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Earthquake intensities are numerical values assigned to the effects of earthquakes
on people and their works, and on the natural environment. Intensities are evaluated
using the Modified Mercalli Intensity Scale of 1931, which contains levels of effects
ranging from intensity I, barely perceptible, to intensity XII, total damage.
Although the development of seismological instruments and instrumental
techniques the past 80 years has been impressive, earthquake intensity
remains among the most acceptable criteria on which to base earthquake
risk factors and to project future seismic activity (Brazee, 1976).
This, in part, is due to the extensive availability of earthquake
intensity data (from the mid-1600s) compared to the availability of
earthquake instrumental data (from about 1897).
Earthquake intensity information is a unique and valuable data source
for the research seismologist, the structural engineer, the actuary, and
the earth science instructor. Although published information on
earthquake effects has been available for many years (see "References"),
compiling comprehensive lists of earthquakes and the intensities
that were experienced at nearby cities still required much time and
effort by the researcher.
In the early 1960s, however, preparing lists of historical earthquakes
became less laborious and time consuming after the U.S. Coast and
Geodetic Survey compiled a computerized data file of worldwide
earthquakes. The file not only contained epicenters of earthquakes but
also held other valuable information on each event, including the
maximum reported intensity. The new file still had limitations,
however, because the names of cities experiencing the earthquakes and
the intensities at these cities were not included in the data base. The
next logical step, therefore, was to compile another file--an
Earthquake Intensity File--that not only would include instrumental
earthquake data, but also would contain the important ancillary data
on earthquake effects.
Description of Database
The Earthquake Intensity File contains more than 157,000 reports on over
20,000 earthquakes that affected the United States from 1638 through 1985.
The principal data included for each earthquake in the file are the names
and geographic coordinates of cities (or localities) that have reported
effects from earthquakes (hereafter called "reporting cities") and the
intensities assigned to those effects. Each intensity has been assigned
using the Modified Mercalli Intensity Scale of 1931 (Wood and Neumann, 1931).
Other information given for each earthquake includes: distance of each
reporting city from the epicenter of the earthquake; number of hours to
subtract from Universal Time (UT) to obtain origin time in local standard
time; reference (authority) codes for reporting cities and intensity values;
and state codes. In addition, the date, origin time, epicenter, magnitude,
and depth (where available) are given for all earthquakes.
Although the Earthquake Intensity File represents an important contribution
to seismology research, it has several limitations that should be mentioned:
- About 25 percent of the 2,500 earthquakes reported from 1638-1928
and 10 percent of the 18,500 events from 1928-80 do not have instrumental
epicenters; this omission is mainly due to the fact that seismological
instruments were not developed until the late 1800s, and further that the
instruments were not widely distributed for many years later.
- Several of the reporting cities (or localities) listed in the file have not
been assigned geographic coordinates.
- The file contains data primarily for those earthquakes that have epicenters
in the United States, nearby U.S. territories, and areas of Canada and Mexico
that border the United States. Data for a few events in the Philippines
(from the late 1930s through 1941) are also included.
Definition of Variables
- Year Mo Da Hr Mn Sec
- The Date and Time are listed in Universal Coordinated Time and are
Year, Month (Mo), Day (Da), Hour (Hr), Minute (Mn), Second (Sec)
- UTC Conv
- Number of hours to subtract from the Date and Time given in Universal
Coordinated Time to get local standard time for the epicenter. In
general:
-
- 4 = 60 degree meridian (Atlantic Standard Time)
- 5 = 75 degree meridian (Eastern Standard Time)
- 6 = 90 degree meridian (Central Standard Time)
- 7 = 105 degree meridian (Mountain Standard Time)
- 8 = 120 degree meridian (Pacific Standard Time)
- 9 = 135 degree meridian (Alaska Standard Time)
- 10 = 150 degree meridian (Hawaii-Aleutian Standard Time)
Use care when converting Universal Coordinated Time to local standard time.
It is uncertain how some of the conversion factors in this database were
determined--for example, whether Daylight Saving Time or Standard Time was
used. Many, but not all, areas represented in this database have observed
Daylight Saving Time during the summer.
Prior to 1966, states and communities that observed Daylight Saving used
whatever beginning and ending dates they chose. The Uniform Time Act
of 1966 provided that any area that observes Daylight Saving Time must
begin and end on specified federal dates.
