Why We Need Strong Motion Sensors
Earth scientists know from geologic evidence, historical records,
and recent experience that Washington and Oregon are susceptible
to large tectonic earthquakes. While our region has been home
to a formal seismic network since 1970, we did not have the
kind of instruments that stay on-scale and accurately record
the large amplitude ground motion generated by strong earthquakes.
To address this problem, the PNSN, along with several other regional
networks, joined the efforts of the United States Geological Survey
(USGS) to create an Advanced National Seismic System (ANSS). In
cooperation with the ANSS, the PNSN has installed dozens of strong
motion sensors in Washington and Oregon since June 2000. Today,
data from these sensors provide valuable information that is used
by earth scientists and other professionals to reduce earthquake-related
hazards in the Pacific Northwest.
Before the wide use of strong motion instruments, scientists attempted
to estimate the shaking from strong earthquakes by extrapolating
(scaling up) the observed effects of smaller earthquakes (magnitude 2.5-5.0).
This method works well for many applications and has improved with the
use of data from strong motion instruments. However, this approach is
not applicable in every situation. Some geologic materials and
structures do not respond to strong shaking in a simple, predictable
manner that can be accurately scaled upward. In these situations,
scientists need actual data generated by strong ground motion to better
understand the processes at work. Strong motion sensors have been installed
in different areas of geologic interest throughout the Pacific Northwest
to provide this type of data.
Using strong motion data, earth scientists hope to gain a better understanding of:
1) ground response near fault ruptures of large earthquakes
2) effects of severe shaking on different subsurface structures and geologic materials.
3) ground response in areas that undergo liquefaction.
Until the fine details of subsurface geology in Washington and Oregon are
better known, strong motion sensors will play a valuable role in the assessment
of earthquake damage for future earthquakes.
