Magnitude and Intensity
Intensity
 How Strong Earthquake Feels to Observer



Qualitative assessment of the kinds of damage
done by an earthquake
Depends on distance to earthquake & strength of
earthquake
Determined from the intensity of shaking and
damage from the earthquake
Magnitude
 Related to Energy Release
Release.



Quantitative measurement of the amount of
energy released by an earthquake
Depends on the size of the fault that breaks
Determined from Seismic Records
Measuring
g Earthquakes
q

Seismogram is visual record of arrival time and
magnitude
g
of shaking
g associated with seismic
wave. Analysis of seismogram allows
measurement of size of earthquake.

Mercalli Intensity scale
 Measured
by the amount of damage caused in
human terms
terms-- I (low) to XII (high); drawback:
inefficient in uninhabited area

Ri ht S
Richter
Scale
Scalel - (logarithmic
(l
ith i scale)
l )
 Magnitude
Magnitude--
based on amplitude of the waves
 Related
R l t d to
t earthquake
th
k total
t t l energy
Intensity
How Strong Earthquake Feels to Observer
Depends On:
 Distance to Quake
 Geology
 Type of Building
 Observer!
Varies from Place to Place
 Modified Mercalli ScaleScale- 1 to 12
E th
Earthquake
k Magnitude
M
it d

ML - Local (Richter) magnitude

MW - Seismic Moment magnitude
g

MS - Surface wave magnitude

MB- Body wave magnitude
Richter Scale

Richter Scale




Amplitude scale is logarithmic (10
(10
10--fold increase for
every whole number increase)
Scale 1 ---- 0.001 mm; 2---- 0.01 mm; 5---- 10
10mm;
mm;
7---- 1 meter
Earthquake Energy: Each whole number
represents a 33
33--fold increase in Energy; Energy
diff
difference
b
between 3 & 6 means ~1000
~1000 times
i
Drawbacks:
 Based on Antiquated WoodWood-Anderson
Seismographs
 Measurement Past Magnitude 7.0 ineffective –
Requires Estimates
Local Magnitude of Earthquake

Magnitude
 Richter scale measures the magnitude of an earthquake, based on
seismogram independent of intensity
 Amplitude of the largest wave produced by an event is corrected for
distance and assigned a value on an openopen-ended logarithmic scale
 The
Th equation
ti for
f Richter
Ri ht Magnitude
M it d is:
i
ML = log10A(mm) + (Distance correction factor)
ML = log
l 10A(mm)
(
) +3 log
l 10[8 t (sec)]-2.93
(
)] 2 93
Here A is the amplitude, in millimeters, measured directly from the
pphotographic
g p paper
p p record of the Wood-Anderson seismometer,, a
special type of instrument. The distance factor comes from a table given
by Richter (1958).
Richter’s Local Magnitude
Right side diagram (nomogram)
demonstrates how to use
Richter's
Richter
s original method to
measure a seismogram for a
magnitude estimate After you
measure the wave amplitude
you have to take its logarithm
and scale it according
g to the
distance of the seismometer
from the earthquake, estimated
b th
by
the S
S-P
P ti
time diff
difference. Th
The
S-P time, in seconds, makes t.
The equation behind this
nomogram, used by Richter in
Southern California, is:
ML = log10A(mm) +3 log10[8 t (sec)]-2.93
Richter Scale: Related to intensity

M=1 to 3: Recorded on local seismographs, but
M=1
generally not felt

M= 3 to 4: Often felt, no damage

M=5
M=
5: Felt widely, slight damage near epicenter

M=6: Damage to poorly constructed buildings and
M=6
other structures within 10
10's
's km

M=7: "Major" earthquake, causes serious damage
M=7
up to ~
~100
100 km .

M=8: "Great"
M
M=8
"G t" earthquake,
th
k greatt destruction,
d t ti
loss
l
of life over several 100 km

M=9
M=
M
9: Rare great earthquake, major damage over
a large region over 1000 km
Surface Wave Magnitude
g
Richter’s local magnitude does not distinguish between different
types of waves
waves.
At large distances from epicenter, ground motion is dominated
by surface waves.
Gutenberg and Richter (1936) developed a magnitude scale
based on the amplitude of Rayleigh waves.
S f
Surface
wave magnitude
it d Ms = log
l 10A + 1.66
1 66 log
l 10 +2
A = Maximum ground displacement in micrometers
= Distance of seismograph from the epicenter
epicenter, in degrees
degrees.
Surface wave magnitude is used for shallow earthquakes
Body
y Wave Magnitude
g
For deep focus earthquakes, reliable measurement of amplitude
of surface waves is difficult
difficult.
Amplitudes of P-waves are not strongly affected by focal depth.
Gutenberg (1945) developed a magnitude scale based on the
amplitude
lit d off the
th first
fi t few
f
cycles
l off PP waves, which
hi h iis useful
f l ffor
measuring the size of deep earthquakes.
Body
y wave magnitude
g
Mb = log
g10A – log
g10T +0.01  + 5.9
A = Amplitude of P-waves in micrometers
T = Period of P wave
= Distance of seismograph from the epicenter, in degrees.
Seismic - Moment Magnitude
A Seismograph Measures Ground Motion at
One Instant But - A Really Great Earthquake Lasts Minutes
 Releases Energy over Hundreds of
Kilometers
 Need to Sum Energy of Entire Record
 Moment magnitude scale based on seismic
moment (Kanamori, 1977)
1977) and doesn’t
depend upon ground shaking levels
levels.
 It’s the only magnitude scale efficient for any
size of earthquake
earthquake.
Moment Magnitude
g

Moment-Magnitude Scale
Moment Seismic Moment = Strength of Rock x Fault Area x Total amount of
Sli along
Slip
l
R
Rupture
t
M0 =  A D
Moment Magnitude Mw = 2/3 x [log10M0(dyne
(dyne--cm) –16
16]]

Measurement Analysis requires Time
Displacement in horizontal direction A strike-slip fault
Seismic Energy
Both the magnitude and the seismic moment are related to the
amount of energy that is radiated by an earthquake. Dr.
G
Gutenberg
b
and
d Richter
Ri h 1956),
19 6) developed
d
l
d a relationship
l i
hi b
between
magnitude and energy. Their relationship is:
logES = 11.8 + 1.5Ms
Energy ES in ergs from the surface wave magnitude Ms. ES is not
the total ``intrinsic'' energy of the earthquake, transferred from
sources such as gravitational energy or to sinks such as heat
energy. It is only the amount radiated from the earthquake as
seismic waves
waves, which ought to be a small fraction of the total
energy transferred during the earthquake process.
Local Magnitude - Seismic Energy correlation
Gujarat (2001)
Size of an earthquake
q
using
g the Richter’s Local Magnitude
g
Scale is shown on the left
hand side of the figure above. The larger the number, the bigger the earthquake. The
scale on the right hand side of the figure represents
the amount of high explosive
This figure was produced in cooperation with the US
required to produce the energy released by the
earthquake.
Geological
Survey, and the University of Memphis private
i
Frequency of earthquakes
Last 83 earthquakes
Aug 28,
28 2010 - Sep 28,
28 2010
Shiraz
Earthquake
Mon, Sep. 27
27,, 2010
2010,,
Mehr,5
Mehr,
5,1389
ML=5.9
Shiraz
Earthquake
Mon, Sep. 27
27,, 2010
2010,,
Mehr,5
Mehr,
5,1389
ML=5.9
Shiraz
Earthquake
Mon, Sep. 27
27,, 2010
2010,,
M h 5,1389
Mehr,5
Mehr,
ML=5.9