GEO 102: Earthquakes and Earth's Interior

Understanding Earth’s Interior: Astronomy

Used the average acceleration of the Earth to figure out average density

Kimberlite Pipes

Narrow, vertical, pipe-shaped bodies of rock that form deep in the mantle, at depths between roughly 150 and 450 kilometers, and rise toward the surface very rapidly and violently

Xenoliths

Fragments of the rock from deep below the Earth carried upward by kimberlites

Ophiolite

A slice of oceanic crust and the upper mantle beneath it that has been lifted up onto a continent

Understanding Earth’s Interior: Earthquakes

• By far the most powerful tool for probing the Earth’s interior

• Vibrations of Earth produced by rapid release of energy 

• Energy release due to plate tectonics and fault zones 

• Energy radiates in waves from the source (focus) 

• Epicenter: surface expression of the focus 

Elastic Rebound Theory

• Two blocks of rock along a fault are forced past each other; they bend and store energy

• When the stress finally exceeds the strength of the rock, the fault slips suddenly and the rock snaps back toward its original shape

• Elastic Region: Once stress is removed object returns to original size/shape 

  Plastic Region: permanently deformed by the stress 

Where Earthquakes Occur

• Divergent boundaries: where plates pull apart, earthquakes are frequent but shallow and modest in size (mid ocean ridges)

• Transform boundaries: where plates grind horizontally past each other, earthquakes are shallow but can be large

• Convergent boundaries: where one plate is forced beneath another, most powerful earthquakes of all

Benioff Zone

• Slopping band the earthquake foci trace

• Begins shallow at the ocean trench and follows the descending slab down as deep as 700 kilometers

Intraplate Earthquakes

• Earthquakes that occur within the stable interior of a plate

• Usually along ancient faults buried beneath younger rock

• EX: New Madrid Seismic Zone- where Missouri, Tennessee, Arkansas, and Kentucky meet in the Mississippi River valley

Understanding Earth’s Interior: Drilling

Crust is around 40 kilometers thick 

• Deepest hole ever dug was 12km deep 

• Kola Superdeep Borehole (in Russia) 

• Had to abandon the project because drill was melting 

Body waves

Travel through the interior, or body, of the Earth, and because they sample the deep planet they are the waves that reveal its structure

• P and S Waves

Surface waves

Travel along the surface, like ripples spreading across a pond, and although they cause much of an earthquake's destruction, they tell us less about the deep interior

Primary Waves (P-Waves)

• Compressional (particle motion parallel to wave propagation) 

• Travel through all materials 

• Fastest wave 

• Smallest amplitude 

Moves by alternately compressing and expanding the material it travels through, so the rock vibrates back and forth in the same direction the wave is moving, much as a sound wave travels through air

Secondary Wave (S-Wave)

• Shear waves (particle motion perpendicular to wave propagation) (moves the rock from side to side, at right angles to the direction the wave is traveling)

• Only travel through solids 

• Second to arrive at seismic station 

• S waves shake the ground more strongly than P waves and cause much of the damage in an earthquake

• They have one property that turns out to be extraordinarily useful for studying the interior: an S wave can travel only through solids

Surface Waves

• Travel along the ground itself

• Love waves and Rayleigh waves

• Cause the most damage

• Can move in all directions

• Most damaging waves

• Last to arrive at seismic station

• Greatest amplitude

travel more slowly than body waves and arrive last, but because their energy is concentrated near the surface they often produce the most violent shaking and the greatest damage to buildings

Love Waves

move the ground from side to side in a horizontal, snaking motion,

Rayleigh Waves

roll the ground in a circular, up-and-down motion, like an ocean swell passing beneath a boat

Wave Speed & Rock

• Loose, water-logged sediment and soil carry seismic waves slowly

• Sedimentary rocks such as sandstone and shale are faster, solid crystalline rocks such as granite and basalt faster still, and the dense rock of the mantle faster than any rock found at the surface

• Wave speed also increases with depth, because the growing pressure squeezes rock into a stiffer, denser state

• A sudden jump in wave speed marks a boundary between two kinds of rock, and this is exactly how the base of the crust was first detected

Reflection

• When a seismic wave meets such a boundary, part of its energy bounces back

• The return of a wave off a boundary, much as light reflects from a mirror

Refraction

• When a seismic wave meets such a boundary part of its energy continues onward in a new direction

• The bending of the wave that continues across the boundary, a change in the direction and speed of a wave as it passes from one material into another

Locating Earthquakes

• Three recording stations used to pinpoint epicenter

• A station close to the earthquake records the P and S waves arriving close together; a distant station records a long gap between them

• The size of that gap tells a recording station how far away the earthquake was, though not in which direction

Seismic Shadow Zones

• The vast S-wave shadow is therefore direct proof that a large part of the Earth's interior, the outer core, is molten. A single missing wave reveals a liquid layer thousands of kilometers down

• The existence and exact width of the P-wave shadow zone reveal both the size of the core and the depth of its boundary with the mantle

Measuring Earthquake Size

Three Scales:

• Richter: records amplitude of seismic waves ( a measure of energy); logarithmic, no fixed limit

• *Moment Magnitude: records energy from fault area and slip, based on energy and physical observations (best for large quakes) (logarithmic, no fixed limit)

• Mercalli: records intensity of shaking and destruction at a place (I to XII)

Richter Scale

• Magnitude (energy) 

• Measures amplitude of largest wave 

• 1 digit increase is a 30-fold increase in energy 

Mercalli index

• Assesses damage 

• Based on human observation 

Moment Magnitude

Based on energy released and physical observations

Each step up the scale means about 10 times stronger shaking and about 32 times more energy released 

• A magnitude of 7 unleashes roughly 1,000 times the energy of a magnitude 5 

• Anything <3 is not felt by humans 

• More low magnitude earthquakes than high magnitude 

Tsunamis

• Waves created by earthquakes 

• Move at speeds between 500 and 950 km/hr (300-600 mil/hr) 

• Waves reach height of over 30 meters (100ft) 

• Occur near subduction zones/ fault lines 

Liquefaction

• Pores of sand contain liquid, when pressure increases liquid pushes grains apart 

• Sand is no longer solid, acts like a fluid 

• Quicksand 

Time Interval Analysis 

• Statistics is used to generate a “best-fit line” which can be used to predict the time of the next earthquake 

• Not reliable; earthquakes are not linear 

Seismic Gaps

• Areas that have had earthquakes recently are less likely to have them again because the energy has already been released 

Why this is unreliable: 

Smaller earthquakes can release stress so larger more devastating earthquakes don’t occur