Bremerton Earthquake Intensity Map

The following map is an attempt to display the felt-form data obtained by our Web forms responses. This map was generated by averaging the responses over all responses from one zip-code and using that average at the geographic center of the zip code (dots on the map) and then producing a smooth contour of those values. We plan to try and do a similar thing for the exact location of each response; however, it is quite difficult to obtain a geographic coordinate from a full address so this may take much longer.

About the isoseismal map:

The contours shown on this map represent equal strength of shaking and are known as isoseismal lines. The numbers (ranging from 3.3 to 5) are intensity values and should not be confused with earthquake magnitude. Earthquake magnitude is a measure of the relative size of the earthquake source and can be determined from seismographs often located far from the earthquake epicenter. In theory, each earthquake will have a single magnitude value. In practice, different ways of calculating magnitude and the use of different seismic instruments will cause some variation in the magnitude numbers. However, these variations are usually pretty small and the final magnitude calculations for the Bremerton earthquake all fall between 4.2 and 5.0. Earthquake intensities, in contrast to magnitude, always show considerable variation. The intensity is usually strongest near the earthquake epicenter and decreases with distance until eventually, at some point far away, the earthquake is no longer felt by anyone. Other factors such as the local geology beneath a particular site, the regional geology and the orientation of the earthquake fault can all affect intensities. Intensities are determined from the reports of individuals and assessments of damage caused by the earthquake. The numbers on the map represent relative shaking strength and can by qualitatively interpreted as follows:

• less than 3.5: Weak. Not felt or weakly felt.
• 3.5 - 4.4: Mild. Felt by most people inside, no items knocked off shelves no damage, few people frightened.
• 4.5 - 5.4: Moderate. Felt by almost everyone inside and many outside, occasional reports of a few objects toppled over or knocked off shelves, no damage, some people frightened, some people run out of buildings
• 5.5 - 6.4: Strong. Felt be everyone inside and most outside, awakens all sleepers. Most people report some items knocked from shelves. A few reports of minor damage like cracks in plaster, occasional broken windows. Many are frightened.

Some individual reports were for shaking stronger than this; however the map shows averages over a zip-code district and so will not represent the worst shaking in an area. Although there are no higher averages on the Bremerton map, other earthquakes are quite capable of producing stronger shaking. Higher values have the following general characteristics:

• 6.5 - 7.4: Severe. Felt by all, awakens all, felt by persons driving cars, heavy furniture displaced, many reports of minor damage, major damage rare. Most people are frightened.
• 7.5 - 8.4: Violent. Major damage to poorly built structures such a unreinforced masonry buildings. Houses likely to move off foundations if not secured. Anything not firmly secured will be displaced including water heaters, wood burning stoves, pianos etc.
• 8.5 - 9.4: Extreme. Damage to some structures designed to resist strong ground shaking. This is the intensity level experienced in the communities of Northridge, Grenada Hills and Sherman Oaks from the 1994 Northridge earthquake.

Descriptive text adapted from report by Lori Dengler of Humbolt State University