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Caveat
Erroneous statistical conclusions can be drawn from the
numbers of earthquakes taken from Catalog of Significant Earthquakes,
2150 B.C. to the present. The reporting of large or destructive earthquakes
is not homogeneous in space or time, particularly for periods prior to
the 1900s. Because this publication mainly lists those earthquakes that
have caused death or damage, the number of earthquake reports is dependent
on the written history available for a particular region, as well as on
the rate of development of population centers and related structures.
Therefore, it is misleading to use the numbers of significant earthquakes
in that publication to suggest statistically that there has been an
increase in worldwide seismic activity since 1900 or for any time period.
that "apparent" increase in activity:
Instrumental seismology is a young science. The first calibrated
instruments to measure seismic waves traveling through the earth did not
appear until the late 1800s. At that time, seismologists became aware of
the vast numbers of earthquakes occurring throughout the world, but
because of the insensitivity of their instruments they were able to
locate only the large magnitude events.
The 1960s saw two major advances. First, a network of seismological
observatories, the Worldwide Standardized Seismograph Network (WWSSN),
was installed by the United States Government, principally
to monitor underground nuclear tests. These sensitive instruments could
detect and identify earthquakes anywhere in the world from about
magnitude 4.5.
Second computers became available in the late 1960s. Computers allowed
seismologists to leave inaccurate and cumbersome graphical methods of
locating earthquakes, and to process the increasing volume of new network
data more rapidly than ever before. Prior to 1962, only hundreds of
earthquake epicenters were determined each year by Government and academic
institutions, but the number increased to the thousands using computerized
location methods. In some special local studies, more than 100,000
earthquakes per year were identified and located.
In summary, using the data in Catalog of Significant Earthquakes,
2150 B.C. to the present to suggest that there has been an increase in
worldwide earthquake activity is misleading and erroneous. The above
observations and reporting factors must also be considered when making
statistical studies based on that historical data report.
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Uncertainties in the Significant Earthquake and Tsunami Databases
The same problems that can lead to erroneous statistics
discussed in the Caveat, also introduce uncertainties
in the earthquake and tsunami databases for
events prior to the late 1800s, and in some areas for events prior to the
installation of the WWSSN in 1962. Before the invention of seismographs
in the late 1800s magnitudes, times and locations of earthquakes
and tsunami sources were
determined from descriptions of earthquake damage and tsunami effects.
Earthquake epicenters and tsunami source locations may have been assigned
to the wrong places when the names of localitieis were incorrectly
transcribed or when some localities had identical or very similar names.
Errors may have also been introduced when the local times of earthquakes
and tsunami reports were incorrectly converted to Universal Coordinated Time
by catalogers.
Earthquake and tsunami events can also be listed twice when the
dates are recorded in different sources according to the Julian or the
Gregorian calendars.
As a result, the earthquake and tsunami databases may include listings
of events on different dates that are actually descriptions of the same
event. For a more complete discussion of these errors
please see Historical Seismograms and Earthquakes of the World,
edited by W.H.K. Lee, H. Meyers and K. Shimzaki, 1988, Academic
Press, Inc., San Diego, California,
513 p.
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