9 Crustal Deformation and Earthquakes

9 Crustal Deformation and Earthquakes

• Example of normal faulting in the Pennsylvanian Honaker Trail Formation near Moab, Utah.

KEY CONCEPTS

• By the end of this chapter, students should be able to:

  • Differentiate between stress and strain.

  • Identify the three major types of stress.

  • Differentiate between brittle, ductile, and elastic deformation.

  • Describe the geological map symbol used for strike and dip of strata.

  • Name and describe different fold types.

  • Differentiate the three major fault types and describe their associated movements.

  • Explain how elastic rebound relates to earthquakes.

  • Describe different seismic wave types and how they are measured.

  • Explain how humans can induce seismicity.

  • Describe how seismographs work to record earthquake waves.

  • From seismograph records, locate the epicenter of an earthquake.

  • Explain the difference between earthquake magnitude and intensity.

  • List earthquake factors that determine ground shaking and destruction.

  • Identify secondary earthquake hazards.

  • Describe notable historical earthquakes.

9.1 Stress and Strain

• Crustal deformation occurs when applied forces exceed the internal strength of rocks, physically changing their shapes.

• Stress: The force exerted per unit area.

• Strain: The physical change that results in response to that force.

• When applied stress exceeds the internal strength of rock, strain results in the form of deformation, which may include changes in rock volume, shape, or fractures.

• Three types of stress:

  • Tensional Stress:

  • Forces pulling in opposite directions.

  • Results in strain that stretches and thins rock.

  • Compressional Stress:

  • Forces pushing together.

  • Causes rock folding and thickening.

  • Shear Stress:

  • Transverse forces causing opposing regions to move past each other.

Table of Types of Stress and Associated Strain

9.2 Deformation

• Different materials deform differently when stress is applied:

  • Material A: Relatively little deformation, then plastic deformation, and finally brittle failure.

  • Material B: Only elastically deforms before brittle failure.

  • Material C: Undergoes significant plastic deformation before brittle failure.

• Types of deformation:

  • Elastic Deformation:

  • Strain that is reversible after stress is released (e.g., stretching a rubber band).

  • Ductile Deformation:

  • Permanent shape changes after sufficient stress (e.g., bending a metal bar).

  • Yield Point: The point where elastic deformation transitions to ductile deformation.

  • Brittle Deformation:

  • Occurs when rock integrity fails, leading to fractures.

• Factors influencing deformation:

  • Pore pressure: Pressure exerted by fluids in rock pores.

  • Strain rate: Speed of deformation (e.g., slow stress application allows wood to bend without breaking).

  • Rock strength: Ability of rock to resist deformation (e.g., shale vs granite).

  • Temperature: Increased temperature tends to increase ductility.

Table of Factors Affecting Rock Strain

9.3 Geological Maps

• Geological maps are 2D representations of geological formations and structures at the Earth’s surface, showing formations, faults, folds, and rock types.

• Formations: Recognizable rock units marked by color and labeled on maps.

  • Labels indicate the geological time period and formation name.

9.3.1 Cross Sections

• Depict subsurface geology in the vertical plane versus the horizontal plane on geological maps.

9.3.2 Strike and Dip

• Strike: Orientation of rock layers relative to North/South and horizontal.

• Dip: Angle at which a bed plunges into the Earth.

• Strike and dip symbols resemble 'T'; the trunk represents dip direction, top line denotes strike.

• Example: N 30° E means a line at an angle of 30° northeast from true north.

• The dip is the angle of descent, e.g., 45° SE indicates a downward slope in that direction.

• Horizontal rock bed: 0° dip; vertical bed: 90° dip.

9.4 Folds

• Geological folds are layers of rock that have been bent by ductile deformation, typically due to compressional forces at depth.

• Axial plane: Splits the fold into two halves; fold axis: Line of bending; hinge line: Line of most bending.

Types of Folds

1. Anticline: Convex-upward folds with the oldest rocks in the center.

   • Antiform: Same shape as anticline, but relative ages cannot be determined.

2. Syncline: Trough-like folds with the youngest rocks in the center.

   • Synform: Shape similar to a syncline without age distinction.

3. Monocline: Step-like folds where flat rocks are warped.

4. Dome: Upwarped rock beds shaped like an inverted bowl.

5. Basin: Bowl-shaped depression with beds dipping towards the center.

9.5 Faults

• Faults are fractures where blocks of rocks move relative to one another, typically categorized by motion type.

• Normal Faults: Hanging wall moves down relative to footwall, created by tensional forces (common at divergent plate boundaries).

• Reverse Faults: Hanging wall moves up relative to footwall due to compressional forces (include thrust faults).

• Strike-Slip Faults: Side-to-side motion associated with transform plate boundaries.

• Common terms: Fault scarp (surface exposure), slickensides (polished rock surfaces), joints (fractures without displacement).

9.5.1 Normal Faults

• Vertical movement and common in the Basin and Range Province.

• Normal faulting creates grabens (lowered blocks), horsts (raised blocks), and half-grabens (asymmetrical blocks).

9.5.2 Reverse Faults

• Reverse faults include low-angle thrust faults and megathrust faults, which can trigger significant earthquakes and tsunamis.

9.5.3 Strike-slip Faults

• Side-to-side motion with sinistral (left-lateral) and dextral (right-lateral) designations, creating compressional or tensional features, such as flower structures.

9.6 Earthquake Essentials

• Earthquakes are felt when energy is released from blocks sliding past each other along faults, creating seismic waves. Most earthquakes occur along active plate boundaries.

9.6.1 How Earthquakes Happen

• Elastic Rebound Theory explains the build-up and release of strain at faults. Stored energy leads to brittle failure, with released energy resulting in seismic waves.

