The Modified Mercalli Intensity Scale
The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series
of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally total destruction. 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. It was developed in 1931 by the American seismologists Harry Wood and Frank
Neumann. This scale, composed of increasing levels of intensity that range from imperceptible shaking to
catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is
an arbitrary ranking based on observed effects.
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.
The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by
people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually
contribute information for assigning intensity values of VIII or above.
The following is an abbreviated description of the levels of Modified Mercalli intensity.
Intensity Shaking
Description/Damage
I
Not felt Not felt except by a very few under especially favorable conditions.
II
Weak
Felt only by a few persons at rest, especially on upper floors of buildings.
Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many
III
Weak
people do not recognize it as an earthquake. Standing motor cars may rock slightly.
Vibrations similar to the passing of a truck. Duration estimated.
Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes,
IV
Light
windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking
building. Standing motor cars rocked noticeably.
Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects
V
Moderate
overturned. Pendulum clocks may stop.
Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen
VI
Strong
plaster. Damage slight.
Damage negligible in buildings of good design and construction; slight to moderate in
Very
VII
well-built ordinary structures; considerable damage in poorly built or badly designed
strong
structures; some chimneys broken.
Damage slight in specially designed structures; considerable damage in ordinary
VIII
Severe
substantial buildings with partial collapse. Damage great in poorly built structures. Fall of
chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
Damage considerable in specially designed structures; well-designed frame structures
IX
Violent thrown out of plumb. Damage great in substantial buildings, with partial collapse.
Buildings shifted off foundations.
Some well-built wooden structures destroyed; most masonry and frame structures
X
Extreme
destroyed with foundations. Rails bent.
Richter Scale
What is the Richter Scale
The Richter magnitude scale (often shortened to Richter scale) is the most common standard of measurement
for earthquakes. It was invented in 1935 by Charles F. Richter of the California Institute of Technology as a
mathematical device to compare the size of earthquakes. The Richter scale is used to rate the magnitude of an
earthquake, that is the amount of energy released during an earthquake.
Image: What is the Richter Scale
The Richter scale doesn't measure quake damage (see: Mercalli Scale) which is dependent on a variety of
factors including population at the epicenter, terrain, depth, etc. An earthquake in a densely populated area
which results in many deaths and considerable damage may have the same magnitude as a shock in a remote
area that does nothing more than frightening the wildlife. Large-magnitude earthquakes that occur beneath the
oceans may not even be felt by humans.
The magnitude of an earthquake is determined using information gathered by a seismograph.
Seismograph (Seismometer)
What is a seismograph?
A seismograph, or seismometer, is an instrument used to detect and record seismic waves. Seismic waves are
propagating vibrations that carry energy from the source of an earthquake outward in all directions. They travel
through the interior of the Earth and can be measured with sensitive detectors called seismographs. Scientists
have seismographs set up all over the world to track the movement of the Earth’s crust.
Seismic waves are divided into two types: body waves and surface waves.
Body waves include P (compressional or primary) waves and S (transverse or secondary) waves. An earthquake
radiates P and S waves in all directions and the interaction of the P and S waves with the Earth's surface and
shallow structure produces surface waves. Near an earthquake the shaking is large and dominated by shearwaves and short-period surface waves. These are the waves that do the most damage to our buildings, highways,
etc.
At farther distances the amplitude of the seismic waves decreases as the energy released by the earthquake
spreads throughout a larger volume of Earth. Also with increasing distance from the earthquake, the waves are
separated apart in time and dispersed because P, S, and surface waves travel at different speeds.
Image: Body Waves and Surface Waves
Love waves and Rayleigh waves are surface waves. Love waves are transverse waves that vibrate the ground in
the horizontal direction perpendicular to the direction that the waves are travelling. They are recorded on
seismometers that measure the horizontal ground motion.
Image: Love waves are transverse and restricted to horizontal movement
Rayleigh waves are the slowest of all the seismic wave types and in some ways the most complicated. Like
Love waves they are dispersive so the particular speed at which they travel depends on the wave period and the
near-surface geologic structure, and they also decrease in amplitude with depth. Typical speeds for Rayleigh
waves are on the order of 1 to 5 km/s.
Generally, a seismograph consists of a mass attached to a fixed base. During an earthquake, the base moves and
the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical
voltage. The electrical voltage is recorded on paper, magnetic tape, or another recording medium. The record
written by a seismograph in response to ground motions produced by an earthquake or other ground-motion
sources is called seismogram.
How does a seismograph work?
Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the
instrument. This record is proportional to the motion of the seismometer mass relative to the earth, but it can be
mathematically converted to a record of the absolute motion of the ground. These are examples on how
seismographs work. Nowadays they are much more modern but the physical principles are the same:
Video: How a seismograph works https://youtu.be/eE54rBak8vA
Video: How a seismograph works https://youtu.be/MAkX4mjyKyU
The magnitude of an earthquake is determined by readings of the seismic waves resulting from the vibrations
generated by the seismic source. Sensitive seismographs, which greatly magnify these ground motions, can
detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an
earthquake can be determined from the data recorded by seismograph stations.
Image: Seismograph
How is the Richter Scale calculated
The Richter magnitude involves measuring the amplitude (height) of the largest recorded wave at a specific
distance from the seismic source. Adjustments are included for the variation in the distance between the various
seismographs and the epicenter of the earthquakes.
The Richter scale is a base-10 logarithmic scale, meaning that each order of magnitude is 10 times more
intensive than the last one. In other words, a two is 10 times more intense than a one and a three is 100 times
greater. In the case of the Richter scale, the increase is in wave amplitude. That is, the wave amplitude in a level
6 earthquake is 10 times greater than in a level 5 earthquake, and the amplitude increases 100 times between a
level 7 earthquake and a level 9 earthquake. The amount of energy released increases 31.7 times between whole
number values.
Image: How is the Richter Scale calculated
In fact, it is to point out that, while it is correct to say that for each increase in 1 in the Richter magnitude there
is a tenfold increase in amplitude of the wave, it is incorrect to say that each increase of 1 in Richter magnitude
represents a tenfold increase in the size of the earthquake (as is commonly incorrectly stated by the press). A
better measure of the size of an earthquake is the amount of energy released by the earthquake, which is related
to the Richter Scale by the following equation: Log E = 11.8 + 1.5 M (where Log refers to the logarithm to the
base 10, E is the energy released in ergs and M the Richter magnitude).
Richter Scale formula
This above principle is expressed by the original Richter Scale formula as:
where A is the maximum excursion of the Wood-Anderson seismograph, the empirical function A0 depends
only on the epicentral distance of the station, . In practice, readings from all observing stations are averaged
after adjustment with station-specific corrections to obtain the ML value.
Richter Scale rating explained
On the Richter scale, magnitude is expressed in whole numbers and decimal fractions. For example, a
magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as
magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude
represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the
magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the
preceding whole number value.
Image: Richter Scale Graphic Representation
At first, the Richter scale could be applied only to the records from instruments of identical manufacture. Now,
instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the
record of any calibrated seismograph.
The Richter Scale has no upper limit. Recently, another scale called the moment magnitude scale MMS has
been devised for more precise study of great earthquakes.
The Richter scale Explained in a video
Video: Learn about Richter Scale - What is it? https://youtu.be/wVqYg4NPvC4
Video: Bill Nye the Science Guy- Earthquakes and Richter Scale https://youtu.be/wVqYg4NPvC4
Video: How things work: How Does the Richter Scale Work? https://youtu.be/wVqYg4NPvC4
Highest magnitude ever recorded
Tectonic earthquakes can range in size from magnitudes less than zero, resulting from fault slippage of a few
centimeters, to the largest events (magnitude greater than 9), where fault displacements are on the order of
many meters. The size of an earthquake is not only a function of the amount of displacement but also the area of
the fault plane that ruptures. Hence the larger the rupture area, the larger is the earthquake. A magnitude 7
earthquake ruptures a fault area of about 1000 km2 or about 50 km long and 20 km wide.
Also depth is an important factor influencing earthquake severity. We know that earthquakes can originate at
various depths within the Earth’s solid core. The deeper the earthquake, the more powerful it is, but it is also far
less likely to reach the surface. That’s why shallow earthquakes are more common and more dangerous,
because the shallower an earthquake, the more damage to surface structures it can cause.
The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to
the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude
earthquake in Japan in 2011 (as of March 2011), and it was the largest Japanese earthquake since records began.
Image: Photo from March 2011 earthquake in Japan
Fortunately most earthquakes are extremely small. A majority of quakes register less than 3 on the Richter
scale; these tremors, called micro-earthquakes, aren’t generally felt by people and are usually recorded only on
local seismographs. With every whole point the magnitude rises, the strength of the waves increases tenfold.
Events with magnitudes of about 4.5 or greater - there are several thousand such shocks annually - are strong
enough to be recorded by sensitive seismographs all over the world. Only a tiny portion of earthquakes (15 or so
of the 1.4 million quakes that register above 2.0) register at 7 or above, which is the threshold for a quake to be
considered major.
