O Earthquakes have a greater effect on society than most people
think. O These effects range from economical to structural to
mental. O An earthquake only occurs for a few brief moments. O But
the aftershocks can continue for weeks; the damage can continue for
years.
Slide 6
Loss due to Bhuj 2001Earthquake
Slide 7
Origin of Earthquakes O An earthquake is a vibration of the
Earth produced by a rapid release of energy. O The main features
include the focus, the location within the Earth where the
earthquake rupture starts, and O The epicenter, which is on the
surface of the earth, at the top of the focus.
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Slide 9
O The earliest seismologists were the Chinese who worked hard
to record their quakes in detail. O They even developed a means to
predict earthquakes by filling a ceramic jar to the brim with water
and leaving it set. O If the water overflowed the jar, then an
earthquake was imminent. Of course, this means of prediction was
unreliable and uncertain.
Slide 10
O It is thought that some animals may feel vibrations from a
quake before humans, and that even minutes before a quake dogs may
howl and birds fly erratically. O Aristotle was one of the first
Europeans to create a theory about the origin of Earthquakes. He
thought that they were the result of heavy winds.
Slide 11
The First Seismograph O The first seismograph was invented by
the Chinese astronomer and mathematician, Chang Heng. He called it
an "earthquake weathercock. O It had eight dragons and each of the
eight dragons had a bronze ball in its mouth below the dragons, at
the base of the weather cock are eight toads with their mouths open
representing eight directions.
Slide 12
O Whenever there was even a slight earth tremor, a mechanism
inside the seismograph would open the mouth of one dragon. O The
bronze ball would fall into the open mouth of one of the toads,
making enough noise to alert someone that an earthquake had just
happened. O The direction from which the earthquake came by seeing
which dragon's mouth was empty.
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O In the 1850s Robert Mallet, figured out a means to measure
the velocity of seismic waves. O Meanwhile, in Italy, Luigi
Palmieri invented an electromagnetic seismograph, one of which was
installed near Mount Vesuvius and another at the University of
Naples. O These seismographs were the first seismic instruments
capable of routinely detecting earthquakes.
Slide 16
O In 1872 a U.S. scientist named Grove Gilbert figured out that
earthquakes usually center around a fault line. O It was after the
1906 earthquake in San Francisco that Harry Reid hypothesized that
earthquakes were likely the result of a build-up of pressure along
these faults.
Slide 17
O It was about 1910 that Alfred Wegener published his theory of
plate tectonics to explain volcanic and seismic activity. O Since
then, seismologists have continued to work at a furious pace,
building better instruments, computer models, theories and forecast
to study the causes and effects of earthquakes.
Slide 18
Modern Seismographs O Most seismographs today are electronic,
but a basic seismograph is made of a drum with paper on it, a bar
or spring with a hinge at one or both ends, a weight, and a pen. O
The one end of the bar or spring is bolted to a pole or metal box
that is bolted to the ground. O The weight is put on the other end
of the bar and the pen is stuck to the weight.
Slide 19
O The drum with paper on it presses against the pen and turns
constantly. O When there is an earthquake, everything in the
seismograph moves except the weight with the pen on it. O As the
drum and paper shake next to the pen, the pen makes squiggly lines
on the paper, creating a record of the earthquake. This record made
by the seismograph is called a seismogram.
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Slide 23
O By studying the seismogram, the seismologist can tell how far
away the earthquake was and how strong it was. O This record does
not tell the seismologist exactly where the epicenter was, just
that the earthquake happened so many miles or kilometers away from
that seismograph.
Slide 24
O In a seismogram, there will be wiggly lines all across it. O
These wiggly lines are seismic waves that the seismograph has
recorded. O Most of these waves were so small (microseisms) that
nobody felt them. O At the time of earthquake, the P- wave will be
the first wiggle, which is bigger than the microseisms.
Slide 25
O P-waves are the fastest seismic waves, and are usually the
first ones that a seismograph records. O The next set of seismic
waves on the seismogram will be the S-waves and these are normally
bigger than the P- waves. O The surface waves (Love and Rayleigh
waves) are often larger waves marked on the seismogram.
Slide 26
Slide 27
O Surface waves travel a little slower than S-waves, so they
tend to arrive at the seismograph just after the S- waves. O For
shallow earthquakes, the surface waves may be the largest waves
recorded by the seismograph. O Often they are the only waves
recorded at long distance, from medium-sized earthquakes.
Slide 28
MEASURING EARTHQUAKES O The Richter scale O The Mercalli Scale
O The Modified Mercalli Intensity Scale
Slide 29
The Richter scale O The magnitude of most earthquakes is
measured on the Richter scale, invented by Charles F. Richter in
1934. O The Richter magnitude is calculated from the amplitude of
the largest seismic wave recorded for the earthquake, no matter
what type of wave was the strongest.
