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Restless Earth: Earthquakes and Mountain Building

Chapter 6 Lecture

Natalie Bursztyn Utah State University

Foundations of Earth Science Eighth Edition

• Sketch and describe the mechanism that generates most earthquakes.

Focus Question 6.1

• The sudden movement of one block of rock slipping past another along a fault

• Most faults are locked until a sudden slip • Seismic waves radiate out from the focus

(hypocenter), where slip begins – Earth’s surface directly above the hypocenter is the

epicenter

What Is an Earthquake?

• Weak earthquakes can be generated by – Volcanoes, landslides, and meteorite impacts

• Destructive earthquakes occur because tectonic motion builds up stress – Friction keeps the fault from slipping – Slip initiates when stress overcomes friction – Elastic rebound causes deformed rock to spring back

to undeformed position

Discovering the Causes of Earthquakes

• Convergent plate boundaries generate compressional forces – Mountain building and faulting associated with large

earthquakes – Subducting plates form a megathrust fault

• Produce the most powerful earthquakes • Vertical motion underneath the ocean generates tsunamis

Faults and Large Earthquakes

• Transform faults have many branches and fractures – Offset occurs in distinct segments

• Some segments displace slowly with little shaking during fault creep

• Some segments produce numerous small earthquakes • Some segments are locked for hundreds of years and

rupture in large earthquakes – Earthquakes occur in repetitive cycles – Rupture propagates in a discrete time period

Faults and Large Earthquakes

What Is an Earthquake?

• Compare and contrast the types of seismic waves. • Describe the principle of the seismograph.

Focus Questions 6.2

• The study of earthquake waves is known as seismology

• Waves are measured by seismometers – Inertia keeps a weighted arm from moving while

ground motion moves the instrument – Amplifies ground motion – Generates seismograms

Seismology: The Study of Earthquake Waves

Instruments That Record Earthquakes

• Two main types of seismic waves generated by earthquakes: – Surface waves travel in rock layers just below Earth’s

surface – Body waves travel through Earth’s interior

Seismic Waves

• Two types of body waves: – Primary or P waves

• Push/pull rocks in direction that wave is traveling • Temporarily change volume of material • Travel through solids, liquids, and gasses

– Secondary or S waves • Shake particles at right angles to direction that wave is

traveling • Change shape of material • Do not travel through liquids or gasses

Seismic Waves

• Two types of surface waves: – Some travel along Earth’s surface like rolling ocean

waves – Others move Earth materials from side to side

• Most damaging type of ground motion

Seismic Waves

• Speed of travel is very different for each type of wave

Seismic Waves

• Locate Earth’s major earthquake belts on a world map.

• Label the regions associated with the largest earthquakes.

Focus Questions 6.3

• Circum-Pacific belt convergent boundaries experience 95% of earthquakes – Megathrust faults generate largest earthquakes

• Earthquakes along Alpine-Himalayan belt because of continental collision

• Weak earthquakes along oceanic ridge system because tension pulls plates apart

• Transform and strike-slip faults generate large, cyclical earthquakes

Earthquake Associated with Plate Boundaries

• Less frequent earthquakes in central and eastern United States

• Produce large areas of structural damage – Underlying bedrock is older and more rigid – Waves travel greater distances with less attenuation

• Intraplate earthquakes occur away from plate boundaries

Damaging Earthquakes East of the Rockies

• P waves travel faster than S waves • Difference in arrival time is exaggerated by

distance – Greater interval between P and S wave arrivals

indicates greater distance to epicenter

Locating the Source of an Earthquake

• Distinguish between intensity scales and magnitude scales.

Focus Question 6.4

• Intensity measures the amount of ground shaking based on property damage

• Magnitude is a quantitative measure of energy released in an earthquake – Developed more recently

Determining the Size of an Earthquake

• Used for historical records • Modified Mercalli Intensity Scale developed in

California in 1902 • Community Internet Intensity Map

– Website developed by U.S.G.S. – Generated by user input

Intensity Scales

• Richter scale is related to the amplitude of the largest seismic wave – Logarithmic scale

• 10-fold increase in wave amplitude corresponds to increase of 1 on the scale

• Each unit of increase equates to a 32-fold increase in the energy released

– Not adequate for describing large earthquakes

Magnitude Scales

• Moment magnitude measures total energy released based on amount of slide, area of rupture, and strength of faulted rock – Better at estimating the relative size of very large

earthquakes

Magnitude Scales

• List and describe the major destructive forces that earthquake vibrations can trigger.

