Download Earth / Environmental Science Ch. 8 – EARTHQUAKES AND

Earth / Environmental Science
Ch. 8 – EARTHQUAKES AND EARTH’S INTERIOR
What is an Earthquake?
z The vibration of Earth produced by the rapid release of energy.
z Often caused by slippage along a break in Earth’s crust.
Focus and the Epicenter
z Focus – the point within the Earth where the earthquake starts.
z Epicenter – the location on the Earth’s surface directly above the
focus.
z Seismic waves radiate out in all directions.
Cause of Earthquakes
z See the following figure:
z Forces within the Earth slowly deform the crustal rocks on both sides
of a fault.
z These forces cause the rock to bend and store elastic energy.
z Eventually, the resistance caused by internal friction that holds the
rocks together is overcome. The rocks slip at the weakest point (the
focus).
Elastic Rebound Hypothesis
z Elastic Rebound – the springing back of the rock into its original place.
z When rocks are deformed, they first bend, and then break. (releasing
energy).
z Most earthquakes are produced by the rapid release of elastic energy
stored in rock that has been subjected to great forces. When the
strength of the rock is exceeded, it suddenly breaks, causing the
vibrations of an earthquake.
Aftershocks and Foreshocks
z Aftershocks – movements that follow a major earthquake – producing
smaller earthquakes.
z Foreshocks – small earthquakes that come before a major quake.
z Fault creep - slow, gradual movement that is usually fairly smooth.
z Some segments stay locked and store energy for hundreds of years
before they break and cause great earthquakes.
Measuring Earthquakes
z Seismology – the study of earthquake waves.
z Seismographs – instruments that record earthquake waves.
z Seismogram – a recording of the ground motion.
Earthquake Waves
z Two main types of waves are produced:
z Surface Waves – seismic waves that travel along Earth’s outer layer.
z Travel along the ground causing the ground and anything on it to
move.
z Move in an up-and-down motion as well as sis-to-side.
z Side-to-side motion is especially damaging to building foundations.
Body Waves
z Body waves are either P waves or S waves.
z P waves – push-pull waves – compression and expansion in the
direction the waves travel (known as compression waves).
z S waves – vibrate at right angles to the direction of propagation.
Transverse waves.
z P waves temporarily change the volume of the material they pass
through by compression and expansion.
z S waves temporarily change the shape of the material they pass
through.
z Gases and liquids will not transmit S waves because they do not
rebound elastically to their original shape.
Seismogram
z A seismogram shows all three types of seismic waves – surface waves,
P waves and S waves.
z P waves arrive at the recording station first, followed by S waves,
followed by the surface waves.
z These waves travel at different speeds.
z Generally, P waves travel 1.7 times faster than S waves.
z Surface waves travel the slowest – about 90% of the speed of the S
waves.
Locating an Earthquake
z The difference in the velocities of the P and S waves enables us to
locate the epicenter.
z The greater the interval measured between the first P wave and the
first S wave, the greater the distance to the earthquake source, the
focus.
Earthquake Distance
z We can determine the distance from the recording station to the
earthquake in two steps:
1. Find the time interval between the arrival of the first P wave and the
first S wave.
2. Find the equivalent time spread between the P and S wave curves.
Earthquake Direction
z The precise location of the earthquake can be found when the distance
from three (or more) seismic stations is known.
z The epicenter is the point where the three circles intersect.
Earthquake Zones
z Most earthquakes occur around the outer edge of the Pacific Ocean –
the circum-Pacific belt (the Ring of Fire).
z Earthquake active areas include:
z Japan, the Philippines, Chile,
Alaska’s Aleutian Islands.
z A second zone of activity is the Mediterranean-Asian belt.
z A third: The oceanic ridge.
Measuring Earthquakes
z Earthquake measurements involve intensity and magnitude.
z Intensity – is the amount of earthquake shaking at a given location
based on amount of damage. (not quantitative)
z Magnitude – quantitative measurements rely on calculations using
seismograms. It is a measure of the size of seismic waves or the
amount of energy released at the source of the earthquake.
Richter Scale
z Based on the amplitude of the largest seismic wave (P, S, or surface
wave).
z Logarithmic scale - since quakes vary greatly in strength.
z Seismic waves weaken as the distance between the quake focus and
the seismograph increases. Richter scale is only useful for small,
shallow earthquakes within 500 km of the epicenter.
z This scale is not longer used by scientists.