Slide 30
O The Richter magnitudes are based on a logarithmic scale (base
10). O What this means is that for each whole number you go up on
the Richter scale, the amplitude of the ground motion recorded by a
seismograph goes up ten times. O On this scale, a earthquake of
magnitude 5 would result in ten times the level of ground shaking
as a earthquake of magnitude 4 and 32 times as much energy would be
released.
Slide 31
Magnitude Vs Ground Motion & Energy MAGNITUDE CHANGEGROUND
MOTION CHANGE (DISPLACEMENT) ENERGY CHANGE 1.010.0 TIMES32 TIMES
0.53.2 TIMES5.5 TIMES 0.32.0 TIMES3 TIMES 0.11.3 TIMES1.4
TIMES
Slide 32
O For example, a magnitude of 6.0 earthquake produces 10 times
more ground motion than a magnitude of 5.0 earthquake. O The energy
difference is about 32 times. O The energy release is the best
indicator of destructive power of earthquake.
Slide 33
O Because of the logarithmic basis of the scale, each whole
number increase in magnitude represents a tenfold increase in
amplitude. O Each increase in magnitude scale, corresponds to the
release of 32 times more energy.
Slide 34
Bhuj 2001 and Sumatra 2004 O Bhuj - Magnitude 7.7 & Sumatra
Magnitude 9.1. O The magnitude scale is logarithmic scale. O (10
9.1 /10 7.7 ) = 10 1.4 = 25.1189 O (i.e.) Sumatra earthquake is
25.1189 times greater than Bhuj earthquake. O In other words,
Sumatra earthquake is equal to 25.1189 Bhuj earthquakes.
Slide 35
Energy difference calculation O Based on empirical formula log
(E) is proportional to 1.5M. O Where, E is energy and M is
magnitude. O 10 1.5 is approximately 32 times. O ((10 1.5 ) 9.1
)/((1o 1.5 ) 7.7 ) = 10 (1.5*1.4) = 125.8925 times energy
released.
Slide 36
Richter Scale (Magnitudes) Earthquake Effects Less than 3.5
Generally not felt, but recorded. 3.5-5.4 Often felt, but rarely
causes damage. 5.5 - 6.0 At most slight damage to well-designed
buildings. Can cause major damage to poorly constructed buildings
over small regions. 6.1-6.9 Can be destructive in areas up to about
100 km across where people live. 7.0-7.9 Major earthquake. Can
cause serious damage over larger areas. 8 or greater Great
earthquake. Can cause serious damage in areas several hundred
kilometers across.
Slide 37
The Mercalli Scale O Another way to measure the strength of an
earthquake is to use the Mercalli scale. O Invented by Giuseppe
Mercalli in 1902, this scale uses the observations of people who
experienced the earthquake to estimate its intensity. O The
Mercalli scale is not considered as scientific as the Richter
scale. O Some witnesses of the earthquake might exaggerate just how
bad things were during the earthquake. O Therefore, the amount of
damage caused by the earthquake may not accurately record how
strong it was either.
Slide 38
The Modified Mercalli Intensity Scale O The effect of an
earthquake on the earth's surface is called the intensity. O
Although numerous intensity scales have been developed over the
last several hundred years to evaluate the effects of earthquakes,
the one currently used in the United States is the Modified
Mercalli (MM) Intensity Scale. O It was developed in 1931 by the
American seismologists, Harry Wood and Frank Neumann.
Slide 39
O This scale, composed of 12 increasing levels of intensity
that range from imperceptible shaking to catastrophic destruction,
is designated by Roman numerals. O It does not have a mathematical
basis; instead, it is an arbitrary ranking based on observed
effects. O The Modified Mercalli Intensity value assigned to a
specific site after an earthquake has a more meaningful measure of
severity to the nonscientist than the magnitude because intensity
refers to the effects actually experienced at that place.
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O I) Not felt, except by a very few, under especially favorable
conditions. O II) Felt only by a few persons at rest, especially on
upper floors of buildings. O 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 are similar to the passing of a truck.
Slide 42
O 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. O V) Felt by nearly
everyone; many awakened. Some dishes, windows broken. Unstable
objects overturned. Pendulum clocks may stop. O VI) Felt by all,
many frightened. Some heavy furniture moved; a few instances of
fallen plaster.
Slide 43
O 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. O 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.
Slide 44
O 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. O X. Some well-built wooden structures destroyed; most
masonry and frame structures destroyed with foundations. Rails
bent. O XI. Few, if any (masonry) structures remain standing.
Bridges destroyed. Rails bent greatly. O XII. Damage total. Lines
of sight and level are distorted. Objects thrown into the air.
