Explanation:

The three acceleration time-series shown below were recorded on broadband seismic systems (Green Mountain, Liberty Mine, and Longmire) that are part of the Pacific Northwest Regional Seismic Network. Unlike conventional accelerograms which are recorded on low-gain accelerometers, these accelerations are obtained by differentiating velocity recordings.

A seismogram is shown for each of the three components of motion at each station (North-South, East-West, and Vertical). For example, the North-South component measures horizontal accelerations in the North-South direction.

The time scale is in seconds. In each case, the seismogram begins at the mainshock origin time and 90 seconds are shown. Since it takes longer for seismic energy to propagate to stations that are farther away, the amount of time until the P wave (the first arriving energy) is proportional to the distance between the earthquake and the station. The acceleration amplitudes are shown in centimeters per second squared (1 g = 980 cm/s^2).

The abrupt increase of acceleration that arrives about 10 seconds after the P (primary) wave (at distances around 100 km) is called the S (shear) wave arrival. The S wave is most obvious on the horizontal components and typically generates the largest strong ground motions in an earthquake. The ground motion is sampled 50 times each second.