The Uniform Time Act of 1966, P.L. 89-387, section 4(c), also established
four time zones in Alaska:
- Pacific Standard Time (120th meridian--southeastern Alaska--8 hours behind UTC)
- Yukon Standard Time (135th meridian--Yakutat--9 hours behind UTC)
- Alaska-Hawaii Standard Time (150th meridian--most of Alaska, from
west of Yakutat to just east of Cold Bay, and northward--10 hours behind UTC)
- Bering Standard Time (165th meridian--westward from just east of Cold Bay, at the western
end of the Alaska Peninsula--11 hours behind UTC)
On October 30, 1983, the number of time zones in Alaska was reduced from four to two,
which placed 90 percent of Alaska on the same time, just one hour behind the U.S. West
Coast; the boundary between the two zones is just west of Umnak Island in the Aleutian
chain:
- Alaska Standard Time (9 hours behind UTC)
- Hawaii-Aleutian Standard Time (10 hours behind UTC)
- U/G
- Unpublished or grouped intensity
- U = Intensity (MMI) assigned that was not listed in the source document.
- G = Intensity grouped I-III in the source document was reassigned intensity III.
- EQ Lat / EQ Long
- This is the geographic latitude and longitude of the epicenter expressed as decimal
numbers. The units are degrees. The latitude range is +4.0 to +69.0, where "+" designates
North latitude (there are no South latitudes in the database). The longitude range is
-179.0 to +180.0, where "-" designates West longitude and "+" designates East longitude.
Most of the epicenters are West longitude (from -56 to -179), but a few epicenters in the
Philippines and Aleutian Islands are East longitude (from +120 to +180).
- Mag
- These are magnitudes as listed in United States Earthquakes, Earthquake
History of the United States (either mb, MS, or ML), or the equivalent derived from
intensities for pre-instrumental events. The magnitude is a measure of seismic energy. The
magnitude scale is logarithmic. An increase of one in magnitude represents a tenfold increase
in the recorded wave amplitude. However, the energy release associated with an increase of one
in magnitude is not tenfold, but thirtyfold. For example, approximately 900 times more energy
is released in an earthquake of magnitude 7 than in an earthquake of magnitude 5. Each increase
in magnitude of one unit is equivalent to an increase of seismic energy of about 1,600,000,000,000 ergs.
- Depth (km)
- Hypocentral Depth (positive downward) in kilometers from the surface.
- Epi Dis
- Epicentral Distance in km that the reporting city (or locality) is located from the epicenter of the earthquake.
- City Lat / City Long
- This is the geographic latitude and longitude of the city (or locality) where the Modified Mercalli
Intensity was observed, expressed as decimal numbers. The units are degrees. The latitude range is +6.0 to +72.0,
where "+" designates North latitude (there are no South latitudes in the database). The longitude range is
-177.0 to +180.0, where "-" designates West longitude and "+" designates East longitude. Most of the reporting
cities (or localities) are West longitude (from -29 to -177), but a few reporting cities (or localities) in
the Philippines and Aleutian Islands are East longitude (from +119 to +180).
- MMI
- Modified Mercalli Scale Intensity (MMI)
is given in Roman Numerals. Values range from I to XII.
(Roman Numerals were converted to numbers in the digital database.
Values range from 1 to 12.) Macroseismic information is compiled from
various sources including newspaper articles, foreign broadcasts, U.S.
Geological Survey Earthquake reports and seismological station reports.
- State Code
- Numerical identifier for state, province, or country in which the earthquake
was reported (felt) by residents:
01 Alabama 02 Alaska 03 Arizona
04 Arkansas 05 California 07 Colorado
08 Connecticut 09 Delaware 10 District of Columbia
11 Florida 12 Georgia 14 Hawaii
15 Idaho 16 Illinois 17 Indiana
18 Iowa 19 Kansas 20 Kentucky
21 Louisiana 22 Maine 23 Maryland
24 Massachusetts 25 Michigan 26 Minnesota
27 Mississippi 28 Missouri 29 Montana
30 Nebraska 31 Nevada 32 New Hampshire
33 New Jersey 34 New Mexico 35 New York
36 North Carolina 37 North Dakota 38 Ohio
39 Oklahoma 40 Oregon 41 Pennsylvania
42 Puerto Rico 43 Rhode Island 45 South Carolina
46 South Dakota 47 Tennessee 48 Texas
49 Utah 50 Vermont 51 Virginia
52 Virgin Islands 54 Washington 55 West Virginia
56 Wisconsin 57 Wyoming 58 West Indies
74 Panama 75 Philippine Is. 80 Mexico
81 Baja California 90 Canada 91 Alberta
92 Manitoba 93 Saskatchewan 94 British Columbia
95 Ontario 96 New Brunswick 97 Quebec
98 Nova Scotia 99 Yukon Territory
- City Name
- City (or locality) in which the earthquake was reported (felt) by residents.