Slide 45
Locating Earthquakes
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O Measure the distance between the first P wave and the first S
wave. O In this case, the first P and S waves are 24 seconds apart.
O Find the point for 24 seconds on the left side of the chart below
and mark that point. O According to the chart, this earthquake's
epicenter was 215 kilometers away.
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O Measure the amplitude of the strongest wave. O The amplitude
is the height of the strongest wave. O On this seismogram, the
amplitude is 23 millimeters. O Find 23 millimeters on the right
side of the chart and mark that point.
Slide 50
O Place a ruler (or straight edge) on the chart between the
points you marked for the distance to the epicenter and the
amplitude. O The point where your ruler crosses the middle line on
the chart marks the magnitude (strength) of the earthquake. O This
earthquake had a magnitude of 5.
Slide 51
Finding the Epicenter O Check the scale of a map. It is
different for different maps. On the map, one centimeter could be
equal to 100 kilometers or something like that. O Figure out how
long the distance to the epicenter (in centimeters) is on the map.
For example, say a map has a scale where one centimeter is equal to
100 kilometers. If the epicenter of the earthquake is 215
kilometers away, that equals 2.15 centimeters on the map.
Slide 52
O Using compass, draw a circle with a radius equal to the
number came up with in Step #2. O The radius is the distance from
the center of a circle to its edge. O The center of the circle will
be the location of the seismograph. O The epicenter of the
earthquake is somewhere on the edge of that circle. O Do the same
thing for the distance to the epicenter that the other seismograms
recorded. O All of the circles should overlap. O The point where
all of the circles overlap is the approximate epicenter of the
earthquake.
Slide 53
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Focus of an earthquake O It is otherwise called as hypocenter.
O The position where the strain energy stored in the rock is first
released, marks the point where the fault begins to rupture. O The
focal depth can be calculated from measurements based on seismic
wave phenomena.
Slide 55
O As with all wave phenomena in physics, there is uncertainty
in such measurements that grows with the wavelength. O So the focal
depth of the source of these long-wavelength waves is difficult to
determine exactly.
Slide 56
O Very strong earthquakes radiate a large fraction of energy is
released in seismic waves. O This is associated with very long
wavelengths. O Therefore a stronger earthquake involves the release
of energy from a larger mass of rock.
Slide 57
Depth of an earthquake O Earthquakes can occur anywhere between
the Earth's surface and about 700 kilometers below the surface. O
For scientific purposes, this earthquake depth range of 0 - 700 km
is divided into three zones: O Shallow, Intermediate, and
Deep.
Slide 58
O Shallow earthquakes are between 0 km and 70 km deep. O
Intermediate earthquakes, 70 - 300 km deep and O Deep earthquakes,
300 - 700 km deep. O In general, the term "deep-focus earthquakes"
is applied to earthquakes deeper than 70 km.
Slide 59
Calculating Depth O The most obvious indication on a seismogram
that a large earthquake has a deep focus is the small amplitude of
the recorded surface waves. O The surface waves does generally
indicate that an earthquake is either shallow or may have some
depth.
Slide 60
O The most accurate method of determining the focal depth of an
earthquake is to read a depth phase recorded on the seismogram. O
The depth phase is the characteristic phase pP. O pP initially goes
up from the earthquake source, reflects off the Earth's
surface.
Slide 61
O Then it follows closely behind the P wave to arrive at the
seismograph. O At distant seismograph stations, the pP follows the
P wave by a time interval that changes slowly with distance but
rapidly with depth. O This time interval, pP-P (pP minus P), is
used to compute depth-of- focus tables.
Slide 62
O Then the additional travel time for pP is simply twice the
vertical travel time from hypocenter to the surface. O (i.e.) The
extra travel time as (pP - P)=2d/v, O Where, (pP - P) is the travel
time difference, d is hypo central depth, and v is the average P
wave velocity above the source.
Slide 63
O Using the time difference of pP-P as read from the seismogram
and the distance between the epicenter and the seismograph station,
the depth of the earthquake can be determined from published
travel-time curves or depth tables.
Slide 64
Epicenter O The epicenter is the point on the Earth's surface
that is directly above the hypocenter or focus, the point where an
earthquake or underground explosion originates. O In the case of
earthquakes, the epicenter is directly above the point where the
fault begins to rupture, and in most cases, it is the area of
greatest damage.
Slide 65
O But for larger events, the length of the fault rupture is
much longer, and damage can be spread across the rupture zone. O
For example, in the magnitude 7.9, 2002 Denali earthquake in
Alaska, the epicenter was at the western end of the rupture. O But
the greatest damage occurred about 330 km away at the eastern end
of the rupture zone.
