Revised 10 / 10 (Monroe 6th ed.)

Earthquakes and the Earth's Interior - Chapters 10 and 11




Case Histories

The United States



Europe and the Middle East

South America

Occurrence and Causes of Earthquakes

Measurements of Seismic Waves

Intensity vs. Magnitude

Seismic Waves and the Earth's Interior

People and Earthquakes

Prediction & Control of Earthquakes



Cause of many of the earth's best natural processes (or disasters)

Remember, I love natural processes

Only a disaster when there is damage to humans and/or their stuff

And there is plenty of human impact!

Approx. 15,000,000 fatalities in last 4000 years

Approx. 1,000,000 quakes per year

Most relatively minor

Approx. 50 cause significant damage

Approx. 10 "major" events

Always have a great effect on people

Destruction in a "moment's time"

But a much slower process building to the event

See Strickler's 2nd Law of GeoFantasy

Sinister events - no warning

A great quake is completely overpowering

We are helpless in every respect

And the magnitude of the destruction can be immense

Beyond our capacity to restrict or control

Stay out of the way or hang on tight


Case Histories...

The United States

We've actually been extremely fortunate

Relatively minimal damage during recorded history

San Francisco - The "great American quake"

April 18, 1906 at 5:12 a.m.

Affected nearly 400,000 sq. mi.

Estimated at 8.25 (Richter)

Extensive damage from quake and following fire

Loss of life reported at 700 people

May be B.S. - most likely quite a bit higher

Easy to underestimate casualties due to living conditions at that time

Also, evidence that the city downplayed the loss for PR reasons

Severity of the quake varied

Due in part to subsurface conditions (Monroe; fig. 10-12, pg. 312)

Solid rock - Hard shake and then stop

Loose fill - like jello; keeps shaking

Liquefaction in extreme cases

The Fire - actually caused the most damage

Mostly caused by severed gas lines

Hard to fight

Wooden structures

Broken water lines

Dazed & confused survivors

Tried leveling whole blocks to minimize roof-top advance

Inexperienced blasters

Scattered burning debris to new areas

Like what happened in Hawaii when the plague hit the Chinese

Significant event

California Earthquake Commission

"When in doubt, form a committee"

Rupture of the San Andreas fault (Monroe; pg. 316)

Right lateral strike-slip movement

Approx. 300 miles involved in the 1906 quake

Max. offset 21' at Tomales Bay

Average nearly 15'

New Madrid, Missouri (1811-12)

Read eyewitness account (Monroe, page 306)

Three quakes (late Dec. 1811 to early Feb. 1812)

Many aftershocks for one year

Estimated by book at 7.5 (in Table 9.1)

Other estimates much higher

In any event, some of the most severe land quakes in U.S. in 200 years

Felt over wide area (1,000,000 sq. mi.)

North America east of the Rockies

Rang church bells in Boston

Scarcely populated area at the time

Still fairly well documented

Several "noteworthy" effects

Vertical scarps up to 6' high

Waterfalls still evident in Mississippi River up to 2 years later

Uplift of swampland

Fifteen mile long areas raised up to 20'

Sunken areas

150 mile long stretch along the Mississippi River

Formed 2 new lakes

An unusual event

Three extremely large quakes in a very short time

Not along a plate boundary

"Usually a quiet area"

According to whose time table?

Lots of recent study in this densely populated area

These events would result in a bit more impact today!

Imagine the devastation

DIGRESS TO: rigid vs. non-rigid bedrock and the propagation of seismic waves

Broad structurally unstable area

Deep seated crustal feature

Cause uncertain

Many like to place the cause on the weight of delta sediments at mouth of Mississippi

I like the idea of a craton split?

Like 1.5 X 109 years ago in Montana


Lots of quakes - Why?

25 quakes greater than San Francisco since 1900

Dense population patterns increase risk

Over 200,000 fatalities since 1700's

Many quakes are offshore - Why?

Tsunami's common (DEFINE) (Monroe; figs. 10-18 to 10-20, pg. 320-322; Table 10.4, pg. 322)

1896 - 110' tsunami north of Tokyo

27,000 fatalities


Seem to get nailed regularly (Monroe; Table 10-1, pg. 300)

Largest recorded loss of life

1556 - approx. 1,000,000 deaths

From quake as well as famine & disease after

As recent as 1976 - up to 600,000 deaths

Many different reports on fatalities from this one

Major embarrassment for Chinese (EXPLAIN IN BRIEF - see 1975 quake)

Europe & Middle East

Mediterranean area extremely active - Why?

Lisbon, Portugal - 11/1/1755 (All Saints Day)

Three large quakes

30,000 dead, fire, tsunami

Generated profound philosophical debate

Rousseau - "Tout est bien" (all is good)

Civilization is what is bad

"If we were not cooped up in cities, quakes wouldn't kill us."