- Data Source
- This is a code referring to the source of one or more of the reported parameters (e.g., epicenter, city and intensity).
A = Source unknown; 1925 earthquake in Boston area (reports not listed in source H).
B = Report by Bollinger and Stover, 1976.
C = Quarterly Seismological Reports, 1925-27.
D = Source unknown; 1937-1977 earthquakes in Hawaii, California, and eastern U.S.
H = Earthquake History of the United States (Coffman and others, 1982).
K = Report by Carnegie Institution, 1908, 1910.
M = Source unknown; 1899-1912 earthquakes in Alaska.
N = Report by Nuttli, 1973.
Q = Abstracts of Earthquake Reports for the United States, 1933-70.
S = Unpublished report by Nina Scott, 1965.
T = Source unknown; 1872-1904 earthquakes along U.S. west coast.
U = United States Earthquakes, 1928-85.
W = Monthly Weather Service Seismological Reports, 1914-24.
History of Earthquake Intensity Scales
Although the history of the development of earthquake intensity scales
is long and interesting, only a brief summary will be presented here.
Poarid, an Italian, made the first known attempt to classify earthquakes
by intensity in 1627; he used a scale of four levels of intensity to
describe effects of earthquakes experienced at different towns.
Although many additional attempts were made in the 18th and 19th
centuries to develop comprehensive scales for measuring earthquake
intensity, none was more widely used than that formulated in 1873 by
M. S. de Rossi of Italy and F. A. Forel of Switzerland. But this scale
also had severe limitations, and therefore was superseded in the early
1900s by Mercalli's revised intensity scale; it contained 12 levels of
intensity.
In 1931, a modified version of Mercalli's scale was published by H. O.
Wood and Frank Neumann. Known as the Modified Mercalli Intensity
Scale of 1931, it has become the standard used by the United States
engineering seismology community, which includes the National
Geophysical Data Center and the U.S. Geological Survey.
An interpretation of the values is listed in Table 1.
Table 1.--Modified Mercalli Intensity Scale of 1931
| I. |
Not felt except by a very few under especially
favorable circumstances. |
| II. |
Felt only by a few persons at rest, especially
on upper floors of buildings. Delicately suspended objects may swing. |
| III. |
Felt quite noticeably indoors, especially on
upper floors of buildings, but many people do not recognize it as an
earthquake. Standing motor cars may rock slightly. Vibration like passing
truck. Duration estimated. |
| IV. |
During the day felt indoors by many, outdoors by
few. At night some awakened. Dishes, windows, and doors disturbed; walls make
creaking sound. Sensation like heavy truck striking building. Standing
motorcars rock noticeably. |
| V. |
Felt by nearly everyone; many awakened. Some
dishes, windows, etc., broken; a few instances of cracked plaster; unstable
objects overturned. Disturbance of trees, poles, and other tall objects
sometimes noticed. Pendulum clocks may stop. |
| VI. |
Felt by all; many frightened and run outdoors.
Some heavy furniture moved; a few instances of fallen plaster or damaged
chimneys. Damage slight. |
| VII. |
Everybody runs outdoors. Damage negligible in
buildings of good design and construction slight to moderate in well built
ordinary structures; considerable in poorly built or badly designed structures.
Some chimneys broken. Noticed by persons driving motor cars. |
| VIII. |
Damage slight in specially designed structures;
considerable in ordinary substantial buildings, with partial collapse; great
in poorly built structures. Panel walls thrown out of frame structures. Fall
of chimneys, factory stacks, columns, monuments, walls. Heavy furniture
overturned. Sand and mud ejected in small amounts. Changes in well water.
Persons driving motor cars disturbed. |
| IX. |
Damage considerable in specially designed
structures; well-designed frame structures thrown out of plumb; great in
substantial buildings, with partial collapse. Buildings shifted off foundations.
Ground cracked conspicuously. Underground pipes broken. |
| X. |
Some well-built wooden structures destroyed; most
masonry and frame structures destroyed with foundations; ground badly cracked.
Rails bent. Landslides considerable from river banks and steep slopes.
Shifted sand and mud. Water splashed over banks. |
| XI. |
Few, if any (masonry), structures remain standing.
