SHIPS - Seattle - January 26 - May 27 2002

Goals

The 2002 "Seattle SHIPS" experiment was designed to study how the Seattle sedimentary basin influences ground shaking during earthquakes. The Seattle basin is a large (30 km by 80 km) depression, 8 km deep, filled with relatively soft rocks, muds and silts. The Seattle basin extends from Hood Canal to the Cascade foothills, and lies beneath Bremerton, Seattle, Bellevue and Redmond.

During the 1999 " Dry SHIPS" experiment, low-frequency (0.2 to 0.8 Hz) seismic waves from the M7.6 Chi-Chi, Taiwan, earthquake were amplified by factors of 12 to 16 over the Seattle basin, whereas high frequencies (7 to 10 Hz) showed lower amplitudes over the basin than at sites on bedrock. These results suggest that the Seattle basin has a large effect on the amount of ground shaking during earthquakes, but more detailed measurements are needed to better define this effect and to understand the underlying causes. The Seattle SHIPS experiment was designed to put a large number of seismometers over and around the Seattle basin to study the ground shaking during local earthquakes, quarry blasts, and distant earthquakes.

The Seattle SHIPS Experiment

During the Seattle SHIPS experiment, 87 seismometers were deployed in a long east-west line, three north-south lines, and a grid throughout the Seattle urban area (Figure 1). The instruments were installed by hardy volunteers during a snowstorm on January 26 and 27, 2002, and were retrieved gradually between April 18 and May 27. Each of these seismometers consisted of a buried motion sensor connected to a small recording computer provided by the Incorporated Research Institutes for Seismology (IRIS). The computer was powered by a car battery, which was connected to a small battery charger. The instruments were placed at homes and businesses, whose owners allowed us to draw electrical power from their outdoor outlets. The instruments recorded ground motion continuously for the entire length of the experiment, with their internal clocks being synchronized by satellite Global Positioning System (GPS) receivers.

Puget Sound area map. The red dots are locations of seismometer installations. Click on map for larger image.

Figure 1
Puget Sound area map. The red dots are locations of
seismometer installations. Click on map for larger image.

Seattle SHIPS Seismometer Deployment Photos

installing gps antenna
installing GPS antenna
installing gps antenna
installing GPS antenna
digging hole for sensor installation
digging hole for sensor
digging,sensor in the foreground
digging,sensor in the foreground
burying sensor
burying sensor
setting up equipment
setting up equipment
testing equipment
testing equipment
"stomp" testing sensor
"stomp" testing sensor
Click thumbnails for larger photo.

Earthquakes Recorded During Seattle SHIPS

During the four months of the Seattle SHIPS experiment, there were 66 small earthquakes in the Puget Sound region with magnitudes of 1 to 3.2. Nearly all of these local earthquakes were too small to be felt, but our instruments made good recordings of about 15 of these earthquakes. There were also six earthquakes greater than magnitude 6.5 somewhere on Earth during the experiment, and our instruments made particularly good recordings of the four earthquakes with magnitudes greater than 7. Earthquake information for local events can be found at the Pacific Northwest Seismograph Network (PNSN) and for world wide events at the USGS National Earthquake Information Center (NEIC).

The earthquake recordings displayed on this web page show two of the better recordings. The local earthquake is a magnitude 2.5 earthquake that occurred 17.3 km ENE of Tacoma on March 26th, 2002. The distant earthquake is a magnitude 7.1 earthquake that struck near the northeast coast of Taiwan on March 31, 2002.

Recording of earthquake in TacomaRecording from eathquake in Taiwan.

Click on figure for larger image.

Each figure shows graphs of the ground motion at about 75 sites (not all of our instruments were working at all times). The graphs of ground motion are arranged as "traces": vertical lines that record the ground motion as a wavy line showing ground motion versus the time it was recorded. The graphs are grouped in three sets, with one set showing vertical ground motions, one set showing ground motions in a north-south direction, and the third set showing ground motions in an east-west direction.

Although the individual traces are too small to see individually, the sets of traces on the figures show the arrival of two types of seismic waves from the earthquakes. The first seismic waves to reach our instruments are the "Primary", or "P" waves. These waves causes the ground to move in the direction the wave is traveling. The second strong seismic wave to arrive is the "Seconday", or "S" wave. S-waves cause the ground to move sideways, or perpendicular to the direction the wave is traveling. For local earthquakes, these waves move horizontally and reach the closest station first, forming the sloped line. For distant earthquakes, the waves pass deep into the Earth and arrive from beneath, reaching all of our stations at nearly the same time.

Analysis of Ground Motions

To date, we have collected the data and organized it by earthquake and site. This has not been a small task, as we collected over 300 gigabytes of data! This winter, we plan to look at each of the recordings to examine the amount of ground motion at each site, to see where the ground shaking was strong and where it was weak. Once we have this information, we will use computer models to try and understand what causes these changes in the strength of ground shaking. This information will ultimately allow engineers to design safer buildings and bridges. Stay tuned...