Moment Magnitudes
z Derived from the amount of displacement that occurs along a fault
zone.
z Calculated using several factors:
1. Avg. amt. of movement along the fault
2. Area of the surface break
3. Strength of the broken rock
Surf. Area x avg. displacement x rock rigidity
Destruction from Earthquakes
Seismic Vibrations
z The damage to buildings and other structures from earthquake waves
depends on:
1. Intensity and duration of the vibrations
2. Nature of material on which structure is built
3. Design of the structure.
Building Design
z More flexible buildings are damaged less
Liquefaction
z Where loosely consolidated sediments are saturated with water,
earthquakes can cause a process known ad liquefaction.
z Stable soil turns into a liquid that cannot support buildings or bridges.
z Underground storage tanks and sewer lines may float to the surface.
Tsunamis
z Tsunamis – seismic sea waves.
Causes of Tsunamis
z An earthquake-triggered tsunami occurs where a slab of the ocean
floor is displaces vertically along a fault.
z Can also occur when the vibration of a quake sets an underwater
landslide into motion.
z Travels at 500-950 km/hour
z When waves reach the shore, they are slowed and water piles up to
heights of 30 meters or more.
Other Dangers
z Vibrations cause landslides, fire and ground subsidence
Predicting Earthquakes
Short-range predictions:
z Scientists measure uplift, subsidence, strain in the rocks near active
faults, water levels and pressure in wells, radon gas emissions and
small changes in the electromagnetic properties of rocks.
Long-range Predictions
z 30-100 year range
z Based on the idea that earthquakes are repetitive or cyclic.
z A seismic gap is an area along a fault where there has been no
earthquake activity for a long time.
Earth’s Layered Structure
z Layers are Defined by Composition
z The increase in speed of seismic waves with depth is due to increased
pressure, which changes the elastic properties of deeply buried rock.
Thus, the paths of the seismic rays through earth are refracted, or
bent, as they travel.
Earth’s Interior
z Crust
z Mantle
z Core
Crust
z Divided into oceanic and continental crust.
z Oceanic crust – 7 km thick; composed of the igneous rocks basalt and
gabbro.
z Continental crust – 8-75 km thick (avg. 40 km). Consists of many rock
types. Average composition is granitic rock called granodiorite with an
average density of 2.7 g/cm3; and are 4 billion years old.
z Oceanic crust rocks are younger (<180 million year) and have an
average density of 3.0 g/cm3.
Mantle
z 82% of Earth’s volume is contained in the mantle – extends to a depth
of 2890 km.
z Dominant rock type in uppermost mantle is peridotite (density of 3.4
g/cm3)
Core
z Iron-Nickel alloy – density = 13 g/cm3.
Layers Defined by Physical Properties
z
z
z
z
Lithosphere
Asthenosphere
Outer Core
Inner Core
Lithosphere and Asthenosphere
z Earth’s outermost layer consists of the crust and the uppermost
mantle, and forms a relatively cool, rigid shell called the Lithosphere.
z Average thickness is 100 km
z Asthenosphere – soft, comparatively weak layer beneath the
lithosphere. Hot enough to easily deform rock
Lower Mantle
z 660 km to base of mantle is a more rigid layer called the lower mantle.
z Rocks are still very hot and capable of flow.
z Bottom few hundred km of the mantle lays on top of the hot core, and
contains softer more flowing rock like that of the asthenosphere.
Inner and Outer Core
z Outer Core – liquid layer 2260 km thick
z Flow metallic iron in this zone generates the Earth‘s magnetic
field.
z Inner Core – sphere with a radius of 1220 km.
z Compressed into the solid state by the immense pressure
Discovering Earth’s Layers
z 1909 – Andrija Mohorovicic – discovered that the velocity of seismic
waves increases abruptly below about 50 km of depth. This boundary
separates the crust from the underlying mantle and is known as the
Mohorovicic discontinuity, Moho for short.
Shadow Zone
z Boundary between the mantle and the outer core.
z P waves were bent around the liquid outer core beyond about 100
degrees away from an earthquake. The outer core also causes P waves
that travel through the core to arrive several minutes later than
expected. This region where the P waves arrive is called the shadow
zone.
z The P waves bend around the core. So rather than stopping the P
waves in the shadow zone. The outer core bends them.
z It was also shown that S waves could not travel through the outer
core. Therefore, geologists concluded that this region is liquid.
Discovering Earth’s Composition
z Continental crust – lighter granitic rock
z Oceanic floor – basaltic composition