Download Essentials of Geology, 3rd edition

What is an Earthquake?


Earth shaking caused by a rapid release of energy.

Due to tectonic stresses that cause rocks to break.

Energy moves outward as an expanding sphere
of waves.

This waveform energy can
be measured around the
globe.
Earthquakes destroy buildings and
kill people.

3.5 million deaths in the last
2000 years.
Seismicity

Seismicity (earthquake activity) occurs due to…
 Motion
along a newly formed crustal fracture (or
fault).
 Motion
on an existing fault.
 Inflation
 Volcanic
 Giant
of a magma chamber.
eruption.
landslides.
 Meteorite
 Nuclear
impacts.
detonations.
Earthquake Concepts


Hypocenter (or focus) - The spot within the Earth where
earthquake waves originate.

Usually occurs on a fault surface.

Earthquake waves expand outward from the hypocenter.
Epicenter – Land surface above the hypocenter.
Faults and Earthquakes

Most earthquakes occur along faults.
 Common
crustal fractures that move
rock masses.
 The
amount of movement is termed
displacement.
 Displacement
is also called offset, or
slip.

Markers reveal the amount of offset.
Faults and Fault Motion

Faults are like planar breaks in blocks of crust.

Most faults slope (although some are vertical).

On a sloping fault, crustal blocks are classified as:
 Footwall
(block
below the fault).
 Hanging
wall
(block above the
fault).
Fault Types

Fault type based on relative block motion.
 Normal
fault
Hanging wall moves down relative to footwall.
Result from extension (stretching).
Fault Types

Fault type based on relative block motion.
 Reverse
fault
Hanging wall moves up relative to footwall.
Results from compression (squeezing or
shortening).
Slope (dip)
of fault is
steep.
Fault Types

Fault type based on relative block motion.
 Thrust
fault
Special kind of reverse fault.
Slope (dip) of fault surface is not steep.
Fault Types

Fault type based on relative block motion.
 Strike-slip
fault
Blocks slide laterally past one another.
No vertical block motion.
Fault
surface is
nearly
vertical.
Fault Types

Fault type based on relative block motion.
 Oblique-slip
fault
A combination of dip-slip and strike-slip
displacement.
Most faults display
an oblique-slip
character.
Pure dip-slip or
strike-slip faults are
rare.
Fault Initiation

Tectonic forces add stress to unbroken rocks.


The rock deforms slightly (elastic strain).
 Continued
stress will cause growth of cracks.
 Eventually,
cracks grow to the point of failure.
When the rock breaks, elastic strain transforms into brittle
deformation, releasing earthquake energy.
Fault Motion

Motion along faults occurs in jumps called stick-slip
 Once
movement starts, it quickly stops due to
friction.
 Eventually,
strain
builds up again
causing failure.
 This
behavior is
termed stick-slip
behavior.
Seismic Waves

Body waves – Pass through Earth’s interior.

Compressional, or primary (P), waves
Push-pull (compress and expand) motion.
Travel through solids, liquids, and gases.
Fastest.
Seismic Waves

Body waves – Pass through Earth’s interior.
 Shear,
or secondary (S), waves
“Shaking" motion.
Travel only through solids, not liquids.
Slower.
Seismic Waves

Surface Waves – Travel along Earth’s surface.

Love waves – S waves intersecting the surface.
Move back and forth like a writhing snake.

Rayleigh waves – P waves intersecting the surface.
Move like ripples on a pond.

These waves are the slowest and most destructive.
Seismology

Seismology is the study of earthquake waves.

Seismographs – Instruments that record seismicity.


Worldwide, they detect earthquakes anywhere on Earth.
Seismology reveals size and location of earthquakes.
Seismographs

Measure wave arrivals and magnitude of motion.
 Straight
line = background.
 1st
wave causes frame
to sink (pen goes up).
 Next
vibration
causes opposite
motion.
Seismograph Operation

Waves always arrive in sequence.
P
waves first
S
waves second
 Surface

waves last.
Wave arrivals are captured by the seismograph.
Locating an Epicenter

p and s waves travel at different velocities.

1st arrivals of p and s
waves vary with distance.

A travel-time graph
plots the distance of
each station to the
epicenter.
Locating an Epicenter

Data from 3 stations can pinpoint the epicenter.
A
circle is drawn around each station.
The radius is equal to the distance to
epicenter.
Circles around 3 or
more stations will
intersect.
 The
point of intersection
is the epicenter.
Earthquake Size

Size is described by either intensity or magnitude.

Mercalli intensity scale Degree of shaking damage.

Roman numerals assigned to
different levels of damage.

Damage occurs in zones.

Damage diminishes in
intensity with distance.
Earthquake Size

Magnitude – The amount of energy released.
 Maximum
seismogram motion.
 Normalized

for distance.
Several magnitude scales.
 Richter.
 Moment.

