EARTH SCIENCE CHAPTER 6
I.
INTRODUCTION -- SUMMARY OF 2 EARTHQUAKES
- Loma Prieta (SF), 1989
- Northridge (LA), 1994
• Destruction from ground shaking, but also gas and water line
rupture
-Fires and no water to fight them
• Note differences in deaths from similar magnitude
-Kobe, Japan: 3000 dead
-LA : 50 dead
Building codes in LA probably helped.
• 75 earthquakes/year do “significant” damage
(How many earthquakes occur each year total?)
II.
WHAT IS AN EARTHQUAKE?
• Def’n: “vibration produced by rapid release of energy”
• Waves radiate from source like those in water when stone is
dropped (Fig. 6.2)
A. Earthquakes and Faults
• huge amount energy released during earthquake, but
earthquakes are infrequent
• movement along faults appears to be trigger (Fig. 6.3)
B. Elastic Rebound
• apparent mechanism that causes eqks
• Reid studied San Andreas after 1906 earthquake
-msrd 15 ft of N-ward displacement of Pacific plate relative
to N.
American plate
Eth Sci wk5 p.2
-Reid showed that land surface on either side of San
Andreas fault had slowly, steadily moved 10 feet
-Reid figured that there was a rapid release of built up
energy as rocks “snap back” to their pre-strain state (like
bending and releasing a stick)
• Mechanism as in Fig 6.4
-rocks bend and store energy as plates shift
- slippage occurs at weakest spot, which is fault plane
-this is “elastic rebound”, where rocks snap back
• 2 types fault motion
energy
1. fault creep - slow gradual slip; little energy built up or
released
2. stick-slip - fault sticks for long time, finally slips; lots
built up, then released
-fault segments that stick -slip on San Andreas estimated
to store energy for 50-200 years before slippage
C. Foreshocks / aftershocks
after
weakened
• biggest risk with aftershocks is they can cause damage
main quake has weakened structures
-example- Armenian aftershock collapsed many
structures, caused thousands of deaths
• foreshocks precede main quake by days, sometimes
years
II.
EARTHQUAKE WAVES
A. Terms: Seismology - study of waves in Earth
Seismographs - instruments that record waves Fig. 6.5
Seismogram - the actual record Fig. 6.6
Types:
1. “Body waves” - travel through Earth
a. Primary (“P”) waves
b. Secondary (“S”) waves
2. “Surface waves” - travel around outer layer
Eth Sci wk5 p.3
1. Body waves
a. P waves
• P waves are compressional
• Push-pull, like a slinky Fig. 6.7 A
• Particle motion in rock in same direction as wave
travel
Propagation mechanism: solids, liquids, gas all resist a
change in volume when compressed, spring back when
compressional force removed. So “P” waves can travel through
these materials, because molecules in materials first compress,
then expand back
b. Shear waves
• S waves are shear waves
• Particle motion transverse to direction of travel
Fig. 6.7B
• Analogous to shaking a rope
• S. waves do NOT act to change volume, but they do
induce
shear motion. Gases, Liquids don’t act to
resist shear
motion, so shear motion not transmitted
through
these...whatever shear energy enters
them is completely
absorbed.
-Interesting application:
In O & G exploration, we shoot both p and s waves into
the ground. In theory, areas that hold oil or natural gas
show shear wave attenuation, while p waves go right
through - so areas that show differences in p waves vs
shear waves can be good targets, may hold oil and gas.
2. Surface waves
• Motion complex
• Resemble ocean waves, only in rock
• Side-to-side motion as well - very damaging to
buildings
Eth Sci wk5 p.4
B. Sequences of arrivals at a location Fig 6.6
• P, then S, then surface waves
Pwave veloc = 6 km/sec,
S veloc = 3.5 km/sec
Surf wave veloc = 3.15 km/sec.
III
LOCATING EARTHQUAKE EPICENTERS
• “Epicenter” = surface location of the earthquake focus
• Use diffs in travel times between p and s waves (Fig 6.8)
- Procedure - assume you have recorded a seismogram (red
line) at your station….place seismogram on travel-time
graph, fit it to match change of time between p and s wave
arrival
- Read distance to epicenter (3800 mi) off x-axis
- Draw circle from your origin with radius of 3800 miles
- Assume you can find two people at two different stations
elsewhere. They each plot circles with their station as origin
Intersection of 3 circles will give you epicenter.
- This is the one pt on surface of earth that satisfies the
distances recorded at each station Fig. 6.9
IV
EARTHQUAKE MAGNITUDE
• One system is Modified Mercaslli Intensity Scale
This based on damage, is subjective
• Richter scale much more objective Table 6.1
-Introduced 1935
- Magnitude determined by measuring amplitude of largest
wave on seismogram
-Largest earthquake recorded are magnitude 8.6 (Equiv.
energy to ___tons of TNT)
-Ones less than 2 usually can’t be felt
- Logarithmic scale
Eth Sci wk5 p.5
incr.
