TRINET'S --- SHAKEMAP ---
-- A Broadcaster's Guide to Reporting Earthquake Intensity --
Earthquake Information Summary:
Date: Fri Jun 13, 2003
Time: 07:34:00 AM EDT
Location: N47.70 W70.09: Charlevoix, Quebec
Name:Unnamed Earthquake (Event ID 0)
Magnitude: 4.1
ShakeMap on the world-wide web: http://www.trinet.org/shake
INSTRUMENTAL INTENSITY SCALE:
ESTIMATED INTENSITYPERCEIVED SHAKINGPOTENTIAL DAMAGE
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INot FeltNone
II-IIIWeakNone
IVLightNone
VModerateVery Light
VIStrongLight
VIIVery StrongModerate
VIIISevereModerate/Heavy
IXViolentHeavy
XExtremeVery Heavy
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TRINET is the seismic network in southern California operated
cooperatively by:
United States Geological Survey (USGS)
California Institute of Technology (Caltech)
California Division of Mines and Geology (CDMG)
CONTACTS:
Dr. David Wald, USGS, Pasadena
Dr. Bruce Worden, USGS, Pasadena
Mr. James Goltz, Caltech
What is earthquake intensity? How does ShakeMap display intensity?
ShakeMap is a geographic representation of the ground shaking produced by
an earthquake. Intensity is one of the ways that ground shaking is
expressed, along with more quantitative measures like velocity and
acceleration. The information ShakeMap presents is different from the
earthquake magnitude. Magnitude is the number (for example, 7.1) that
represents the energy released in an earthquake; a single number
representing magnitude is assigned to each earthquake. Intensity, on the
other hand, is a measure of how the ground shook at a particular site. So,
while an earthquake has one magnitude and one epicenter, it produces a
range of ground shaking levels at sites throughout the region. These
different intensities depend on distance from the earthquake, the rock and
soil conditions at geographical sites, and variations in the propagation
of seismic waves from the earthquake due to complexities in the structure
of the Earth's crust. ShakeMap focuses on the ground shaking produced by
the earthquake, rather than the characteristics of the earthquake source.
How are earthquake intensities expressed?
Intensities portray the effects of an earthquake in a particular
location. These effects include potential damage, perception of shaking
and permanent changes in topography. Whether there was damage, and the
severity of observed damage, is one element of intensity. Intensity scales
also take into account how the earthquake was perceived by persons in a
geographic location ranging from "not felt" to "unable to stand." The
impact of the earthquake shaking on the ground, whether, for example,
cracks or displacements occurred, or in some cases of severe shaking,
landslides, is also a feature of measured intensity.
The most popular intensity scale used in the United States is the Modified
Mercalli Scale (MMI) first developed in 1931. This scale uses Roman
Numerals to represent progressively greater shaking from MMI I in which
"people do not feel any earth movement" to MMI X in which "most buildings
and their foundations are destroyed, bridges and dams are severely damaged
and large landslides occur." Historically, intensities were derived in
the months following an earthquake through questionnaires sent to Post
Offices in the impacted area. Postal officials were asked to report the
effects of shaking in their district and their observations were combined
with those of scientists and engineers. When all the questionnaires and
observations were combined, they were used to construct an intensity map
of the earthquake.
With the development of the new seismic network under the TriNet project,
intensity (ShakeMaps) maps can be generated automatically from measured
ground acceleration and velocity. These "instrumental intensities" are
calculated and mapped within 5 minutes of the earthquake and are thus
termed "real-time" maps. To the extent possible, shaking expressed as
intensity in ShakeMap correspond to the observed intensity of the older
Modified Mercalli scale. To provide news broadcasters guidance in the
interpretation of intensities reported in ShakeMap, we have provided below
the ten intensity levels the and human perception, damage and topographic
change associated with each level.
It is important to note in reporting intensity information contained in
ShakeMap that shaking and damage vary considerably from location to
location, and only some structures may exhibit the effects noted below in
an area assigned a particular intensity.
MODIFIED MERCALLI INTENSITY SCALE
MMI
Value Full Description
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IPeople do not feel any earth movement.
II Felt by persons at rest, on upper floors of tall buildings
IIIFelt by people indoors. Hanging objects swing back and forth.
Vibration from the earthquake may seem like the passing of light
trucks. May not be recognized as an earthquake.
IV Hanging objects swing. Vibration may seem like the passing of heavy
trucks or a jolt, like a heavy ball striking the walls. Parked
vehicles may rock noticeably. Windows, dishes, doors may rattle and
glasses clink. In the upper range of IV, walls of wood frame
buildings may creak.
VAlmost everyone feels movement whether inside or outdoors. Sleeping
people are awakened. Liquids in containers are disturbed; some are
spilled. Small unstable objects are displaced or overturned. Doors
swing, close, or open. Shutters, pictures on the wall move.
VIFelt by all; some are frightened and take cover. People have
difficulty walking due to motion. Objects fall from shelves and
dishes, glassware and ceramics may be broken. Pictures fall off
walls. Furniture moves or is overturned. Weak plaster and masonry
cracked. Damage slight in poorly constructed buildings. Trees, bushes
shaken visibly or are heard rustling.
VII People have difficulty standing. Drivers on the road feel their cars
shaking. Furniture may be overturned and broken. Loose bricks fall
from buildings and masonry walls and cracks in plaster and masonry
may appear. Weak chimneys may break at the roofline. Damage is slight
to moderate in well-built structures; considerable in poorly
constructed buildings and facilities.
VIII Drivers have trouble steering. Tall structures such as towers,
monuments and chimneys may twist and fall. Wood frame houses that
are not bolted to their foundations may shift and sustain serious
damage. Damage is slight to moderate in well-constructed buildings,
considerable in poorly constructed buildings. Branches are broken and
fall from trees. Changes occur in flow or temperature of springs and
wells. Cracks appear in wet ground and on steep slopes.
IXMasonry str...