Magnitude scales are
logarithmic.
 Increases
of 1 unit = 10 fold
increase in ground motion.
Measuring Earthquake Size

Earthquake energy release can
be calculated.
 M6.0
– Energy of the
Hiroshima bomb.
 M8.9
– Annual energy
released by all other
earthquakes.
Measuring Earthquake Size

Small earthquakes are frequent.
 ~100,000

magnitude 3 / year.
Large earthquakes are rare.
 32
magnitude 7 earthquakes /
year.
Earthquake Occurrence

Earthquakes linked to plate tectonic boundaries.
 Shallow
– Divergent and transform boundaries.
 Intermediate
and deep – Convergent boundaries.
Earthquake Focal Depths

Shallow – 0-20 km.
 Along
the mid-ocean
ridge.
 Transform
 Shallow
boundaries.
part of trenches.
 Continental
crust.
Earthquake Focal Depths

Intermediate and deep
earthquakes occur along the
subduction trace, the BenioffWadati zone.
 Intermediate
– 20-300 km Downgoing plate still brittle.
 Deep
- 300-670 km - Mineral
transformations?

Earthquakes rare below
670 km (mantle is ductile).
Convergent Boundaries

Cities near subduction zones have frequent
earthquakes.
 Most
are minor.
 Periodically,
Garnet Muscovite Schist
they are devastating.
Continental Earthquakes

Earthquakes in continental crust. 
Continental transform faults (San Andreas, Anatolian).

Continental rifts (Basin and Range, East African Rift).

Collision zones (Himalayas, Alps).

Intraplate settings (ancient crustal weaknesses).
San Andreas Fault

The Pacific plate meets the North American plate.

The San Andreas is a very active strike-slip fault.

A very dangerous fault; hundreds of earthquakes per year.

San Francisco – Destroyed in 1906.

Loma Prieta, 1989, World Series.
Intraplate Earthquakes

5% of earthquakes are not near plate boundaries.

Intraplate earthquakes are not well understood.


Remnant crustal weakness in failed rifts or shear zones?

Stress transmitted inboard?

Isostatic adjustments?
Clusters:

New Madrid, Mo.

Charleston, S.C.

Montreal, P.Q.
Earthquake Damage

Earthquakes kill people and destroy cities.

The damage can be heartbreaking and horrific.

Knowledge improves odds of survival.
Earthquake Damage


Ground Shaking and Displacement

Earthquake waves arrive in a distinct sequence.

Different waves
cause different
motion.
P waves are the 1st to
arrive.

They produce a rapid
up and down motion.
Earthquake Damage

S waves arrive next (2nd).
 They
produce a pronounced back and forth motion.
 This
motion is
usually much
stronger than
from P-waves.
S
waves cause
extensive
damage.
Earthquake Damage

Surface waves lag
behind S waves.
 Long
waves are
the first to follow.
 Ground
writhes like
a snake.
Earthquake Damage

R waves are the last to arrive.
 The
land surface behaves
like ripples in a pond.
 These
waves may last
longer than others.
 Cause
extensive damage.
Earthquake Damage

Severity of shaking and damage depends on…
 Magnitude
(energy) of the earthquake. More = more.
 Distance
from the hypocenter.
 Intensity
and duration of the vibrations.
 The
nature of the subsurface material.
Bedrock transmits waves quickly = less damage
Sediments bounce waves = amplified damage.
Earthquake Damage

Effects on buildings:
 Buildings
“pancake.”
Earthquake Damage

Effects on buildings:
 Bridge
supports crush.
 Masonry
walls break apart.
Earthquake Damage

Landslides and Avalanches.
 Shaking
causes
slopes to fail.
 Hazardous
slopes
bear evidence of
ancient slope failures.
 Rockslides
 Mount
and avalanches follow earthquakes in uplands.
St. Helens erupted via an earthquake landslide.
Earthquake Damage

Liquefaction – Waves liquefy H2O-filled sediments.

High pore pressures force grains apart, reducing
friction.

Liquefied sediments flow as a
slurry.

Sand becomes “quicksand;” clay
becomes “quickclay.”
Sand dikes.
Sand volcanoes.
Contorted layering.
Liquefaction

Water saturated sediments turn into a mobile fluid.

Land will slump and flow.

Buildings may founder and
topple over intact.
Earthquake Damage

Fire is a common hazard following earthquakes.

Shaking topples stoves, candles, and power lines.

Broken gas mains and fuel tanks ignite a conflagration.

Earthquakes destroy critical infrastructure such as water, sewer,
telephone, and electrical lines, as well as roads.

Firefighters powerless.
No road access.
No water.
Too many hot spots.