• Increase of 1.0 on Richter scale results from 10x increase in
measured wave amplitude
• 10 times increase in measured wave amplitude results in 30 x
in energy released.
Examples
-earthquake 5.5 releases 30 times more energy than 4.5
-earthquake 6.5 releases 900 times more energy than 4.5
V.
DESTRUCTION FROM EARTHQUAKES
• Most violent in N. American in 20th century was_________
A. Factors dictating degree of destruction
1. Magnitude of earthquake
2. Proximity to populated area
3. Geologic setting, eg, bedrock vs. loose sediments
B. Seismic vibrations cause building damage
• Ground shaking complex
• Amount of structural damage due to vibration controlled
by:
1.
2.
3.
4.
Intensity (amplitude) of waves
Duration of quake
Nature of ground material
Structural design
C. 1964 Anchorage Earthquake Information gained on structures
• All multistory building damaged
-wooden (flexible) fared BEST
-steel (good) vs rigid concrete (bad) Fig 6.11
• Many buildings damaged even though built to California
code
• What was unusual?
-Large quake
-Long duration (3-4 Min) vs. more typical 15-40
seconds
so: 1). intensity and 2). duration were very important
but, 3). ground material also very important
-Anchorage buildings on unconsolidated sediments
-Sediments respond by amplifying seismic signal
Earth Sci wk5 p.6
-Whittier, AK closer to epicenter than Anchorage, but
suffered
less damage WHY?
-Most Whittier bldgs on granite, which transmit seismic
waves
but don’t distort or amplify them (Bedrock doesn’t
make wave
worse)
D. Tsunami Damage
• “tsunami” a.k.a. “tidal wave” Fig 6.13
• caused by fault or sea floor displacement
• unnoticeable in open ocean, but rise to huge height in
shallow water (up to 100 feet)
• 107 deaths in Alaskan Earthquake caused by tsunami,
only 9
due to bldg collapse in Anchorage
E. FIre Damage
electric
• 1906 San Francisco quake
• Huge amount of fire damage due to severed gas and
lines
• Water lines broken, no water available
• Fires burned for 3 days over 500 blocks
• Japan, 1923 fires killed 100,000 people
F. Landslide, subsidence damage
• major cause of bldg. damage in Alaska
• earthquake waves trigger landslides in unconsolidated
sediments
• liquefaction (like quicksand) a big problem
• waterfront towns of Seward and Valdez “carried away”
-Valdez “relocated” to more stable ground
• most Anchorage damage caused by landslides (Fig. 6.14)
• subsidence of 10 feet occurred downtown
VI. EARTHQUAKES EAST OF THE ROCKIES
• not many historically, but generally big
• New Madrid, MO 1811, 1812
-3 quakes, mag 7.5, 7.3, 7.8
-widespread damage
-Boston felt quake 1100 miles away
(bedrock in interior US carries energy a LONG WAY)
Earth Sci wk5 p.7
• Charleston, SC - 1886
-largest quake in eastern states
-felt in Chicago, St. Louis
others include:
• Plymouth, Mass 1683
• Cambridge, Mass - 1755
• NY State - many
VII
EARTHQUAKE PREDICTION
• Research under way in Japan, China, US Soviet Union
• Some precursors to earthquakes
-uplift or subsidence
-fault zone movement
-quiescence followed by activity
• Short Range Predictions
-Methods not yet reliable
- Some successes, however
-1975 - Liaonig Province, China
-Followed 1 year later by quake NOT predicted
-False alarms in China too
• Long range Predictions
-As soon as one is over, assumption is that continuing
plate motion starts to build strain again...a repeating
cycle
past -
- One way to predict occurrence is by studying the
look at the repeat frequency
-Example of prediction by probability Fig 6.17
-Next most likely spot on San Andreas?
S. portion of San Andreas
Why?
No earthquake since 1857, strain energy
always building up
VIII.
Earth Sci wk5 p.8
EARTHQUAKES AND INTERIOR OF THE EARTH
• our measurement of p and s waves tells us about internal
Earth Structure Fig 6.18
• 4 layers:
1.
2.
3.
