EARTHQUAKE
MEASURING SCALES
CEELA230 (Earthquake Engineering)
3rd Term AY2024-2025
Engr. Arleen Sacdalan-Pablo
Faculty COE-CE Dept. 02/04/2025 1
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Concern:
➢ How are earthquakes recorded?
➢ How are earthquakes measured?
➢ How is the magnitude of an earthquake
determined?
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Recap:
❑ Magnitude measures the energy released at the
source of the earthquake.
➢ Magnitude is determined from measurements
on seismographs.
❑ Intensity measures the strength of shaking produced
by the earthquake at a certain location.
➢ Intensity is determined from effects on people,
human structures, and the natural
environment.
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How are earthquakes recorded?
❑ Earthquakes are recorded by a seismograph
network. Each seismic station in the network
measures the movement of the ground at that site.
The slip of one block of rock over another in an
earthquake releases energy that makes the ground
vibrate. That vibration pushes the adjoining piece of
ground and causes it to vibrate, and thus the
energy travels out from the earthquake hypocenter
in a wave .
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How Earthquake Magnitude is measured?
The following are the Scales used to measure
Earthquake Magnitude.
a. Richter Scale
b. Body Wave Magnitude Scale
c. Surface Wave Magnitude Scale
d. Moment Magnitude Scale
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RICHTER MAGNITUDE
SCALE
• First scale for measuring earthquake
magnitudes
• Developed by Charles F. Ricther in
1934
• Logarithmic scale, each unit
represents a ten-fold increase in the
amplitude of the seismic waves.
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MOMENT MAGNITUDE SCALE
- Was introduced by Hanks and Kanamori in 1979
- It is based on the total moment release of the earthquake.
- Moment is a product of the distance a fault moved and the
force required to move it.
- It is preferred because it works over a wide range of
earthquake sizes and is globally.
- It is capable of measuring magnitude 8 and greater events
accurately than Richter’s.
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EARTHQUAKE MAGNITUDE
CALCULATION
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Earthquake magnitude
calculations
• The magnitude of an earthquake is a number that relates to the
amplitude of the earthquake.
• Earthquake magnitude scales are logarithmic (i.e. a 1 unit increase in
magnitude corresponds to a 10 fold increase in amplitude).
• Scientists can only estimate the true magnitude of an earthquake by
measuring its effects, this leads to earthquakes appearing to have
different magnitudes depending on what method is used for estimating
the magnitude and which datasets have been used to make this
estimate.
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Earthquake magnitude
calculations
• Press reporters love the Richter scale and will report any earthquake
magnitude as a 'magnitude on the Richter scale'. However, for any
large earthquake that has made the news, it is very unlikely that a
Richter (or Local magnitude) is an appropriate scale.
• Due to the complexities of the calculations the reported magnitude
of events can change as more data gets analyzed. For the
devastating 24 December 2004 event in Sumatra the original
magnitude of Mw 9.0 was recalculated several months later to Mw
9.3 as more data was analyzed.
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Richter magnitude (Local Magnitude ML)
❑ Originally earthquake magnitudes were based on the
amplitude of ground motion displacement as measured by a
standard seismograph. The best known of these is the Richter
Magnitude which was defined for local earthquakes in southern
California.
ML = logA + 2.56logD - 1.67
Where A is the measured ground motion (in micrometres) and D is the
distance from the event (in km). This is still used for measuring the magnitude of
shallow events at distances less than 600 km (today called the Local
Magnitude). For events larger than magnitude 8 this scale saturates and gives
magnitude estimates that are too small.
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Moment Magnitude Scale
(Mw) = 2/3log(Mo) - 6.06
❑ Nowadays the moment magnitude scale is the one used by
seismologists to measure large earthquakes. The historic
Richter magnitude is calculated by measuring the deflection
on a seismometer corrected for distance from the event.
