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Introduction

One seismic station can give information about how far away the earthquake occurred, but yields little other information. How does having more stations help us?

Research has shown that P-waves travel about 5 to 14 Km/sec within the Earth's crust and mantle; whereas S-waves travel about 3 to 7 Km/sec. If we look at the arrival times of the P and S waves on a seismogram, we see that the time difference (S - P) is linked to the distance the station is from the epicenter.

Four-station Seismograph Network

We use a cow and a tree in this narrated cartoon for fun and to emphasize that seismic waves traveling away from an earthquake occur everywhere, not just at seismic stations. A person would feel a large earthquake only at station A near the epicenter. Stations B, C, D, and the cow are too far from the earthquake to feel the seismic waves. Both the scale of the buildings (and cow) and the amplitude of the movements are exaggerated. The cartoonish amplified ground motions show the compressive (up-down in this case) P wave, the shearing (back-forth) S wave, and the rolling surface wave motions recorded by sensitive instruments. Notice that Station D does not record an S wave because shear waves cannot travel through Earth's liquid outer core.

Direct Link to Four-station Seismic Network - 1.7MB Quicktime

Direct Link to Four-station Seismic Network (En Español) - 716KB Quicktime

Four-station with No Cartoon Bounce

The above animation showed the buildings and cows bouncing and rolling above the surface of the Earth. This motion is exaggerated. The intent was to illustrate the nature of wave movement, not mimic reality. This second cartoon, not narrated, was designed as a better approximation to reality. It shows the arrival of seismic waves through select wave paths through the Earth (P and S waves) and over the surface of the Earth. The movement at distant stations occurs at a microscopic scale. While that doesn't result in noticeable movements of the buildings, the arrivals are recorded on sensitive seismometers.

Direct Link to Four-station With No Cartoon Bounce - 1.1MB Quicktime

What's going on here?

Additional Teaching Materials and Activities

This animation shows the amplified ground motions as the seismic waves arrive at each seismic station, but does not show the paths the waves follow through the Earth. The purpose is to have students observe the ground motions and seismograms and infer where the earthquake occurred using the amplitudes and arrival times of the seismic waves at the four stations. A leading questions might be: "Why does Station D only experience two waves, the up-down motion from the first seismic wave and rolling motion from the last wave and not the back- and-forth ground motion from a second wave like the other stations?"

Direct Link to Four-station With No Wave Paths - 1.7MB Quicktime

Video Lecture

Video lecture on P, S, and surface waves from a workshop for middle school teachers in Portland, OR.

Direct Link to Video Lecture - 5.1MB Quicktime