Bridges destroyed. Broad fissures in ground. Underground pipelines completely
out of service. Earth slumps and land slips in soft ground. Rails bent greatly. |
| XII. |
Damage total. Waves seen on ground surfaces. Lines
of sight and level distorted. Objects thrown upward into the air. |
Reference: Harry O. Wood and Frank Neumann, in Bulletin of the
Seismological Society of America, Vol. 21, No. 4, December 1931.
es Geophysicae, 4B, (6), Paris, France, 1986, pp. 679-702. |
Comparison of Modified Mercalli (MM) and Other Intensity Scales
Modified
Mercalli Rossi-Forel Japanese European
I I 0 I
II I-II I II
III III II III
IV IV-V II-III IV
V V-VI III V
VI VI-VII IV VI
VII VIII- IV-V VII
VIII VIII+ to V VIII
IX-
IX IX+ V-VI IX
X X VI X
XI ----- VII XI
XII ----- ----- XII
Collecting Data on Earthquake Intensity
The U.S. Geological Survey (USGS) is the Federal agency responsible for
collecting earthquake intensity data. The USGS carries out this responsibility
using a questionnaire called "Earthquake Report," and also sends field
investigators to the scene of destructive earthquakes to analyze the resulting
damage. Different versions of the questionnaire have been used since the
mid-1920s by several groups who were responsible for collecting intensity data.
The present questionnaire contains pertinent questions about earthquake effects
that enable a seismologist to evaluate the intensity of the earthquake in all
parts of the shaken area; it also is designed specifically for computer
processing.
In the past, the USGS sent questionnaires immediately after each U.S. earthquake
to postmasters, National Weather Service offices, military installations, and
others, requesting that they report all effects of the earthquake in their area.
If the earthquake was damaging, expert observers travelled to the field to
investigate and photograph the damage incurred. This data-collection program
was supplemented by newspaper accounts of earthquake effects, published
scientific reports, and reports provided by seismology collaborators.
After the completed earthquake questionnaires were returned to USGS, a
seismologist analyzed each report and assigned intensities on the basis of the
effects at each town; intensity (or isoseismal) maps were then constructed for
earthquakes felt over large areas. Descriptions and maps of these events were
published annually in the United States Earthquakes series, and a summary of
the stronger earthquakes (MM intensity >= V) was published periodically in
Earthquake History of the United States.
The USGS now only canvasses selected earthquakes. If you want to report an
earthquake, use the National Earthquake Information Center's Did You Feel it? page.
References
The main sources of data used in compiling the Earthquake Intensity Database
Earthquake History of the United States
This publication is a summary of all earthquakes (intensity V and above)
that have occurred in the United States and its territories from earliest
recorded history (about 1638 in the New England region) through 1980. The
1982 edition of this publication (Coffman and others, 1982) contains revised
epicenters and intensities for several earthquakes. This source, therefore,
is the authority for epicenters of significant earthquakes in the file and
also for most intensities of MM intensity >= V. In addition, pages xi-xii
of "Earthquake History" contain several addenda and corrigenda, which have
been used to update information in the Intensity File.
United States Earthquakes
Much of the intensity data in the Earthquake Intensity Database for 1928-85
were taken from this annual report. Its publication in 1928 began a continuing
program of collecting comprehensive effect reports on all earthquakes in the
United States and its territories. This publication not only contains brief
descriptions of all earthquakes that were felt or damaging, but also includes
MM intensities for most of the cities (or localities) in which the tremors were
observed. For earthquakes in 1928-31, however, published intensities were
assigned using the Rossi-Forel (RF) intensity scale. Where included in the
Intensity File, these intensities are converted to values on the MM scale. In
addition, some descriptions of earthquake effects in United States Earthquakes
were not assigned an intensity because the effect information was insufficiently
detailed (e.g., "slight," "feeble," "felt"). To make the computerized file as
complete as possible, NCEI assigned an intensity of II to earthquake reports
that had only the slightest of details, and an intensity of III to earthquake
reports that were grouped as intensity I-III.
Quarterly Seismological Report
This publication, which was published by the U.S. Coast and Geodetic
Survey, was a source of information on reporting cities and intensities
for the years 1925-27. All Rossi-Forel intensities assigned to
earthquake reports during this period have been converted to the MM
Intensity Scale. Where omitted, intensities have been assigned
according to the method described in the preceding paragraph.
Abstracts of Earthquake Reports for the United States
This quarterly publication includes all the effect data collected for
earthquakes in the United States and its territories during the 1933-70 period.
Because the quarterly "Abstracts" report contains only preliminary earthquake
data, information in United States Earthquakes annual reports and in Earthquake
History of the United States is preferred, for they contain additional
reporting cities and revised intensity information.
Monthly Weather Service Seismological Reports
These reports represent a main source for information on earthquake
effects in the United States and nearby territories for December 1914
through June 1924. As in other pre-1931 publications all intensities
were assigned according to the Rossi-Forel scale; however, minimal
reported effects (e.g., slightly felt) were not assigned intensities.