Focus Question 6.5

• Magnitude and other factors determine degree of destruction

• Area 2050 km surrounding the epicenter experiences equal shaking

• Ground motion diminishes rapidly outside of 50 km • Earthquakes in stable interiors are felt over a

larger area

Earthquake Destruction

• Earthquake damage depends on: – Intensity – Duration – Nature of surface materials – Nature of building materials – Construction practices

• Flexible wood and steel-reinforced buildings withstand vibrations better

• Blocks and bricks generally sustain the most damage

Destruction from Seismic Vibrations

• Soft sediment amplifies vibrations • Vibrations cause loosely packed, waterlogged

materials to behave like a fluid – During liquefaction stable soil becomes mobile and

rises to the surface • Vibrations can also cause landslides, ground

subsidence, and fires

Destruction from Seismic Vibrations

• Megathrust displacement lifts large slabs of seafloor, displaces water, and generates a tsunami – Low amplitude wave travels at very high speed in open

ocean – Amplitude can reach tens of meters in shallow coastal

waters – Arrival on shore is preceded by a rapid withdrawal of

water from beaches, followed by what appears as a rapid rise in sea level with a turbulent surface

Tsunamis

• Tsunami warning system developed after 1946 Hawaiian tsunami – Seismic observatories

report large earthquakes – Deep-sea buoys detect

energy released by earthquakes

– Tidal gauges measure rise and fall in sea level

Tsunamis

• Explain how Earth acquired its layered structure. • List and describe each of its major layers.

Focus Questions 6.6

• Earth has distinct layers: – Heaviest material at the center, lightest at top – Iron core, rocky mantle and crust, water ocean, gaseous

atmosphere • Interior is dynamic

– Mantle and crust are in motion – Material is recycled from surface to deep interior

Earth’s Interior

• Seismic waves are the only way to “see” inside the interior – Waves are reflected at boundaries – Refracted through layers – Diffracted around obstacles – Velocity increases with depth as stiffness and

compressibility of rock change • Can be used to interpret composition and temperature of

rock

Probing Earth’s Interior: “Seeing” Seismic Waves

Earth’s Interior

• Temperature increased as material accumulated to form Earth – Iron and nickel melted and sank to the center to

produce iron-rich core – Buoyant rock rose to the surface and formed crust rich

in O, Si, and Al (+ Ca, Na, K, Fe, and Mg) – Chemical segregation led to iron-rich core, primitive

crust, and mantle

Earth’s Layered Structured

• Earth is divided into three compositionally distinct layers: – Crust – Mantle – Core

• Can be further subdivided into zones based on physical properties – Solid or liquid – Strength

Earth’s Layered Structure

• Thin, rocky crust is divided into: • Oceanic crust

– ~7 km thick – Composed of basalt – Density ~3.0 g/cm3

• Continental crust – ~35 – 40 (up to 70) km thick – Many rock types – Average density ~2.7 g/cm3

Earth’s Layered Structure

• Mantle – Solid layer extending to 2900 km – 82% of Earth’s volume

• Chemical change at boundary between crust and mantle

• Uppermost mantle (first 660 km) is peridotite – Stiff top of upper mantle (+ crust) is lithosphere

• Cool, rigid outer shell • ~100 km thick

• Weaker portion below is asthenosphere – Upper asthenosphere is partially melted – Lithosphere moves independently of asthenosphere

Earth’s Layered Structure

• Core is an ironnickel alloy – Density ~10 g/cm3 – Outer core is liquid

• 2270 km thick • Generates Earth’s magnetic field

– Inner core is solid sphere • 1216 km radius

Earth’s Layered Structure

Earth’s Layers

• Compare and contrast brittle and ductile deformation.

Focus Question 6.7

• Deformation – All changes in shape, position, or orientation of a rock

mass – Bending and breaking occurs when stress exceeds

strength

Rock Deformation

• Elastic deformation – Stress is gradually applied – Rocks return to original size and shape when stress is

removed – Ductile or brittle deformation occurs when elastic limit is

surpassed • Strength of a rock is influenced by temperature,

confining pressure, rock type, and time

Types of Rock Deformation

• Brittle deformation – Results in fractures – Common at/near surface

• Ductile deformation – Solid-state flow at great depths – Produces a change in the size and shape of a rock – Some chemical bonds break and new ones form

Types of Rock Deformation

• List and describe the major types of folds.

