Revised 10 / 10 (Monroe 6th ed.)
Including...
Occurrence and Causes of Earthquakes
Seismic Waves and the Earth's Interior
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
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
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)
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!
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)
Lots of historical speculation
Aristotle - 4th century B.C.
Air escaping from deep within the earth
Japanese
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:
Heat
Pressure
Rock characteristics
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)
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
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
Asthenosphere
Rapidly increased temperature lowers seismic velocities
Possible sliding surface for plate motions
Mantle
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
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
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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