NCEI, therefore, has assigned intensities to all effects and, in
addition, has converted Rossi-Forel intensities to the MM scale.
Bibliography
- Bollinger, G.A. and Carl W. Stover, 1976: List of Intensities for the
1886 Charleston, South Carolina, Earthquake, U.S. Geological Survey
Open-File Report 76-66, 31 p.
- Brazee, Rutlage J., August 1976: Final Report: An Analysis of Earthquake
Intensities with Respect to Attenuation, Magnitude, and Rate of
Recurrence (Revised Edition), NOAA Tech. Memorandum EDS NGSDC-2,
NOAA/National Geophysical Data Center, Boulder, Colorado, 53 p. (plus
appendices).
- Carnegie Institution of Washington, vols. 1-2, 1908, 1910: California
Earthquake of April 18, 1906, Report of the State Earthquake
Investigation Commission, v.1 (by Andrew C. Lawson); v. 2 (by Harry F.
Reid), Washington, D.C., 623 p. (plus Atlas).
- Coffman, Jerry L., Carl A. von Hake, and Carl W. Stover, 1982: Earthquake History of the United States, Publication 41-1, Revised
Edition (with Supplement Through 1980), National Oceanic and Atmospheric
Administration and U.S. Geological Survey, Boulder, Colorado, 258 p.
- Dutton, Clarence E., 1889: "The Charleston Earthquake of August 31,
1886," Ninth Annual Report, 1887-88, U.S. Geological Survey, Washington
D.C., p. 203-528.
- Meyers, Herbert, April 1976: A Historical Summary of Earthquake
Epicenters in and Near Alaska, NOAA Tech. Memorandum EDS NGDC-1,
NOAA/National Geophysical Data Center, Boulder, Colorado, 57 p. (plus
appendices).
- Meyers, Herbert and Carl A. von Hake, 1976: Earthquake Data File
Summary, Key to Geophysical Records Documentation No. 5, NOAA/National
Geophysical Data Center, Boulder, Colorado, 32 p. (plus appendices).
- Meyers, Herbert, R.J. Brazee, J.L. Coffman, and S.R. Lessig, 1976: An
Analysis of Earthquake Intensities and Recurrence Rates in and Near
Alaska, NOAA Technical Memorandum EDS NGSDC-3, NOAA/National Geophysical
Data Center, Boulder, Colorado, 101 p.
- Murphy, Leonard M. (Scientific Coordinator), 1973: San Fernando,
California, Earthquake of February 9, 1971, vols. 1-3,
NOAA/Environmental Research Laboratories, Washington D.C.
- Nuttli, Otto W., 1973: "The Mississippi Valley Earthquakes of 1811 and
1812; Intensities, Ground Motion, and Magnitudes," Bulletin of the
Seismological Society of America, v. 63, No.1, p. 227-248.
- Scott, Nina, 1965: List of Modified Mercalli intensities for the April
8, 1906, San Francisco, California, Earthquake, Prepared under contract for
U.S. Geological Survey, unpublished manuscript.
- Tarr, Ralph S. and Lawrence Martin, 1912: "The Earthquakes at Yakutat
Bay, Alaska, in September 1899," U.S Geological Survey Professional
Paper 69, U.S. Department of the Interior, Geological Survey, Washington
D.C., 135 p.
- U.S. Department of Commerce, Coast and Geodetic Survey, Quarterly
Seismological Report, 1925-27, Washington D.C.
- U.S. Department of Commerce, Environmental Science Services
Administration, vols. 1-3, 1966, 1967, 1969: The Prince William Sound,
Alaska, Earthquake of 1964 and Aftershocks, Publication 10-3 (in three
volumes), Washington, D.C.
- U.S. Department of Commerce, National Oceanic and Atmospheric
Administration (and predecessor agencies), Abstracts of Earthquake
Reports for the United States, quarterly issues 1933-70. (Earlier title:
Abstracts of Earthquake Reports for the Pacific Coast and the Western
Mountain Region).
- U.S. Department of Commerce, National Weather Service, Monthly Weather
Review, December 1914-June 1924 (Seismological reports are usually
described in the issues for the month in which the earthquakes
occurred), Washington D.C.
- U.S. Geological Survey, United States Earthquakes, published
annually by: Coast and Geodetic Survey from 1928-68; National Oceanic
and Atmospheric Administration (NOAA) from 1969-72; NOAA/U.S. Geological
Survey from 1973-80; and U.S. Geological Survey from 1981-1986.
- Wood, Harry O. and Frank Neumann, Dec. 1931: "Modified Mercalli
Intensity Scale of 1931," Bulletin of the Seismological Society of
America, v. 21, No. 4, p. 277-283.
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