MEASURING EARTHQUAKES
Dr. N. VENKATANATHAN
venkatanathan@eee.sastra.edu
Surface Waves
P Waves
S Waves
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.
Loss due to Bhuj 2001Earthquake
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.
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.
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.
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.
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.
O In the 1850’s 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.
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.
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.
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.
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.
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.
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 Pwave will be the first wiggle, which is
bigger than the microseisms.
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 Pwaves.
O The surface waves (Love and Rayleigh
waves) are often larger waves marked
on the seismogram.
O Surface waves travel a little slower
than S-waves, so they tend to arrive at
the seismograph just after the Swaves.
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.
MEASURING EARTHQUAKES
O The Richter scale
O The Mercalli Scale
O The Modified Mercalli Intensity
Scale
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 .
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.
Magnitude Vs Ground Motion
& Energy
MAGNITUDE CHANGE
GROUND MOTION
CHANGE
(DISPLACEMENT)
ENERGY CHANGE
1.0
10.0 TIMES 32 TIMES
0.5
3.2 TIMES 5.5 TIMES
0.3
2.0 TIMES 3 TIMES
0.1
1.3 TIMES 1.4 TIMES
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.
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.
Bhuj 2001 and Sumatra 2004
O Bhuj
- Magnitude 7.7 & Sumatra –
Magnitude 9.1.
O The magnitude scale is logarithmic
scale.
O (109.1/107.7) = 101.4 = 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.
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 101.5 is approximately 32 times.
9.1
7.7
O ((101.5) )/((1o1.5) ) = 10(1.5*1.4) =
125.8925 times energy released.
Richter Scale
(Magnitudes)
Less than 3.5
3.5-5.4
5.5 - 6.0
6.1-6.9
7.0-7.9
8 or greater
Earthquake Effects
Generally not felt, but recorded.
Often felt, but rarely causes damage.
At most slight damage to well-designed buildings. Can
cause major damage to poorly constructed buildings
over small regions.
Can be destructive in areas up to about 100 km
across where people live.
Major earthquake. Can cause serious damage over
larger areas.
Great earthquake. Can cause serious damage in
areas several hundred kilometers across.
The Mercalli Scale
O Another
O
O
O
O
way to measure the strength of an
earthquake is to use the Mercalli scale.
Invented by Giuseppe Mercalli in 1902, this scale uses
the observations of people who experienced the
earthquake to estimate its intensity.
The Mercalli scale is not considered as scientific as
the Richter scale.
Some witnesses of the earthquake might exaggerate
just how bad things were during the earthquake.
Therefore, the amount of damage caused by the
earthquake may not accurately record how strong it
was either.
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.
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.
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.
O
O
O
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.
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.
O
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.
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.
O
O
O
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.
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.
Locating Earthquakes
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.
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.
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.
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.
O Using compass, draw a circle with a radius equal to the
O
O
O
O
O
O
number came up with in Step #2.
The radius is the distance from the center of a circle to
its edge.
The center of the circle will be the location of the
seismograph.
The epicenter of the earthquake is somewhere on the
edge of that circle.
Do the same thing for the distance to the epicenter that
the other seismograms recorded.
All of the circles should overlap.
The point where all of the circles overlap is the
approximate epicenter of the earthquake.
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.
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.
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.
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.
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.
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.
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.
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-offocus tables.
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.
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.
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.
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.