Transcript earthquakes - Montgomery County Schools

EARTHQUAKES
earthquakes = shaking of earth’s crust caused by a
release of energy
1. results from sudden movement of part of earth’s crust
2. rocks under stress suddenly shift along a fault = break in
lithospheric plate where movement can occur
3. friction can prevent rocks from moving for a time → fault is
said to be “locked”
4. most common cause is faulting
elastic rebound theory = rocks that are strained past certain
point will fracture and spring back to original shape
1. when rocks slip to new position seismic waves are released
2. this energy can often cause stress in other rocks causing
those to move
3. aftershocks = series of smaller earthquakes originating
close to focus of large earthquakes
a. large earthquakes can have as many as 1000 per day
b. frequency usually diminishes over time
c. can cause damage to buildings and other structure,
especially if already weaken
Haiti
stress = force that causes pressure in rocks of crust
1. caused by isostatic adjustment and plate movement
2. strain = change in shape or volume of rocks due to stress of
being squeezed, twisted, or pulled apart
3. three main types:
a. compression = crustal rocks are squeezed together
1) reduces volume of rock
2) tends to push rocks higher up or deeper down into
crust
3) rock layers are thicker and shorter
b. tension = force that pulls rocks apart
1) rocks tend to become thinner and longer
2) can cause fractures
c. shearing = pushes rocks in opposite horizontal directions
1) rocks bend, twist, or break apart as slide past each
other
2) tends to distort shape of rocks,
especially along plane
between opposing forces
4. ductile deformation = permanent deformation that occurs
when stress exceeds a certain value
a. material stay deformed even if stress is reduced to zero
b. when stress exceeds strength of material, material
breaks or fails
c. rocks can become ductile at higher temperatures at
greater depths in crust
folding = permanent deformation or bending of a rock
under stress
1. cracks may appear but rock layer is intact
2. folds = wavelike structures in rock layers
a. vary greatly in size
b. layered rocks with large folds often have smaller folds
within the layers
Ex: Appalachian Mountains
c. limbs = two sides of a fold
d. strike = compass direction of fold or of rock layers
exposed at surface along fold
3. general types of folds:
a. anticline = upward fold in layers
1) usually form a ridge
b. syncline = downward folds in layers
1) usually form valleys
c. monocline = gently dipping bends in horizontal rock
layers
focus = actual point on fault where rocks break and move
1. underground point of origin
2. seismic waves energy travel outward in all directions
3. shallow-focus earthquakes = occur within 70 km of
earth’s surface
a. 90% of continental earthquakes
b. usually cause most damage
c. usually at divergent boundaries or transform boundaries
4. intermediate-focus earthquakes = occur at depths between
70 km and 300 km
5. deep-focus earthquakes = occur at depths between 300 km
and 700 km
a. occur at subduction zones
b. usually occur farther inland
6. factors into EQ intensity
epicenter = point on surface directly above focus
1. most violent shaking in this area
2. see greatest damage here
Categories of breaks of rocks:
1. fault = when rocks do move along break
a. fault plane = surface of a fault
b. hanging wall = rocks above a normal fault plane
c. footwall = rocks below a normal fault plane
2. joints = breaks along which no apparent movement has
occurred
a. result of same stresses that lift, tilt, and fold rock
layers into mountains
b. surface is usually a plane although some can be
curved
c. joint plane appears as a line on surface of rock
outcrop
d. joint sets = parallel groups of lines
e. provide channels through
which fluids enter and
move through bedrock
TYPES OF FAULTS
1. normal faults = hanging wall drops in relation to foot wall
a. caused by tension (pulling crust apart)
b. fault plane at a steep angle or almost vertical
c. form fault-blocked mountains
d. occur at divergent plate boundaries
Ex: Sierra Nevadas
Great Rift Valley of East Africa
2. reverse faults = hanging wall moves up in relation to
foot wall
a. caused by compression (plates collide)
b. fault plane is steep or nearly vertical
c. causes rocks to bend and sometimes break
d. occurs at convergent plate boundaries
Ex: Himalayan Mountains
3. thrust fault = hanging wall moves up and slides over foot
wall
a. special type of reverse fault in which fault plane dips 45o
or less from horizontal
b. caused by compression
c. fault plane at low angle or nearly horizontal
d. older rocks are pushed up on top of younger rocks
e. occur at convergent plate boundaries
Ex: Glacier National Park mountains
Rockies and Alps
4. strike-slip faults = rock on either side of fault plane slides
horizontally past each other
a. caused by shearing
b. hanging wall moves parallel to the foot wall but in the
opposite direction
1) each wall moves in opposite directions
2) wall can move in same direction but at different rates
c. move past each other without much upward or
downward movement
d. occur at transform fault boundaries
Ex: San Andreas Fault of California
Major Earthquake Zones
1. along or near edges of the earth’s plates
2. three major areas:
a. Pacific Ring of Fire = includes west coasts of North and
South America, east coast of Asia, western Pacific
islands of Philippines, Indonesia, New Guinea, and New
Zealand
1) 80% occur along the edges of the Pacific plate
2) called the Circum-Pacific belt
b. along mid-ocean ridges
c. Mediterranean-Asian belt = crosses southern Europe &
