Geologists return for seismic survey

As part of a continuing earthquake hazard study in the Seattle urban area, the U. S. Geological Survey (USGS) will be conducting a series of seismic-reflection surveys starting Thursday, July 10 on Mercer Island, as well as in parts of Seattle. The studies will conclude on or around July 19.

USGS scientists will re-survey the area where they conducted the 2008 seismic-reflection profile on Mercer Island. Exact locations include: along 84th Ave. S.E. from S.E. 39th St. to S.E. 29th St., and along 84th Ave. S.E. from approximately S.E. 26th St., to the north tip of Mercer Island.

During the 2008 USGS survey, the energy source used was a small vibration truck.  This truck used two 12-second sweeps to put vibrations into the earth, which reflected off the boundary between rock types and back to the surface. The reflected vibrations were recorded by sensors attached to a cable placed along the ground surface. The vibrations produced were then converted into information about the soil and rock layers underground.

This year’s experiment will use a different method.  Collected data will record vibrations in the ground and along its surface generated by ambient noise.  This ambient noise, sometimes referred to as “cultural noise,” comes from everyday movement of traffic, people, aircraft, ocean waves and more.

“The seismic imaging technique we are attempting to use this year is in development and we required sites with an existing seismic dataset in an urbanized area for comparison,” said USGS scientist Bill Stephenson.

He explained that the new technology would likely require noise from planes, cars and trains to effectively collect a new dataset. A key factor in site selection is Interstate 90, “a dominant essentially constant noise source.”

USGS recording equipment will collect the ambient noise data over about a six-hour period during the day for two to three days, testing different acquisition parameters.

The goal of this first of its kind experiment in an urban area is to determine if seismic imaging using ambient noise sources can produce an image of similar quality to what was achieved using a vibration source. If successful, it could pave the way for future studies where access to a vibration truck would be limited by sensitive environmental areas and more rural, road-less pockets of the state.


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