Richter magnitudes underestimate the size of large events
and are no longer used. However, the constants used in the
definition of Moment magnitude (Mw) were chosen so that
the magnitude numbers for Richter and Moment magnitudes
match for smaller events.
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Body wave magnitude Mb
❑ For earthquakes measured at distances greater than
600 km magnitude can be estimated from the
formula.
Mb = log(A/T) + σ(D,h)
Where A is the maximum amplitude (in
micrometres)of the P-waves measured at period T
(generally about one second) and σ is a calibration term
(in the range 6–8) that depends on distance from the
event D and depth of the event h (tables of σ are used).
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Surface wave magnitude Ms
❑ For shallow earthquakes (i.e. ones that generate
surface waves) magnitude can be estimated using the
formula.
MS = log(A/T) + 1.66logΔ + 3.3
Where A is the maximum amplitude (in micrometres)
of the Rayleigh waves, T is the period (usually about 20
seconds) and Δ is the distance (in degrees).
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SCALES THAT MEASURE EARTHQUAKE INTENSITY
a. Mercalli Intensity Scale
b. Modified Mercalli Intensity Scale
c. Rossi-Forel Intensity Scale
d. PHIVOLCS Earthquake Intensity Scale
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MERCALLI INTENSITY SCALE
- Invented by Giuseppe Mercalli in 1902
- rates earthquakes on a scale of 1 to 12
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MERCALLI
INTENSITY
SCALE
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MODIFIED MERCALLI INTENSITY SCALE
• developed in 1931 by the American seismologists
Harry Wood and Frank Neumann.
• currently used in the United States
• rates earthquakes on a scale of 1 to 10
• has shown that some criteria are more reliable than
others as indicators of the level of ground shaking.
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MODIFIED MERCALLI INTENSITY SCALE
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ROSSI-FOREL INTENSITY SCALE
- one of the first seismic intensity scales,
- developed by Michele Stefano Conte de Rossi of Italy and Francois
Alphonse of Switzerland during the late 19th century,
- has 10 intensity levels
- it was used commonly for about two decades until the introduction of
the Mercalli Intensity Scale in 1902,
- used in some countries, such as the Philippines until 1996 when it was
replaced by the PHIVOLCS Earthquake Intensity Scale (PEIS).
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ROSSI-FOREL INTENSITY SCALE
ROSSI-FOREL INTENSITY SCALE
I. Microseismic tremor. Recorded by a single seismograph or by seismographs of the same model,
but not by several seismographs of different kinds. The shock felt by an experienced observer.
II. Extremely feeble tremor. Recorded by several seismographs of different kinds. Felt by a small
number of persons at rest.
III. Feeble tremor. Felt by several persons at rest. Strong enough for the direction or duration to be
appreciable.
IV. Slight tremor. Felt by persons in motion. Disturbance of movable objects, doors, windows, cracking
of ceilings.
V. Moderate tremor. Felt generally by everyone. Disturbance of furniture, ringing of some bells.
VI. Strong tremor. General awakening of those asleep. General ringing of bells. Oscillation of
chandeliers, stopping of clocks, visible agitation of trees and shrubs. Some startled persons leaving
their dwellings.
VII. Very strong tremor. Overthrow of movable objects, fall of plaster, ringing of church bells. General
panic. Moderate to heavy damage buildings.
VIII. Damaging tremor. Fall of chimneys. Cracks in the walls of buildings.
IX. Devastating tremor. Partial or total destruction of buildings.
X. Extremely high intensity tremor. Great disaster, ruins, disturbance of the strata, fissures in the
ground, rock falls from mountains
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• developed by the Philippine
Institute of Volcanology and
Seismology (PHIVOLCS), as a
response to the 1990 Luzon
Earthquake, - PHIVOLCS cites
seismic scale specifically
developed for the Philippine
setting, the different geography
of each country and other
"geological considerations" led
to the development of PEIS.
• The PEIS was adopted in the
Philippines in 1996 replacing the
Rossi Forel Scale.
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https://youtu.be/HL3KGK5eqaw
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