Download Earthquakes

Kobe, Japan 1995 – 5000 deaths
Earthquakes
It is estimated that there are
500,000 detectable earthquakes
in the world each year.
100,000 of those can be felt,
and 100 of them cause damage.
The world's deadliest recorded
earthquake occurred in 1557 in
central China. It struck a region
where most people lived in caves
carved from soft rock. These
dwellings collapsed during the
earthquake,
th
k killing
killi an estimated
ti t d
830,000 people.
In 1976 another deadly earthquake
struck in Tangshan, China, where
more than 250,000 people were
killed.
Earthquake
epicenters
in the U.S.
1
Seismic risk map for the U.S.
Colors on this map show the level of horizontal shaking
that have a 1-in-10 chance of being exceeded in a 50year period. Shaking is expressed as a percentage of g (g
is the acceleration of a falling object due to gravity).
Turkey, August 17, 1999…in 40 seconds
$20 billion in losses – 10% of Gross National Product
Earthquakes happen
every day, every hour
somewhere in the world.
Listening Team – Turkey, 1999
63,000 damaged buildings, 18,000 deaths, 50,000 injuries,
120,000 families without housing
2
Hawaii Island seismicity, 2000
95% of seismicity
in Hawaii is due to
volcanism –
magma movement.
Other 5% is due to
tectonic forces
on the
seafloor
Plate Tectonic framework for earthquakes
Convergent, Divergent and Transform Boundaries
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Convergent Boundary
Divergent Boundary
Earthquakes and Plate Margins
Circum-Pacific seismic belt – 81% of worlds largest quakes
Alpide Belt – 17% of largest quakes
Mid-Atlantic Ridge
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Elastic rebound
as a cause of
earthquakes
Locating an earthquake
Energy is first released at focus, as slippage.
This exerts strain along the fault, producing more
slippage elsewhere – this propagates as energy
Elastic Rebound –
1. Slow storage of elastic energy
over time (deformation)
2. Forces holding rocks together are
overcome (displacement by elastic
rebound)
Foreshocks – build up
p of strain
Aftershocks – Continued strain release
Landforms – scarps, linear features
diverted
Benioff Zone
beneath the
Tonga Trench
Two types of wave groups generated –
Surface waves (Long Waves)
Travel along outer layer of crust at the surface causing ground roll
like a water wave and lateral shifting…travel slowly and generate
g
the most damage
Earthquake foci
define the subducting
slab.
Body waves (two types)
Primary waves (P) – compress and pull (dilate) rocks in the
direction of movement, involves changing the volume and shape of
material….solids, liquids and gases resist compression and will spring back.
Thus they propagate the waves forward. P waves travel through all types of matter
Fastest wave
Secondary waves (S) – motion is 90 degrees to direction of
propagation (up and down), involves only changing the shape of transmitting
media…fluid and gas do not resist shape change hence they will not spring back
and will not transmit the wave forward. S waves travel only through solids.
Second fastest wave
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Seismic body waves
Recording
earthquakes
P Wave - all types of matter, fastest
S Wave – solids only, second fastest
Typical
Seismograph
record
Three seismographs each plot a circle
of the travel distance calculated
by the S-P delay…where three
circles intersect is the epicenter
Average travel
travel--time
curves
Delay between arrival of S and
P waves is proportional to the
distance traveled by the waves
6
Refraction and reflection of seismic body waves
Increased density allows wave to travel faster…causing slow refraction (bending)
Refraction also happens suddenly when wave crosses density front.
Waves also reflect off density interfaces.
Wave front
Wave ray
Seismic shadow
zones Measuring Earths
Interior
P wave refraction creates
P-wave
a shadow zone
S-wave propagation
creates a shadow zone
P wave
shadow zone
S wave
shadow zone
Network of epicenters around
Earth’s surface defines the
interior zones
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Changes in the seismic
velocity of P and S
waves mark
discontinuities at 100
km (the low velocity
zone – LVZ), 400 km
(base of the upper
mantle), 670 km (top of
the lower mantle), 2900
km (top of the core),
and 4800 km (inner core
boundary).
400 discontinuity - broad depression of the surface (green and
blue) under most parts of the Pacific and Indian Oceans.
Large elevations in the surface (red and yellow) under
continental regions such as Eurasia, North America, Australia,
Antarctica and parts of Africa. In general 400 km discontinuity
correlates well continents and ocean basins.
670 km discontinuity - very different structure from 400 km
surface. Notable features: deep depression in western Pacific,
Tonga, and South America. Basin depths 25 km from average
position of the surface; consistent with effect of subduction in
these regions.
2900 discontinuity - at core-mantle boundary reveals broad
basin under Indian Ocean and eastern Eurasia; western
Africa, southern Indian Ocean, Australia, and western and
central Pacific have a high topography related to thermally
buoyant regions on core surface
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Richter Magnitude –
Energy of seismic wave is a function of both amplitude (X) and duration (T).
Table 13.3 – Frequency of Occurrence of Earthquakes since 1900
M = log X/T + Y (correction factor)
Descriptor
Richter Magnitude
Average Annually
Great
8 and higher
1
Major
7 - 7.9
18
120
Strong
6 - 6.9
Moderate
5 - 5.9
800
Light
4 - 4.9
6,200 (estimated)
Minor
3 - 3.9
49,000 (estimated)
< 3.0
Magnitude 2 - 3: about 1,000
per day
Magnitude 1 - 2: about 8,000
per day
Very Minor
Table 13.2
Modified Mercalli Intensity Scale
I. Not felt except by a very few under especially favorable conditions.
II. Felt only by a few persons at rest, especially on upper floors of buildings.
III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do
not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the
passing of a truck. Duration estimated.
IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes,
windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building.
Standing motor cars rocked noticeably.
V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects
overturned. Pendulum clocks may stop.
VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster.
Damage slight.
VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built
ordinary structures; considerable damage in poorly built or badly designed structures; some
chimneys broken.
VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial
buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory
stacks, columns, monuments, walls. Heavy furniture overturned.
IX. Damage considerable in specially designed structures; well-designed frame structures thrown
out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off
foundations.
X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed
with foundations. Rails bent.
XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.
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