My kind of guy

Caused extreme religious problems

Lisbon was considered "one of the most devout cities"

Extremely pious (and wealthy)

Why suffer at the hands of a loving God?

Why should the good perish along with the evil?

God intended to shock "all of Christiandom" into a state of penitent obedience

Also, the Portuguese were considered the most devout, and were singled out for the honor "of being the first and most severely punished."

Some honor!

South America

West coast very active - Why?

Southern Chile - May 21-22, 1960

Largest series ever recorded

Several mod. to intense foreshocks leading to main quake on the 22nd

Richter: 8.5 to 9.5 (largest single quake ever recorded)

Aftershocks for months

At least 119 recorded (32 were major quakes of their own)

Large area affected (100 X 1000 miles)

No large surface offsets

Other surface effects

Landslides / earth flows

Ground cracks - settling of fill material

Liquefaction (Monroe; fig. 10-15, pg. 315)

DIGRESS TO: Port Royal (1692)

"Earthquake" pg. 51

General settling resulted in local flooding

Sloshing water in lakes - local flooding

Increased volcanic activity in the Andes

Started on May 24th (2 days after main quake)


Occurrence and Causes of Earthquakes

Lots of historical speculation

Aristotle - 4th century B.C.

Air escaping from deep within the earth


Cause by Namazu - a giant catfish ("Earthquake")

"Earthquake" pg. 36-41

Lived in the mud beneath the earth

Namazu is a prankster

Restrained by the God Kashima

When Kashima spaces it out, Namazu can romp & play

We now know they are the result of the slippage of rock along faults

Near surface events

Brittle/Ductile Transition Zone (EXPLAIN)

Global features - regular distribution (Monroe; fig. 10-4, pg. 304)

Linear zones of activity

Related to plate boundaries

Commonly associated with volcanism

Cause of quakes

Lock vs. slip

Slip involves relatively smooth and continuous motion

Not too common - friction is real!

Quakes result from extended locking of plate boundaries

When release comes it results in a quake

In general, the longer the lock, the greater the quake (more on this later)

Elastic Strain Energy

The measure of strain across a locked fault

Elastic Rebound Theory (Monroe; fig. 10-1, pg. 301)

The readjustment of the plates across the fault after the quake

Rarely originate at the surface

Focus - location of rupture (Monroe; fig. 10-3, pg. 303)

In a situation like 1906 on the San Andreas, where 300 miles were involved, how do you define where the focus is?

Depth to focus

Shallow - 0 to 42 miles

Intermediate - 43 to 185 miles

Deep - 186 to 435 miles

Epicenter - point on surface immediately above focus (Monroe; fig. 10-3, pg. 303)

Energy released in two ways

Heat - approx. 50%

Used to heat up adjacent rock materials

Seismic wave motion

Propagate in all directions from focus

Intensity at any location a factor of several things

Amount of energy released

Distance from focus

Generally decreases with distance

Duration of movement

Characteristics of material

Rigid vs. non-rigid bedrock

Solid vs. non-solid bedrock

Klamath Falls vs. Eureka quakes

Velocity directly affected by:



Rock characteristics


Measurements of seismic waves

All quakes result in disturbance of the earth's rocks

Waves can be measured by seismographs (Monroe; fig. 10-2, pg. 302)

Graphical representation of ground motion (Monroe; fig. 10-10, pg. 309)

Often lots of background "noise"

Evans Creek placer study - waterfall interference

Usually not too hard to pick out the "significant" events

Analysis of the wave forms & velocities used to infer internal structure of the earth

Four major wave pulses can be identified (under optimum conditions)

See Monroe; fig. 10-8 and 10-9, pg. 307-308

Body Waves (like at the football game?)

P-Waves - primary waves

First to arrive - must be fastest

Generally low amplitude

Compressional wave

Like sound

Will move thru solid and liquid

Faster through solids

Anything which increases the elasticity of the material increases the speed of the P-waves

Increased pressure increases speed

Increased heat decreases speed

S-Waves - secondary waves

Slower so they arrive after the P-waves

Higher amplitude - shear waves

Will not transmit through non-elastic materials

Air, magma, or liquids

Surface Waves

Love and Rayleigh Waves

Generally travel at the surface

Are the cause of property damage

Higher amplitude and longer wavelength

Composed of both a horizontal and vertical component

Not all quakes give off equal amounts of horizontal & vertical motion

The recent quake in the LA area caused additional damage because structures are designed to withstand one type of motion, not the other

The quake originated on an unknown fault which produced the wrong kind of motion (how rude!)