Focus Question 6.8

• Folds are wavelike undulations that form when rocks bend under compression

• Compressional forces result in shortening and thickening of the crust

Folds: Structures Formed by Ductile Deformation

• Anticlines – Upfolded or arched layers

• Synclines – Associated downfolds or troughs

• Symmetrical – Limbs are mirror images

• Asymmetrical – Limbs are different

Anticlines and Synclines

• Overturned – One or both limbs are tilted beyond the vertical

• Recumbant – Axis is horizontal

Anticlines and Synclines

• Circular or elongated upwarping structures are called domes – Upwarps in basement

rocks deform overlying sedimentary strata

• Downwarping structures are called basins

Domes and Basins

• Monoclines are large, step-like folds – Large blocks of basement

rock displaced upwards – Ductile sedimentary strata

above drape over the fault

Monoclines

• Sketch and describe the relative motion of rock bodies located on opposite sides of normal, reverse, and strike-slip faults.

Focus Question 6.9

• Faults – Fractures in the crust with appreciable displacement – Movement parallel to dip are dip-slip faults

• Rock surface above the fault is the hanging wall block

• Surface below the fault is the foot-wall block • Fault scarps

– Long, low cliffs produced by vertical displacement

Faults: Structures Formed by Brittle Deformation

Dip-Slip Faults

• Normal faults – Hanging wall moves down relative to footwall – Accommodate extension of crust

• Fault-block mountains are associated with large normal faults – Uplifted blocks are elevated topography called horsts – Down-dropped blocks are basins called grabens

Dip-Slip Faults

• Reverse faults – Hanging wall moves up relative to footwall

• Thrust faults – Reverse faults with a dip of less than 45º – Result from compressional stress – Accommodate crustal shortening

Dip-Slip Faults

• Strike-slip fault – Exhibits horizontal displacement – Parallel to strike

Strike-Slip Fault

• Joints – Not a fault – Fractures with no appreciable displacement – Develop in response to regional upwarping and

downwarping

Joints

• Locate and identify Earth’s major mountain belts on a world map.

Focus Question 6.10

• Orogenesis is the set of processes that forms a mountain belt

• Older mountain chains are eroded and less topographically prominent

• Compressional mountains – Large quantities of preexisting sedimentary and

crystalline rocks that have been faulted and folded

Mountain Building

• Sketch an Andean-type mountain belt and describe how each of its major features is generated.

Focus Question 6.11

• Subduction of oceanic lithosphere is the driving force of orogenesis – Volcanic island arcs form where subduction occurs

beneath oceanic lithosphere – Continental volcanic arcs form where subduction

occurs below a continental plate • Also forms mountainous topography along continental

margin

Subduction and Mountain Building

• Volcanic island arcs build mountains by volcanic activity, emplacement of plutons, and the accumulation of sediment from the subducting plate onto the upper plate

Island Arc-Type Mountain Building

• Continental volcanic arcs form at Andean-type convergent zones – Before subduction, sediment accumulates on a passive

continental margin – Becomes an active continental margin when a

subduction zone forms and deformation begins – An accretionary wedge is an accumulation of

sedimentary and metamorphic rocks scraped from the subducting plate

Andean-Type Mountain Building

• Summarize the stages in the development of a collisional mountain belt such as the Himalayas.

Focus Question 6.12

• Cordilleran-type mountain building occurs in Pacific-like ocean basins – Rapid seafloor spreading is balanced by rapid

subduction – Island arcs and crustal fragments (terranes) collide

with a continental margin • Some terranes may have been microcontinents • Small terranes are subducted • Larger terranes are thrust onto the continent

Collisional Mountain Belts

Cordilleran-Type Mountain Belts

• Alpine-type orogenies result from continental collisions

• Himalayas – Collision between Indian and Eurasian plates

Alpine-Type Mountain Belts

[insert Figure 6.47 here]

• Appalachians – Generated by orogenies that lasted a few hundred

million years • One of the stages in assembling Pangaea

– Resulted from three distinct stages of mountain-building

Alpine-Type Mountain Belts