Earthquake shaking in the
Puget Lowland of Washington State:
Results
from the SHIPS experiments and other studies
The frequency, sizes and locations of earthquakes
are only part of the story – ultimately it is how the ground shakes that
matters. The United States Geological Survey (USGS) has carried out a number of
studies in the U. S. Pacific Northwest to estimate how strong the shaking will
be in future earthquakes.
Ground shaking is predictable; Earthquakes are not
Earthquakes do not kill people. Falling objects, collapsing buildings, landslides, fires and other secondary effects from earthquakes are what cause injury and death. We cannot predict when and where an earthquake will strike, but we can control many of these secondary effects with preparation. Bolting bookshelves and other large objects to the wall, shutting off gas valves immediately after an earthquake, and avoiding building in landslide-prone areas, for example, will reduce the number of deaths in the next earthquake.
It is the horizontal shaking during earthquakes that causes building damage or collapse. Most structures are designed to carry heavy loads, so they are strong in the vertical direction. Horizontal stresses on buildings are caused primarily by winds, but the occasional earthquake can cause far more horizontal stress than winds.
We can design buildings to withstand the horizontal ground shaking when the inevitable earthquakes do occur. One method would be to design all buildings to withstand the largest ground shaking we can expect in an area. However, this seismic strengthening can add significantly to the cost of the building construction.
Earthquakes emit energy at different frequencies, so the other way to increase building safety during earthquakes is to design buildings to withstand the specific types and amount of shaking that can be expected at that site. Building the same building at another site could involve a slightly different design because the ground shaking would be different. This method works, and saves building costs, if we can reliably predict the shaking at a site from future earthquakes.
This web document describes work being done by U. S. Geological Survey scientists to better understand how the geology beneath the densely-populated Puget Lowland of Washington State influences ground shaking during earthquakes. The results of this work will guide engineers and architects in estimating what effects need to be considered when designing buildings and other structures.
Sedimentary
basins beneath the Puget Lowland of Washington State
Frequencies, periods, and resonance
Low-frequency
shaking (<1 Hz): Basin effects
Strong-Motion Recordings: Nisqually
Earthquake
PNSN Strong-Motion Recordings: 2002
Denali Earthquake
High-frequency shaking (>1 Hz): Shallow deposits and attenuation
Site Response within the City of
Seattle
For Further Reading
The following general-interest books describe earthquake hazards and research on earthquake hazards:
Earthshaking Science, Susan Hough,…..
Living on an Active Earth: Perspectives on Earthquake Science, National Research Council, The National Academies Press, Washington D.C., 2003, 418 pp.
Living With Earthquakes in California: A Survivor’s Guide, Robert S. Yeats, Oregon State University Press, Corvallis, 2001, 406 pp.
Living With Earthquakes in the Pacific Northwest: A Survivor’s Guide, Robert S. Yeats, Oregon State University Press, Corvallis, 2001, 406 pp.