All quakes generate all 4 types

How they are received at recording station is related to:

Size and nature of rupture

Distance to focus

Depth of focus

Nature of material waves pass through

Difference in P- and S-wave velocities (Monroe; fig. 10-10, pg. 309)

Allow calculation of distance to focus

Does not give actual focus (epicenter)

A circle with distance as radius gives all possible locations

Triangulation is used to get focus (epicenter) (Monroe; fig. 9-10, pg. 310)


Intensity vs. Magnitude

Attempt to express strength of a quake

Intensity - Mercalli scale (Monroe; Table 10-2, pg. 311) or "Earthquake" (pg. 84)

Based on observations of damage caused by quake

Extremely subjective

Twelve levels of damage

Not a true measurement

Rather a description of the event

Richter Scale of Magnitude

Charles Richter - 1932

His sense of scientific objectivity was offended by the totally subjective Mercalli scale

Developed his own scale based on the amount of energy released

Probably one of the most confusing scales in existence

Read "Earthquake" pg. 86

Richter may have not done us much of a favor

Values obtained from seismograph (Monroe; fig. 10-13, pg. 313)

Based on empirical data

Found an incredible increase in energy

A large quake could be up to 10,000,000 times greater than a small one

A linear scale would obviously not do the job

Need to compress the possible range of values

DESCRIBE: Richter values in detail


Seismic Waves and the Earth's Interior

Our only method of investigating the earth's interior (Monroe; fig. 11-1, pg. 336)

We cannot directly observe sub-crustal materials

Can't even get to the base of the crust

Deep Crustal Drilling Program - U.S. (and U.S.S.R.)

Movie "Crack in the World"

Basic theory:

Velocity is affected by the material it is passing through

Reflects and/or refracts when passing from one into another (Monroe; figs. 11-4, 11-5; pgs. 341)

Has allowed geophysicists to identify several layers within the earth

The Crust

Mohorovicic Discontinuity (Monroe; fig. 11-6, pg. 342)

Base of the crust at boundary with mantle

Beneath the crust

Lithosphere - rigid crust and upper mantle


Rapidly increased temperature lowers seismic velocities

Possible sliding surface for plate motions


Convection within this layer may cause plate motions

Kimberlite pipes (Monroe; fig. 11-8, pg. 344)

The Core

Sharp jump in apparent density (4 to 5 times surface rocks)

Differentiation is concentrating the heavy stuff here

2 separate layers have been identified

Liquid outer core

Solid inner core


People and Earthquakes

Unfortunately, people get in the way

Tough to establish population centers away from hazard areas

Quakes common at plate boundaries

Near coast in most cases

That's where the people want to be

Will continue to re-build even after numerous destructive quakes

Managua, San Andreas, etc.

Public awareness may help

Demand better structural integrity of buildings

Vibrate as a single unit

Non-rigid construction

Allow building to flex

Demand intelligent locations

No nukes on faults

No dams on faults

Employment for geologists!


Prediction & Control of Earthquakes - the ultimate goal!

A high priority in many areas of the globe

China - "Quakes are the #1 naturally occurring enemy of the people"

Analyze past patterns

Seismic Risk Maps (Monroe; fig. 10-22, pg. 323)

Really only give a general feel for potential

Seismic Gaps (Monroe; fig. 10-23, pg. 324)

Sections of known faults which haven't ruptured

May indicate increased strain

Only a general feel for potential

DIGRESS TO: Ring of Fire and the Cascadia Gap (Monroe; fig. 10-4, pg. 304)

Measure accumulated elastic strain

Tilt across fault

Magnetic and electrical properties

Again, only a general feel for potential

We need dates and times!!

Seismic monitoring - an ongoing process

Parkfield, California monitoring

Micro-quakes - increase prior to large quake?

Subtle movements of surface

Animal behavior

Dogs & cats freak out just before a big quake

Can give up to a minute or two warning - big deal

In use along San Andreas

"Permanently housed mammals under constant observation"

Who's paying for this? Only in California...

Track record far from sparkling

China - 1975

90,000 evacuated just before large quake

Looked like they had it figured out

China - 1976

Up to 600,000 dead in Tangshan

California - well monitored area, to say the least

Have yet to predict a quake with any certainty

Review Parkfield fiasco

Even if we could predict

Who would take responsibility for ordering evacuations?

Panic, looting, etc.

"The only way to adequately deal with the problem is to control them!"

Many books say it is possible

Usually involve the use of water

Denver, Colorado - 1962

Government began pumping nuclear waste into rock beneath the area

(No more frozen streets in the winter)

Quakes started soon after (Monroe; fig. 10-24, pg. 327)

Rocky Mountain Arsenal denied any connection

Lots of uproar

Stopped in 1965 - so did quakes

Liquids thought to lubricate faults

Set off small slippages before too much elastic strain builds up

"Let's try it on the San Andreas"

The theory: Drill 3 wells to 20,000'

Pump out the outer two to lock fault

Inject water into middle well

Result in minor quake

Continue along fault to relieve entire structure

And they're worried about liability related to "faulty" prediction techniques


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