquakes occur along active plate boundaries
• Intraplate earthquakes occur and are poorly understood

How Earthquakes Happen

• Elastic rebound theory explains seismic energy release
• Rocks deform elastically along faults due to stress
• Stress overcomes friction, rupturing rock and starting fault movement
• Deformed unbroken rocks snap back to original shape in process

Additional Earthquake Facts

• Strain gauges measure bending in earthquake-prone areas
• Fault creep is continuous, gradual displacement in areas where the fault isn't locked
• Foreshocks are small earthquakes before a large earthquake
• Mainshock is the main release of energy
• Aftershocks follow the mainshock to adjust strain and generally decrease over time

Focus and Epicenter

• The earthquake focus, or hypocenter, is the initial rupture point, is always below the Earth's Surface
• The epicenter is the location on the Earth’s surface directly above the focus

Seismic Waves

• Waves describe how energy moves through a medium
• Wave amplitude indicates earthquake motion magnitude or height
• Wavelength is the distance between two successive peaks of a wave
• Wave frequency is the number of repetitions over time
• Period is a wave's travel time for one wavelength

Wave combination

• Constructive interference: waves combine in sync, magnifying each other
• Destructive interference: waves are out of sync, diminishing amplitudes

Body Waves and Surface Waves

• Seismic waves express energy from elastic rebound of rock within displaced fault blocks
• Body waves pass underground through the Earth's interior, including P and S waves
• P waves are the fastest body waves, moving via compression through solids, liquids, plasma, and gases
• S waves travel slower, propagating as shear waves only through solids
• Surface waves are produced when body waves strike the Earth's surface, traveling along it and radiating from the epicenter
• Surface waves include rolling Raleigh Waves and side-to-side Love Waves
• Surface waves travel slower than body waves

Wave Interactions

• Seismic waves refract (bend) and reflect when passing through rocks of differing densities
• S waves are blocked by Earth’s liquid outer core, creating an S wave shadow zone, cannot move through liquid
• P waves pass through the core but refract due to density differences, also refract and create a cone shaped shadow zone

Induced Seismicity

• Induced seismicity: earthquakes near natural gas extraction sites, caused by human activity
• Injection of waste fluids increases pore pressure, lubricating fault planes
• Increased pore pressure can trigger earthquakes

Measuring Earthquakes

• People feel approximately 1 million earthquakes a year
• Major earthquakes( 7.0) are rare

Seismographs

• Seismographs measure ground vibrations
• Early seismographs used a weighted pen suspended by a spring above a rotating drum
• Modern seismographs use magnets, wire coils, electrical sensors, and digital signals

Reading Seismographs

• P waves appear first on a seismogram, followed by S waves, then surface waves
• Arrival time differences determine distance to the epicenter but not direction

Triangulation

• Seismographs at multiple stations pinpoint epicenter
• Circle radii from each station intersect at the epicenter
• Method works in 3D to calculate focus depth

Seismograph Network

• The International Registry of Seismograph Stations lists more than 20,000 seismographs on the planet
• Detects detonations of large explosive devices, and predicts tsunamis
• The Global Seismic Network has 150 stations that meet standards
• The USArray maps subsurface quake activity

Network Purpose

• Monitor earthquakes and related hazards
• detect nuclear weapons testing

Seismic tomography

• Seismic tomography creates images of internal Earth structures, using seismic rays from earthquakes
• The PREM (Preliminary Reference Earth Model) models expected properties of earth materials at every depth within the earth

Additional seismic tomography facts

• Seismic wave transmission velocity depends on rock density and elasticity
• Cooler rocks (in the mantle) transmit waves faster
• Warmer rocks transmit earthquake waves slower
• Seismic anomalies are differences between actual and PREM-predicted arrival times,
• Variations in rock properties allow 3D images to be constructed of the areas through which rays pass

Earthquake Magnitude and Intensity

• Magnitude measures energy released by an earthquake

Richter Scale

• The Richter scale (ML) determined from the maximum amplitude on a seismogram, adjusting for distance
• The Richter Scale is logarithmic, increasing tenfold in seismic-wave amplitude per unit
• The Richter Scale increases energy released 32 times per unit
• Primarily used for earthquakes in Southern California, is limited for larger distances and very large earthquakes