Asia (15% of activity)
3. fault zones = groups of interconnected faults
a. form at plate boundaries
b. movement could occur along one or more of the
individual faults
4. not all result from movement along plate
boundaries
a. can occur in the middle of a continent
b. due to ancient fault zone deep within earth’s crust
Ex: New Madrid, Missouri
TYPES OF SEISMIC WAVES
1. body waves = waves that travel from focus through
Earth
a. each earthquake produces two types: P & S
waves
2. primary waves = P wave
(compressional waves)
a. travel fastest, squeeze and stretch rock materials as pass
through
b. particles move back and forth in direction waves are
moving
c. denser the material, faster
wave travels
d. can move through solids,
liquids, or gases
3. secondary waves = S waves (shear waves)
a. arrive after P waves
b. can travel through solids but not liquids or gases, not
always recorded at all locations
c. speed up through denser material
d. travel little more than half the speed of P waves
through solids
e. particles move at right angles to direction of wave
4. surface waves = L waves
[long]
a. slowest moving wave & travel along Earth’s surface
b. originate on surface at epicenter
c. causes surface to moves up and down like an ocean wave
d. results in most damage since it bends and twists surface
(most destructive when travels through loose earth)
e. two types:
1) Love waves = cause particles of material to
move from side to side, in a direction
perpendicular to waves’ direction of travel
2) Rayleigh waves = cause particles of material
to move in elliptical patterns, travel more
slowly than Love waves
Recording Earthquakes
1. seismologist = scientist that studies earthquakes and
seismic waves
2. seismology = study of earthquake waves
3. seismograph (seismometers) = instrument that detects and
measure seismic waves
a. developed by John Milne in 1893
b. has three separate sensing devices
1) one records vertical motion of ground
2) other two record horizontal
motion in east-west and
north-south directions
c. records motion by
1) tracing wave shaped lines on paper or
2) translating motion into electronic signals
d. distance pen departs from center line is related to amount
of energy released
4. seismogram = seismograph record of waves on paper
a. first major zigzag marks arrival of P waves at station
b. slower S waves arrive next producing a different pattern
Structure of Earth
1. inner core = solid, innermost layer
a. composed of very dense iron and nickel
b. temperature reaches approximately 5000oC
c. very high pressure
d. begins about 5150 km below surface
e. dense iron causes the existence of a magnetic
field around earth = magnetosphere
2. outer core = surrounds the inner core
a. composed of molten liquid iron and nickel
b. begins approximately 2900 km below surface
c. temperature range: 2000oC to 5000oC
3. mantle = layer that lies above outer core; divided into upper
mantle and lower mantle
a. largest layer = 80% of earth’s volume; 68% of earth’s
mass
b. density, temperature, and pressure increases with depth
c. temperature range = 870oC to 2200oC
d. Moho (Mohorovic discontinuity) = boundary between
crust and upper mantle
1) denser layer of lithosphere, will cause seismic waves to
speed up
2) 32 km below continents and
5-10 km below ocean
e. compose of mostly silicon, oxygen, iron, and
magnesium; has plasticity
f. transition zone = region in middle of mantle
1) area where seismic wave velocities increase more quickly
than expected
2) separates upper mantle from denser lower mantle
4. crust = outermost layer (thin)
a. varies in thickness from 60 km in mountainous
regions to 5 km in oceanic regions
b. made of three types of solid rock = igneous,
sedimentary, and metamorphic
c. oceanic crust = beneath ocean
1) mostly silicon, oxygen, iron, and magnesium
2) composition determined by samples taken from ocean
floor by deep sea drilling ship Glomar Challenger
d. continental crust = beneath continents
1) mostly silicon, oxygen, aluminum, calcium, sodium, and
potassium
2) average thickness is about 32 km
EARTHQUAKE MEASUREMENT
1. magnitude = measure of energy released
a. higher the wavy lines on seismogram, stronger the
earthquake
b. height of the tallest wavy line is used to calculate the
strength of the quake
c. Richter Scale = used to describe earthquake magnitudes
1) developed by Charles Richter in 1935
2) based on size of largest seismic waves generated
3) each successive number increase in scale represents
increase in seismic-wave,
or amplitude, of a
factor of 10
4) each increase in
magnitude represents
about a 32-fold
increase in energy
d. moment magnitude scale = includes size of fault rupture,
amount of movement along fault, and rock’s stiffness
1) estimated from size of several types of seismic waves
e. largest EQ so far = 9.6
1) major EQ (widespread damage) = 7 or above
2) moderate EQ = between 6 and 7
3) minor EQ = between 2.5 and 6
4) microquakes =
less than 2.5
(not usually
felt by people)
2. intensity = amount of damage caused to structures
a. modified Mercalli scale = expresses intensity using Roman
numeral and description
b. rating of I has low intensity where rating of XII is total
destruction
c. higher the number, the worse the damage
d. depends primarily on
amplitude of surface
waves generated
e. will decrease as
distance away from
epicenter increases
shadow zone = area on Earth between 105o and 140o from
focus
1. no seismic waves are detected
2. secondary waves are stopped by liquid outer core
3. primary waves are slowed and deflected
4. boundaries between
layers in Earth not only
cause waves to change
in speed, but also causes
waves to bend.