• Initial rupture point known as focus (or hypocenter); the point on the surface directly above is called the epicenter.

9.6.2 Focus and Epicenter

• Focus: Depth at which rupture occurs; seismic waves emanate outwards.

• Epicenter: Vertical projection above the focus on the Earth's surface, affected area of the earthquake.

9.6.3 Seismic Waves

• Seismic waves are waves of energy released during an earthquake, classified as body waves (P and S waves) and surface waves.

  • P Waves: Fastest, compressional waves; can travel through solids, liquids, gases.

  • S Waves: Slower, shear waves; can only travel through solids.

  • Surface Waves: Cause most of the shaking and destruction; include Love waves (horizontal motion) and Rayleigh waves (elliptical motion).

• Seismic ray: Path of a specific point on the wave front, curves with density variations in the Earth.

9.6.4 Induced Seismicity

• Induced seismicity occurs from human actions, such as hydrocarbon extraction and wastewater injection, which increase pore pressure and reduce friction, causing earthquakes.

9.7 Measuring Earthquakes

• Seismographs measure ground vibrations; can detect even small earthquakes.

• Seismogram: A record of seismic waves; measured in 3 axes: north-south, east-west, up-down.

• Location of the epicenter is determined using triangulation based on the arrival times of P, S, and surface waves from multiple stations.

9.7.1 Seismographs

• Seismographs convert ground motion into visual data; can measure the arrival of seismic waves on a seismogram.

• Digital seismographs use modern technology to record seismic data.

9.7.2 Seismograph Network

• The Global Seismic Network provides real-time data from multiple seismic stations globally, aiding in earthquake monitoring and nuclear test detection.

9.7.3 Seismic Tomography

• Seismic tomography creates 3D images of Earth's internal structures using data from many seismic events, akin to a CT scan.

9.7.4 Earthquake Magnitude and Intensity

• Magnitude measures energy release in logarithmic fashion.

• Richter Scale (ML): Historical magnitude scale based on maximum amplitude of seismic waves.

  • Logarithmic scale indicates a 10-fold amplitude increase for each unit.

  • Energy released increases 32 times per magnitude unit.

• Moment Magnitude Scale (MW): More accurate for larger, complex earthquakes, considers area of rupture and rock properties.

• Modified Mercalli Intensity Scale (MMI): Qualitative scale based on observed damage and human response, ranging from I (not felt) to X (total destruction).

• Shake Maps: Computer-generated maps showing areas of intense shaking post-earthquake.

9.8 Earthquake Risk

• Factors influencing shaking:

  • Magnitude: Larger magnitude generally results in stronger shaking.

  • Location and Direction: Closer proximity to the epicenter increases shaking intensity.

  • Local Geologic Conditions: Various material types affect how ground motions are amplified.

  • Focus Depth: Depth of the earthquake influence surface shaking; deeper earthquakes tend to shake less at the surface.

9.8.1 Factors that Determine Destruction

• Building Materials: Flexibility impacts resilience; unreinforced masonry is susceptible.

• Intensity and Duration: Higher shaking intensity and longer duration cause more damage.

• Resonance: When seismic wave frequency matches the building's natural frequency, shaking increases significantly.

9.8.2 Earthquake Recurrence

• Trench studies allow geologists to analyze fault activity over time to assess recurrence intervals and seismic gaps.

9.8.3 Earthquake Distribution

• Earthquake occurrence often clusters around tectonic plate boundaries.

  • Subduction Zones: Largest earthquakes occur here.

  • Collision Zones: Broad zones create large earthquakes.

  • Transform Boundaries: Produce moderate-large earthquakes; notable examples include the San Andreas Fault.

  • Divergent Boundaries: Generally moderate earthquakes occur here.

  • Intraplate Earthquakes: Occur away from plate boundaries; e.g., New Madrid seismic zone.

9.9 Case Studies

• North American Earthquakes:

  • Basin and Range Earthquakes: Normal faults and tensional forces.

  • New Madrid Earthquakes (1811-1812): Intraplate earthquakes that significantly altered landscapes and river courses.

  • Charleston (1886): Major disaster within a historic intraplate seismic zone.

  • San Francisco Earthquake (1906): Magnitude 7.8 catastrophe with aftershocks causing extensive damage.

  • Alaska (1964): Magnitude 9.2 megathrust earthquake causing massive alteration of the landscape.

• Global Earthquakes:

  • Shaanxi, China (1556): Deadliest earthquake, with 830,000 fatalities attributed to cave collapses.

  • Lisbon, Portugal (1755): Magnitude 8-9 earthquake followed by tsunamis causing tens of thousands of deaths.

  • Valdivia, Chile (1960): Most powerful earthquake recorded; generated destructive tsunamis.

  • Tangshan, China (1976): Moment magnitude 7.8 earthquake caused over 240,000 fatalities due to lack of preparedness.

  • Sumatra, Indonesia (2004): Generated significant tsunamis leading to massive coastal devastation.

  • Haiti (2010): Major earthquake with extensive infrastructure damage and high casualty rates.

  • Tōhoku, Japan (2011): Resulted in significant seismic activity and a catastrophic tsunami, including nuclear meltdown consequences.

Summary

• Geologic Stress types: Tension, shear, and compression; results in strain and deformation types: elastic, ductile, and brittle.

• Geological maps depict surface formations; strike and dip symbols orient rock strata.

• Faults: Result from stress overload, categorized into normal, strike-slip, and reverse faults.

• Earthquakes occur due to elastic rebound and release energy as seismic waves (P, S, surface waves).

• Magnitudes measured on scales (Richter, Moment Magnitude) indicate energy release; MMI Scale assesses damage based on perceptions.

• Damage factors: including magnitude, site conditions, and building materials; secondary hazards include liquefaction, tsunamis, and landslides.