Quake Page

When there is a sudden release of energy from the release of stress in rock, the energy propagates as a "seismic wave". When this wave reaches us, and makes the ground shake, we call it an earthquake.

In rock, there are three important kinds of seismic waves. These are:

The P wave. P stands for "primary" because this wave arrives first. This is a longitudinal wave. That means that the shaking is back and forth in the same direction as the directon of propagation. So, for example, if you see that the lampost is shaking in the East-West direction, that means that the P wave is coming from either the East or the West. Some people like to use the memory trick that the P wave is a Pressure wave -- i.e. it is like sound, in being a compression and rarefaction, rather than a transverse motion. The P wave travels at about 6 km/sec. That is a lot faster than the speed of sound (which is 300 meters per second = 0.3 km/sec).

The S wave. S stands for "secondary" because this wave arrives second. This is a transverse wave. That means that the shaking is perpendicular to the direction of propagation. If the wave is travelling from the East, then this implies that the shaking is either North-South, Up-Down, or somewhere in between. Some people like to use the memory trick that the S wave is a Shear wave -- i.e. it can only propagate in a stiff material which does not allow easy shear motion (i.e. sideways slipping). Liquids do not carry shear waves. The way that we know that there is a liquid core near the center of the Earth is because shear waves do not go through it. The S wave travels at about 3.5 km/sec.

The L wave. L stands for "long". These are waves that travel only on the surface of the Earth. Like water waves, they are a combination of compression and shear. They are created near the epicenter when the P and S waves reach the surface. They are called long because they tend to have the longest wavelength of the three kinds of seismic waves. It is the L wave that usually does the most damage, because the wave travelling on the surface often develops the biggest amplitude. The L wave travels at about 3.1 km/sec. Some people like to use the memory trick that the L wave is the Last to arrive. (Careful. The L wave is NOT a pure "Longitudinal" wave!)

To estimate the distance of the quake, as you are ducking under a table, start counting seconds from when you felt the first tremor, i.e. the P wave. (You can get very good at doing this if you live in California long enough.) When the S wave arrives (it is usally a little bigger), then you know that for every second, the epicenter is about 8.4 km away. Thus if there is a five second gap between the two waves, the epicenter was 5x8.4 = 42 km away. You may even be able to estimate the direction from the P wave shaking. If we are lucky enough to have an earthquake during class, then you can watch me do this.

There are small earthquake waves passing by all the time, just as there are small waves everywhere you look on the ocean surface. These waves are recorded, in Bekeley, at the UC Seismographic station. To see the waves recorded for the last few hours, look at the recent UC Berkeley Seismograph record. This is an extremely interesting link to keep on your computer; it is something you can check any time you think you might have felt a quake. You'll see it there, even when it is not reported on the news. The quakes that occur every day in this region are also available on a map.

Those of you in the class who like math: can you see how I got the value of 8.4? It is based on the P and S velocities. Hint: the distance a wave travels is equal to the (velocity) x (time). This calculation is optional (not required).

Don't forget to look at the related page on Whips, Quakes, and Tidal Waves.