LOCATING AN EARTHQUAKE
1. to assess how powerful an earthquake is, scientists must
determine distance of its epicenter from seismograph station
2. analyze difference between arrival times of P and S waves on
travel-time curves
a. P waves travel 1.7 times faster than S waves
b. farther apart waves when measured, farther away
epicenter is
c. plot difference between arrival times of two waves to
determine how far earthquake is from station
d. measuring separation on seismogram or travel-time
graph at which P-curve and S-curve have same
separation → distance to epicenter can be determined
= epicentral distance
3. locating epicenter
a. need information from at least three seismograph
stations at different locations
b. circle is drawn around each station on a map
c. radius of each circle equals that station’s distance from
epicenter
d. point where all three circles intersect is location of
epicenter
4. time of occurrence is determined
from travel-time graph by
subtracting appropriate travel time
from known arrival time of wave
EARTHQUAKES DAMAGE
1. factors that determine amount of damage caused
a. strength of earthquake & how long shaking occurs
b. kind of rock and soil that underlies an area
c. population of area
d. kinds of buildings in an area
e. time at which
it occurs
2. safer in an open,
level field
3. dangers:
a. collapsed buildings and structures
b. from falling objects and flying glass
c. landslides caused by soil liquefaction = loose soil
temporarily takes on properties of liquid
1) soft materials will amplify seismic waves
2) can also cause foundation failures
d. fires, explosions caused by broken electric and gas lines
e. floodwaters released
from collapsing dams
Liquefaction of soil
4. destruction to buildings and property
a. not designed to withstand swaying motion
b. type of ground beneath affects way building responds
1) loose soil and rock cause to sway violently
2) will cause bottom layers to collapse while top layers
fall on top = “pancaking”
c. fault scarps = areas of great vertical offset produced due
to fault movement
5. tsunamis = giant ocean waves [also called tidal waves]
a. caused by two events related to undersea earthquakes
1) faulting - cause sudden drop or rise in ocean floor
2) underwater landslides - water above is thrown into up
and down motion
b. can travel at speeds of 500 to 800 km/hr
c. when reach coast, crest can be as high as 30m
d. cause great destruction and loss of life
e. Seismic Sea Wave Warning System (SSWWS)
1) network of seismograph station around and in
Pacific Ocean
2) alerts to location and magnitude of earthquake
3) if tsunami possible will estimate arrival times at
different locations
4) warnings are issued to these areas immediately
5) those near epicenter
will not have enough
time
Earthquake Safety:
1. can occur in any region of United States
2. study the earthquake history of a region
3. if an area has had them in the past you can expect
them in the future
4. before occurs, be prepared
a. make your home as safe as possible
1) take heavy objects down from high shelves and
place on lower shelves
2) make sure hot water heaters and gas appliances are
held securely to prevent broken gas lines
3) learn how to turn off gas, water, and electricity
4) keep on hand a supply of canned food, bottled
water, flashlights, batteries, and portable radio
b. plan what to do if your are at home, school, work, or
in a car
5. during, stay calm
a. keep away from windows, heavy furniture, and other
objects that may fall
b. stand in doorway or crouch under desk or table to protect
yourself
c. do not run outside
d. if in a car stop in a place away from tall buildings,
tunnels, power lines and bridges
1) remain in car
until tremors stop
6. after, be cautious
a. check for fires and fire hazards
b. stay clear of rubble that could contain many sharp edges
c. wear shoes when walking among broken glass
d. avoid downed power lines and objects touched
by downed wires
Earthquake Preparation
1. most loss of life occurs when people are trapped in and on
crumbling structures
2. seismic-safe structures = resistant to vibrations that occur
3. highways have cement pillars with reinforcing rods wrapped
around them
4. new buildings are anchored to
flexible, circular moorings made
of steel plates filled with
alternating layers of rubber and
steel allowing building to sway
5. underground water and gas pipes
should be replaced with ones that
will bend and not break
Earthquake Warnings and Predictions
1. prediction would include where, when, and how
strong
a. people can be moved from areas in danger
b. could plan better growth of cities
c. buildings could be reinforced better
2. warning signals/ prediction methods
a. one of earliest used was behavior of animals
b. detect changes in earth’s crust
1) instruments places at faults detect changes in rock
movement and increases in strain
2) seismic gaps = sections of active faults that have not
experienced significant earthquakes for long period of
time
a) place where fault is locked and unable to move
b) strain has increased in area
c) no major earthquake in
last 30 years
c. detect slight tilting of ground
d. detect strain and cracks in rocks caused by stress
1) how much strain accumulated in particular part of
fault
2) how much strain was released during last quake
along that section of fault
e. detect small changes in
earth’s magnetic field
f. detect increased natural gas seepage from strained or
fractured rock
g. decrease in speed of local P wave
- longer the decrease, the stronger the earthquake
h. water level in wells often goes up or down
3. controlling force - water injected along a fault reduced
friction and earthquakes were less severe (just being
tested)