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Earthquake Intensity Database
1638 - 1985

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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:
  1. 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.
  2. Several of the reporting cities (or localities) listed in the file have not been assigned geographic coordinates.
  3. 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.
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, NGDC has 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. NGDC, therefore, has assigned intensities to all effects and, in addition, has converted Rossi-Forel intensities to the MM scale.

Bibliography

  1. 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.
  2. 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).
  3. 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).
  4. 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.
  5. 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.
  6. 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).
  7. 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).
  8. 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.
  9. Murphy, Leonard M. (Scientific Coordinator), 1973: San Fernando, California, Earthquake of February 9, 1971, vols. 1-3, NOAA/Environmental Research Laboratories, Washington D.C.
  10. 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.
  11. 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.
  12. 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.
  13. U.S. Department of Commerce, Coast and Geodetic Survey, Quarterly Seismological Report, 1925-27, Washington D.C.
  14. 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.
  15. 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).
  16. 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.
  17. 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.
  18. 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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