1928-1970 Washington and Oregon
Earthquake information and Isoseismal Maps

from: United States Earthquakes, U.S. Department of Commerce, Coast and Geodetic Survey 		PNSN Logo
"United States Earthquakes"- ANNUAL LISTINGS OF EARTHQUAKES
U.S. Earthquakes was series of publications, one per year, that described earthquakes in the U.S. during that year. From 1928 to 1970, U.S. Earthquakes is the most complete source of information on earthquakes in the United States. Before 1928, earthquake information was collected in a somewhat uneven way by weather observers and lighthouse keepers. Beginning in 1928, the U.S. Coast and Geodetic Survey began systematic collection of information on felt earthquakes throughout the United States. Standard forms were filled out by local Postmasters and analyzed by U.S. Coast and Geodetic Survey scientists. A "Modified Mercalli" intensity was assigned to each community where the earthquake was felt, and isoseismal maps were created for some of the larger earthquakes. Larger earthquakes were sometimes recorded on the few seismometers in our region, and origin times with asterisks earthquakes that were recorded instrumentally. After 1970, more seismograph stations were installed in the Pacific Northwest, allowing better location and magnitude estimates.

WHAT ARE EARTHQUAKE INTENSITIES?
Earthquake intensity is a measure of the amount of ground shaking at a particular site, and it is determined from reports of human reaction to shaking, damage done to structures, and other effects. The Modified Mercalli Intensity Scale is now the scale most commonly used to determine intensities for earthquakes felt in the United States.

WHAT ARE ISOSEISMAL MAPS?
An isoseismal map shows countours of earthquake intensities. A single earthquake will produce different intensities at various places.

AN EXAMPLE: The magnitude 6.5 April 29, 1965, Seattle-Tacoma earthquake produced intensity VII to VIII damage near its epicenter, intensity V damage 150 kilometers away, and intensity I and 11 (barely felt) 300 to 500 kilometers from the epicenter. Although the greatest damage, and thus highest intensity, is usually near the earthquake's origin, damage to buildings depends on many factors, such as the type of construction, distance from the epicenter, and type of soil beneath the building. Therefore, maps of earthquake intensity commonly show complex patterns.

MAGNITUDES, LOCATIONS, and TIMES
The magnitudes and locations in the table below are estimates, based on the reports in "United States Earthquakes" and on other data, such as instrumental records, where available. Time in the table below is "UTC" or Greenwich Mean Time. The text accounts from "United States Earthquakes" use Pacific Standard Time. A brief summary of earthquake damage in Washington is given in Washington State Earthquake Hazards, by Linda Noson, Anthony Qamar, and Gerald Thorsen.


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