4.
crust solid (Brittle)
Mantle solid (Brittle and Plastic)
Outercore liquid
Inner core solid
• Moho discontinuity separates crust from mantle
• Outer core known to be liquid (Fig 6.19) because:
-it refracts p waves downward (slows them down)
-S waves can’t propagate through it
• Solid inner core discovered in 1936, size determined in
1960’s
Additional subdivision of upper mantle required for
understanding plate tectonics
• Asthenosphere -lower portion of upper mantle that is hot, weak, plastic
in behavior
• Lithosphere - upper portion of upper mantle and all of crust
-solid, cool brittle
Earth composition • ocean crust is dense and thin
• Granitic crust thick and light
• Assumption that gravity has pulled most of Earth’s dense
minerals toward it’s center, forming very dense core
And gravity has allowed lighter minerals to “Float” to the
surface
Earth Sci wk5 p.9
IX GEOLOGIC STRUCTURES
- Look at rock deformation – 2 main types
A. Folds
B. Faults
A. Folds – bending of rock into “wavelike” features (Fig 6.21), usually due
to compression and horizontal shortening
1. “Anticline” – arching upward of layers (beds convex upward)
2. “Syncline” – arching downward of layers (beds convex
downward)
Folds can be symmetric, assymmetric, or overturned when one limb is
rotated over 90o (past vertical)
other terms:
“Dome” – somewhat circular anticline
(Black Hills, (Fig 6.22),)
“Basin” – large-scale, somewhat circular syncline
(Michigan, Illinois Basins underlie the entire state for which
they’re named)
Even after folds are weathered, you can identify their structure by their
rock patterns
Anticline – oldest rx in middle
Syncline – youngest rx in middle
Ground Surface
Youngest
Oldest
Earth Sci wk5 p.10
B. Faults - “Fractures along which movement has occurred” Fig 6.23
• Range in scale from displacement of few mm to hundreds of
miles
• Areas in which many faults connected together known as
“fault zones”
Types of Faults: Fig 6.24
• Dip-slip vs strike-slip
“Dip-slip” is movement primarily up and down the fault
plane
“Strike-slip” is movement sideways or laterally, along the
trend of the fault plane
• Fault movement in dip-slip faults often described by relative
motion of hanging wall block vs footwall block
-Story of how miners named these block fault blocks
-Which is the upper block?
-Which is the lower block?
• Dip-slip Fault Types Fig 6.24 A-C
1. Normal (tensional stress, high-angle fault plane)
2. Reverse (compressional stress, high-angle)
3. Thrust
(compressional stress, low-angle plane)
displacement in Alps up to 30 miles!
• Strike Slip Faults
at plate boundaries, these are “transform faults”
Fig. 6.24 D
C. Fault motion and stresses
• Can determine stress field from types of faults present
• Normal faults associated with tensional stress, or “pull-apart”,
divergent plate boundaries
-spreading centers
-horsts (highs) and grabens (lows) Fig 6.25
-crust thinned and stretched
Earth Sci wk5 p.11
• Reverse and thrust faults assoc. w/ compressional stress or “pushtogether”, convergent plate boundaries
- subduction zones
- folds also produced
- crust thickened, shortened
X. MOUNTAIN BUILDING
 All mt systems show evidence of major forces at work…
- folding
- faulting
- metamorphism
Young ranges include:
 American Cordillera (Rockies, Andes)
 Alpine-Himalayan chain
 Island arcs:
- Japan
- Phillippines
- Sumatra
Most of these ranges above less than 100 m.y. old
Older ranges include:
 Appalachians (eastern US)
 Urals (between Europe and Asia)
A. Mountain building at convergent boundaries
1. Aleutian and Andean-type boundaries, with a subduction zone
and either ocean-ocean collision or ocean –continent collision
Stages of development:


Fig 6.28
initially a passive margin, sediments deposited on a cont. shelf
subduction initiated


Cont margin becomes deformed and accretionary wedge of
sediment develops near subduc zone
Accretionary wedge evn becomes so large it rises above sea
level
Earth Sci wk5 p.12
Result is 2 parallel mt belts Fig 6.28C
 landward belt is the volcanic arc
volcanoes, plutons, metamorphics

seaward belt is the accretionary wedge
folded, faulted meta-sedimentary rx
volcanic deposits
Example of inactive Andean margin-type pair of ranges - Fig 6.29
is the California Coast Range & Sierra Nevada
accret wedge
remnant of volcanic arc
2. Himalayan-type boundary: continent-continent collision - Fig 6.30
 seds opf the accretionary wedge get caught up in the volcanic
arc
 result is one huge range, not a pair of mt belts
Older examples:
Ural Mts in Russia, where Europe collided with Asia
Appalachians, where Europe collided with N. America
3. Continental accretion Fig 6.31

small fragments get “welded” or “plastered” onto the side of a
continent by being rafted across an ocean
- microcontinents (eg, Madagascar)
- island arcs (Japan, Aleutians)
- submerged crustal fragments (ocean plateaus, undersea
volcanoes, etc)
this is our latest guess (like last 10 years) to explain very confusing,
chaotic geology we see in places like Rocky Mts from California all the
way up to Alaska