Laboratory Manual for Introductory Geology 4e

EXERCISE 11.6
Locating Epicenters of Historic Earthquakes
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With the modern worldwide network of seismic stations, any earthquake epicenter can be located quickly and accurately.
But what about earthquakes that took place before seismometers were invented? How can we define Earth’s zones of
seismic activity if we can’t include earthquakes that occurred as (geologically) recently as 100 or 200 years ago? If there
are records of damage associated with those events, geologists can estimate epicenter locations by making isoseismal
maps based on the modified Mercalli Intensity Scale that show the geographic distribution of damage. First, historical
reports of damage are analyzed for each location and given an approximate intensity value. These data are plotted on a
map, and the map is contoured to show the variation of the damage. Ideally, the epicenter is located within the area of
greatest damage.
The figure below shows Mercalli intensity values for an 1872 earthquake that shook much of the Pacific Northwest.
Draw contour lines the map to show areas of equal damage (isoseismal areas) and indicate with an x the location for the
epicenter of this earthquake.
Modified Mercalli intensity values for the December 1872 Pacific Northwest earthquake.
IV
V
VI
V
VI
VI
IV
Kootenay
VI
VI
VII
Vancouver
BRITISH COLUMBIA
VII
VI
Astoria
VI
V
V
V
VI
Victoria
Portland
V
IV
VII
VI
WASHINGTON
Seattle
VI
VI
VI
V
V
IV
WASHINGTON
OREGON
V
IV
IV
V
IV
Spokane
Walla Walla
V
IV IV
IV
OREGON
IDAHO
MONTANA
Elk City
III
11.4 MEASURING THE STRENGTH OF AN EARTHQUAKE
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