Convenient sources of low frequency energy are large, distant earthquakes in more earthquake-prone areas like Alaska, Japan and the Philippines. These distant earthquakes can produce strong seismic signals on our seismic instruments in Washington State if they have magnitudes greater than about 7, although the signals are still too weak for us to feel. These distant earthquakes are rich in frequencies from 1 Hz to 0.01 Hz (1 second to 100 second periods). These are the frequencies at which 10 to 100 story buildings shake, as well as large bridges, viaducts and dams. We generally cannot measure low frequency shaking from local earthquakes because they emit very little energy at frequencies less than 1 Hz. Large local earthquakes do emit these low frequencies, but they fortunately are rare in western Washington (the last ones were in 1949, 1965, and 2001).
During the few times that a large number of seismometers were in the Puget Lowland when a large local or distant earthquake occurred, we gained valuable information about how the Seattle basin amplifies low-frequency shaking. These studies show that the basin amplifies seismic waves by factors of 10 to 16 at frequencies of 0.2 Hz to 0.5 Hz (5 to 2 second periods), with the largest amplification being at about 0.3 Hz (3.33 sec periods). This increased amplification comes from a combination of larger seismic waves and longer durations of shaking.
The experiments in which we examined low-frequency ground shaking in the Puget Sound region are:
1) The 1999 “Dry” SHIPS experiment (SHIPS 99;
Brocher and others, 2000)
Strong seismic waves from the 1999 Chi-Chi, Taiwan (M7.6) earthquake were recorded on 29 seismometers placed along an east-west line across the Seattle sedimentary basin. The S-waves from this earthquake were amplified up to 16 times due to larger amplitudes of the waves and longer durations of shaking. (details)
2) Recordings by the Pacific Northwest strong motion
network (PNSN-SM) and temporary stations; 2001 Nisqually, Washington,
earthquake
When the magnitude 6.8 Nisqually earthquake occurred beneath southern Puget Sound, there were 35 strong motion instruments in either the PNSN or in a temporary array in Seattle. These strong motion instruments can record ground motions that are too large to record with normal seismometers. The recordings from this earthquake show larger amplitudes and longer times of shaking on the Seattle basin. (details)
3) The 2002 “Seattle” SHIPS experiment (Pratt and
others, 2003)
After the 1999 Dry SHIPS experiment showed large amplifications over the Seattle basin, we designed the 2002 Seattle SHIPS experiment to study the ground shaking over the Seattle sedimentary basin in more detail. We installed 87 seismometers over the basin for a period of 4 months, during which time we recorded 4 large earthquakes: Afghanistan (M7.4), the Marianas (M7.1), Taiwan (M7.1) and the Philippines (M7.5). Results confirmed the amplification at 0.3 to 0.5 Hz we saw during the 1999 experiment, with the largest amplifications over the east side of the basin. (details)
4) Pacific Northwest Seismic Network strong motion
instruments (PNSN-SM), 2002 Denali, Alaska, earthquake
By 2002, there were 46 strong motion stations as part of the Pacific Northwest seismic network and the Advanced National Seismic System (ANSS). These instruments obtained good recordings of the 2002 Denali, Alaska, earthquake (M7.9), from which we can measure the ground shaking during horizontally traveling surface waves, as opposed to the nearly vertically traveling shear waves we recorded in the SHIPS experiments. The results show strong amplification at 0.3 Hz (3.3 second periods). (details)