Good planning is
crucial to saving lives.
Earthquake Damage

Earthquake devastation fuels disease outbreaks.
 Food,
water, and medicines are scarce.
 Basic
sanitation capabilities disabled.
 Hospitals
 Health
 There
damaged or destroyed.
professionals overtaxed.
may be many decaying corpses.
Earthquake Damage

Tsunamis, or seismic sea waves (not tidal waves).

Tsunamis result when earthquakes change the seafloor.

Normal faulting drops the seabed; thrusting raises it.
This displaces the entire volume of overlying water.
A giant mound (or trough) forms on the sea surface.
This feature may be enormous (up to a 10,000 mi2 area).
Feature collapse creates waves that race rapidly away.
Earthquake Damage


Destructive tsunamis occur frequently - about 1/yr.

94 destructive tsunamis in the last 100 years.

51,000 victims (not including 12/26/04)
Future tsunami disasters are inevitable.


Growing human population in low-lying coastal areas.
Education about tsunamis can save many lives.
Tsunami vs. Wind Waves

Wind waves






Influence the upper ~100 m.
Have wavelengths of several tens to
hundreds of meters.
Wave height and wavelength related
to windspeed.
Wave velocity maximum several tens
of km per hour.
Waves break in shallow water and
expend all stored energy.
Tsunami waves
 Influence entire water depth

Have wavelengths of several 10s
to 100s of kilometers.

Wave height and wavelength
unaffected by windspeed.

Wave velocity maximum several
100s of kph.

Waves come ashore as a raised
plateau of water that pours onto
the land.
Tsunami Behavior

Tsunamis race at jetliner speed
across the ocean.

They may be almost imperceptible in
deep water.


Low wave height (amplitude).

Long wavelength (frequency).
As water shallows, waves
slow from frictional drag.

Waves grow in height,
reaching 10-15 m or more.
Tsunami

Tsunami destruction of the coast depends upon…

Offshore bathymetry.
Broad shallows increase amplitude but sap wave
energy.
Quick deep-to-shallow transition – Deadliest condition.
Waves have maximum energy.
Wave heights are modest.
Water pours onto land as a sheet.

Topography of shore.
Broad lowland – Maximum damage.
Steep rise of land – Less damage.
Tsunami Reality

The Indian Ocean Tsunami

On December 26, 2004, a strong megathrust earthquake (M9.0+)
originated in the trench to the west of N. Sumatra.

The earthquake was the largest in 40 years.

Displacement exceeded 15 m; rupture > 1100 km long.

The devastating tsunami killed people in 10 countries surrounding the
Indian Ocean.
The Indian Ocean Tsunami


Killed more people than any tsunami on record.

227,898 were killed or missing and presumed dead.

1.7 million people were displaced (as of 6/4/2009).
Record-setting death toll.

The earthquake was
so large and the
tsunami spread fast.

Coasts were full of
Christmas tourists.
The Indian Ocean Tsunami

Complete devastation below “run-up” elevation.
 Dense
 Entire
coastal development in Banda Aceh hardest hit.
communities were erased – buildings and people.
Earthquake Prediction

Prediction would help reduce catastrophic losses.

Can we predict earthquakes? Yes and no.

They CAN be predicted – long-term (tens to hundreds of
years).

They CANNOT be predicted - short-term (hours to
months).

Seismic hazards are mapped to
assess risk.

This information is useful for…

Developing building codes.

Land-use planning.

Disaster planning.
Long-Term Earthquake Prediction

Probability of a certain magnitude earthquake occurring
on a timescale of 30 to 100 years, or more.

Based on the premise that earthquakes are repetitive.
Long-Term Earthquake Prediction

Require determination of seismic zones, by…
 Mapping
historical epicenters (after ~ 1950).
 Evidence
of ancient earthquakes (before seismographs).
Evidence of
seismicity –
Fault scarps,
sand volcanoes, etc.
Historical records.

Seismic gaps, places that haven’t slipped recently.
Long-Term Earthquake Prediction

Recurrence interval – Average time between events.
 Historical
records.
 Geologic
evidence –
Requires radiometric
dating of events.
Sand volcanoes.
Offset strata.
Drowned forests.
Short-Term Earthquake Prediction

Goal: The location and magnitude of a large earthquake.

Currently, we can’t reliably predict short-range events.

Earthquakes do have precursors.

Clustered foreshocks.

Crustal strain.

Stress triggering.

And, possibly…
Water level changes in wells.
Gases (Rn, He) in wells.
Unusual animal behavior.
Preparing for Earthquakes

We can’t stop them but we can be ready for them.
 Understand
 Map
what happens during an earthquake.
active faults and areas likely to liquefy from shaking.
 Develop
construction codes to reduce building failures.
 Regulate
land-use to control development.