The Pacific Northwest Seismograph Network PNW EARTHQUAKE INFORMATION

Contents

Maps

Seattle area map with strong-motion station locations and peak accelerations from each component of each station.
Western Washington map with broadband station locations and peak accelerations from each component of each station.

Strong Motion Parameters

Parameter Definitions

• STA - The 3 letter station designation. Station locations are shown in the map and are listed in a station table.
• COMP - Component flag. Each station has a tri-axial accelerometer for measuring motions in the vertical (Z), north-south (N) , and east-west (E) directions. The "EL" part of the component indicates that the data are from an accelerometer with a high sample rate (here 100 samples per second). A "BH" or "HH" indicates that the data are from a broadband velocity sensor. These component designations conform to the SEED format specification.
• Dist - Surface distance from the earthquake epicenter to the recording station.
• PGA - Peak ground acceleration. Note that these values are expressed in scientific notation. As an example, a value of 5.1e-02 means 5.1 x 10^(-02) = 5.1 x 0.01 = 0.051 or 51 milli-g.
• Dur_BB - The Bolt-bracketed duration is the measure of time between the first and last exceedence of 0.05 g's. A listed value of 0.0 indicates that the ground motion never exceeded the 0.05g threshold.
• Dur_rel - The relative duration does not depend on the peak values. It is the time interval between the points at which 0.05% and 0.95% of the total energy has been recorded.
• AI - The Arias Intensity is a measure of the total energy at the recording station and is proportional to the sum of the acceleration squared. It is defined as pi/(2g) * dt * sum(a[n]*a[n]), where dt is the sample interval and a[n] is the nth acceleration value.
• SI - The Spectrum Intensity for 5% damping. It is defined as the integral of the psuedo-Spectral velocity curve (also known as the velocity response spectrum), integrated between periods of 0.1 - 2.5 seconds. These quanitities are motivated by the need to examine the response of structures to ground motion. Structures are modeled with a sequence of harmonic oscillators with 5% viscous damping and having varying natural frequencies. The ground motion is used as a forcing function on the oscillators and the resulting displacement is calculated. The peak displacement and natural period of each oscillator form a data point for the Spectral Displacement curve. Multiplying each data point by omega (2 * pi / natural_period ) gives the psuedo-Spectral Velocity curve, or the velocity response spectrum.
• PGV - Peak ground velocity; determined by integrating the acceleration time-series using the trapezoidal rule.
• F_d - Dominant frequency of ground motion; defined as the frequency corresponding to the peak value in the amplitude spectrum. Thus, F_d indicates the frequency for which the ground motion has the most energy. The amplitude spectrum has been smoothed before determining F_d and BW.
• BW - Bandwidth of the dominant frequency; measured where the amplitude falls to 0.707 (1 / square root of 2) of the amplitude of the dominant frequency. Again, this is based on a smoothed amplitude spectrum.

Accelerograms

Click on each station name to view the accelerogram; each accelerogram has peak values listed in the upper right

• SEA, University of Washington, Seattle.
(Vertical scale and peak values are too small by a factor of 2).
• MPL, BPA Maple Valley Power Station.
(Vertical scale and peak values are too small by a factor of 2).
• UPS, University of Puget Sound, Tacoma.
(Vertical scale and peak values are too small by a factor of 2).

One can also obtain accelerograms from broadband sensors by removing instrument response.

• GNW, Green Mountain, west of Bremerton.

Station Locations

Here is a table showing station coordinates and sensor information.

Station Photographs

UPS - University of Puget Sound, Tacoma.
SPW - Seward Park, Seattle.

The information presented in this document is preliminary. It will be modified after additional analysis of the accelerograms.

Back to
Main Page for this Earthquake.
PNSN Strong Motion Home Page.
Dept. of Earth and Space Sciences.