Moment Magnitude Scale

• The Moment Magnitude scale depicts the absolute size of earthquakes, comparing information from multiple locations that measure actual energy released
• Estimates of moment magnitude can take days or even months to calculate
• Both scales are used for reporting earthquake magnitude

Modified Mercalli Intensity Scale

• The Modified Mercalli Intensity Scale (MMI) rates ground-shaking intensity based on observable damage and perception, ranges from I (lowest) to X (highest)
• Historically, scientists used the MMI Scale to categorize earthquakes before quantitative measurements of magnitude

Shake Maps

• Shake maps show areas of intense shaking using seismograph data
• Shake maps are useful in the minutes after an earthquake

Earthquake Risk

Factors That Determine Shaking

• Earthquake magnitude impacts potential damage, where larger magnitude, stronger & longer shaking
• Proximity relative to epicenter
• Local Geologic Conditions, seismic waves are affected by ground materials
• Focus depth impacts potential damage, where deeper earthquakes cause less surface shaking

Earthquake Magnitude Scale

• Magnitude 1-3 is only felt by a very few
• Magnitude 3-4 is noticeable indoors, especially on upper floors
• Magnitude 4-5 dishes, doors, cars shake and possibly break
• Magnitude 5-6 everyone feels it, with some items knocked over or broken and building damage possible
• Magnitude 6-7 frightening amounts of shaking, with signifcant damage
• Magnitude > 7 significant destruction of buildings

Factor that determine destruction

• Building materials are key for earthquake survival, unreinforced masonry is easily devastated
• Intensity and duration, where greater shaking and duration results in more devastation
• Resonance is where seismic wave frequency matches its natural motion and increases shaking

Earthquake Recurrence

• A long hiatus in activity on along a fault segment with a history of recurring earthquakes is known as a seismic gap
• Lack of activity on a fault segment may increase the chance of an earthquake recurring due to the buildup of stress
• Geologists dig earthquake trenches across faults to estimate the frequency of past earthquake occurrences

Earthquake Distribution

• Earthquakes aggregate around active boundaries of tectonic plates
• Subduction Zones, found at convergent plate boundaries, are where the largest and deepest earthquakes, called megathrust earthquakes, occur
• Collision Zones between converging continental plates create broad earthquake zones & generate deep, large earthquakes
• Strike-slip Faults, created at transform boundaries, produce moderate-to-large earthquakes
• Continental Rifts and Mid-ocean Ridges, found at divergent boundaries, generally produce moderate earthquakes
• Intraplate Earthquakes are not found near tectonic plate boundaries, but generally occur in areas of weakened crust or concentrated tectonic stress

Secondary Hazards Caused by Earthquakes

• Liquefaction happens when water-saturated, unconsolidated sediments become fluid-like from shaking
• Tsunamis induced by offset sea floors by an earthquake can cause massive destruction
• Shaking can trigger landslides
• Seiches are waves generated in lakes by earthquakes
• Elastic rebound and displacement along the fault plane cause significant land elevation changes

Case Studies

North American Earthquakes

• Basin and Range earthquakes are found in the Basin and Range Province, occur primarily in normal faults created by tensional forces
• New Madrid earthquakes damaged houses in St. Louis, affected the stream course of the Mississippi River, and leveled the town of New Madrid.
• Charleston was an intraplate earthquake; caused significant liquefaction
• Great San Francisco Earthquake and Fire that occurred near San Francisco California along the San Andreas fault, resulted in the city being destroyed
• Alaska earthquake originated in a megathrust fault along the Aleutian subduction zone, caused large areas of land subsidence and uplift
• Loma Prieta earthquake caused deaths, buckled portions of freeways, and collapsed part of the San Francisco-Oakland Bay Bridge

Global Earthquakes

• Shaanxi, China was attributed to the collapse of cave dwellings (yaodong) built in loess deposits
• Lisbon was followed by a tsunami, which brought the total death toll to between 30,000-70,000 people
• Valdivia was the most powerful earthquake ever measured; triggered tsunamis that destroyed houses across the Pacific Ocean
• Tangshan was when most buildings used unreinforced masonry
• Sumatra the resultant tsunamis created massive waves when they reached the shore, killed more than an estimated 200,000 people
• Haiti's widespread infrastructure damage and crowded conditions contributed to a cholera outbreak
• Tōhoku was when most Japanese buildings are designed to tolerate earthquakes, the tsunami it created was more destructive