Earthquake waves

Emergency earthquake warnings are broadcast before an earthquake actually occurs, right? How do they predict it?

Have you ever heard that earthquake waves consist of P-waves and S-waves?

P-waves and S-waves?

P stands for Primary wave, meaning the first wave, and S stands for Secondary wave, meaning the second wave.

Does that mean that earthquake waves have waves with different speeds?

That's right. The Earthquake Early Warning system predicts the arrival time and magnitude of the S-waves based on the information from the initial P-waves.

Does that mean S-waves are larger in scale?

That's right. Take a look at this diagram. In the Earth's crust, P-waves travel at a speed of approximately 6.5 km/s, while S-waves travel at a speed of approximately 3.5 km/s.

Wow, the speed difference is about double. Looking at this diagram, the shapes of the P wave and S wave are different, aren't they?

That's right. The reason the speeds are different is that although the direction in which the waves travel is the same, the direction in which they oscillate is different. P-waves are compressional waves, meaning they travel by repeatedly passing through areas of high density and low density...

That's the same as how sound travels through the air, isn't it?

That's right. Sound waves propagate by creating alternating areas of high and low density in the air. That's called a longitudinal wave.

So, does that mean S waves are transverse waves?

That's right. As you can see from the diagram, the wave is vibrating perpendicular to the direction in which it is propagating. Waves that propagate in this way are called transverse waves.

But earthquakes can sometimes cause vertical shaking, right? Is that a P-wave?

That's right. If the epicenter is directly below, the shaking will be vertical due to P-waves. The shaking caused by P-waves is usually not very strong, as it's called the initial tremor. The subsequent S-waves are called the main tremors, and they cause much stronger shaking. The amplitude is especially large when the ground moves from a hard layer to a softer layer.

Why is that?

That's right. For example, imagine a soft block of tofu on a hard board. If you vibrate the hard board, the tofu will shake quite a bit, won't it?

I see, so the size of the S-waves is related to the structure of the geological layers.

And there's another type called surface waves. These are waves that travel along the Earth's surface after an earthquake reaches it, and when they have a long period of several seconds to more than ten seconds, they can cause stronger shaking than S-waves.

Does the ground condition have anything to do with that?

That's right. Tokyo has a thick layer of sediment, so surface waves are easily amplified. If that coincides with the period of vibration of high-rise buildings, it can result in a large tremor.

It would be great if earthquake early warnings could predict not only the time of an earthquake but also the approximate time and magnitude of the earthquake in advance.

That's right. Research into earthquake prediction seems to have progressed quite a bit. If we can do that, we'll probably be able to mitigate earthquake damage to some extent.