A temblor with a magnitude of 8.0 or higher is considered a "great earthquake." On the average, one earthquake
of such size, such as the 1964 Good Friday Earthquake in Alaska and the 2011 Japanese Earthquake and
Tsunami, occurs somewhere in the world each year. The biggest quake in recorded history was the 9.5 quake
that struck Chile in 1960. It killed nearly 1,900 people and caused about $4 billion in damage in 2010 dollars.
Generally, you won't see much damage from earthquakes that register below 4 on the Richter scale.
Largest Earthquakes ever recorded
The 10 largest magnitudes in recorded history
1. Valdivia, Chile 22 May 1960 (magnitude 9.5)
This earthquake killed 1655 people, injured 3000 and displaced two million. It caused US$550 million damage
in Chile, while the tsunami that it spawned caused deaths and damage as far away as Hawaii, Japan and the
Philippines. The 'rupture zone' of the quake was more than 1000 km long. Two days after the initial quake, the
nearby volcano Puyehue erupted, sending ash and steam up to 6 km into the atmosphere over a period of several
weeks.
2. Prince William Sound, Alaska 28 March 1964 (magnitude 9.2)
Compared to the Chilean earthquake, this earthquake was less damaging: the resulting tsunami took 128 lives
and caused overall US$311 million in damage. The earthquake was felt mainly over Alaska, as well as some
places in Canada, while the tsunami created by it caused damage as far away as Hawaii. The most damage was
sustained by the city of Anchorage, 120 km north-west of the epicentre. Shaking from the quake itself is
reported to have lasted for three minutes.
3. Sumatra, Indonesia 26 December 2004 (magnitude 9.1)
In terms of damage and loss of life, the scale of the disaster caused by the resulting Boxing Day Tsunami was
enormous. In total, 227,900 people were killed or presumed dead, with around 1.7 million displaced over 14
countries in South Asia and East Africa. The epicentre was 250 km south-east of Band Aceh, Indonesia, at a
depth of 30 km. Several days later on 28 December, a mud volcano began erupting near Baratang, Andamar
Islands, which is thought to have been associated with the earthquake.
4. Sendai, Japan 11 March 2011 (magnitude 9.0)
So far the official death toll stands at several thousand from the combined effect of the powerful earthquake,
aftershocks and the tsunami. However, the total is expected to rise, with some estimates of a final toll of over
10,000. Economic impacts are expected to be huge, with the shutting down of nuclear reactors which many
industries rely on for power.
5. Kamchatka, Russia 4 November 1952 (magnitude 9.0)
This earthquake generated a tsunami that caused widespread damage in the Hawaiian Islands. Property damage
was estimated at around US$1,000,000. Some reports describe waves of over 9 m high at Kaena Point, Oahu. A
farmer on Oahu reported the loss of six cows to the tsunami, but no people were reported killed.
6. Bio-bio, Chile 27 February 2010 (magnitude 8.8)
This earthquake and subsequent tsunami killed at least 521 people, with 56 missing and 12,000 injured. More
than 800,000 people were displaced with a total of 1.8m people affected across Chile, where damage was
estimated at US$30 billion. The epicentre was 335 km south-west of Santiago, at a depth of 35 km. A minor
tsunami travelled across the Pacific causing damage to boats as far away as San Diego, California.
7. Off the coast of Ecuador 31 January 1906 (magnitude 8.8)
This earthquake caused a tsunami that is reported to have killed between 500 and 1,500 in Ecuador and
Colombia. The tsunami travelled as far north as San Francisco, on the west coast of the US, and west to Hawaii
and Japan. The tsunami took roughly 12 hours to cross the Pacific to Hilo, Hawaii.
8. Rat Islands, Alaska 2 April 1965 (magnitude 8.7)
The worst of the damage attributed to this earthquake was caused by a tsunami, reported to be about 10 m high
on Shemya Island. The wave caused flooding on Amchitka Island, causing US$10,000 in property damage. No
deaths or injuries were reported.
9. Sumatra, Indonesia 28 March 2005 (magnitude 8.6)
This earthquake killed 1313, with over 400 people injured by the tsunami as far away as Sri Lanka. The
epicentre was 205 km north-west of Sibolga, Sumatra, at a depth of 30 km. This region, also the site of the 2004
Boxing Day Tsunami, is particularly geologically active, with three of the 15 biggest known earthquakes having
happened here.
10. Assam - Tibet 15 August 1950 (magnitude 8.6)
This inland earthquake caused widespread damages to buildings as well as large landslides. 780 people were
killed in eastern Tibet, with many villages and towns affected across Assam, China, Tibet and India.
Oscillations to lake levels occurred as far away as Norway. The total death toll is likely to be higher, as no
definitive total was ever estimated. While the earthquake itself is known as the Assam Earthquake, it is believed
the epicentre may have been in Tibet.