地震の波
Oto-kun
Emergency earthquake alerts, as obvious as it is, are broadcast before an earthquake occurs. How are they predicted?
father
Have you heard of P-waves and S-waves in seismic waves?
Oto-kun
P waves and S waves?
father
P stands for Primary wave, meaning the first wave, and S stands for Secondary wave, which is the second wave.
Oto-kun
Are there different types of waves in an earthquake that travel at different speeds?
father
I see. The earthquake early warning system predicts the arrival time and magnitude of the S-wave based on the information from the first P-wave.
Oto-kun
Does that mean that S waves are larger in scale?
father
Yes. Look at this diagram. In the crust, the speed of P-waves is about 6.5 km/s, while S-waves travel at a speed of about 3.5 km/s.
Oto-kun
Wow, the speed is about twice as different, huh? Looking at this diagram, the shapes of the P-wave and S-wave are different, right?
father
Yes. The difference in speed is due to the fact that while the direction of wave propagation is the same, the direction of the wave oscillation is different. P-waves, known as compressional waves, are waves that travel by repeatedly moving through regions of high and low density...
Oto-kun
That's the same as the way sound travels in the air, right?
father
That's right. Sound waves propagate through alternating areas of high and low air density. This is called a longitudinal wave.
Oto-kun
That means, are S-waves transverse waves?
father
Yes. You can see from the diagram that the wave vibrates perpendicular to the direction it is moving. Waves that move in this way are called transverse waves.
Oto-kun
But earthquakes can sometimes shake vertically, right? Is that the P-wave?
father
That's right. When the epicenter is directly below, it results in vertical shaking caused by P-waves. The shaking caused by P-waves is usually not very strong, as it is referred to as the initial tremor. The subsequent S-waves, known as the main shock, cause larger shaking. The amplitude increases particularly when transitioning from a hard layer of ground to a softer layer.
Oto-kun
Why is that?
father
That's right. For example, let's say there is soft tofu on a hard board. If you vibrate the hard board, would the tofu shake a lot?
Oto-kun
I see, so the increase in S-wave amplitude is related to the structure of geological layers.
father
There is also something called surface waves. These are waves that travel along the surface after the earthquake reaches the ground, and when the period is long, ranging from a few seconds to several seconds, they can cause larger tremors than S-waves.
Oto-kun
Is that also related to the ground conditions?
father
That's right. Tokyo has thick sediment layers, so surface waves are easily amplified. When the sway period of skyscrapers coincides with that, it can result in significant shaking.
Oto-kun
It would be great if earthquake early warning systems could predict not only just before an earthquake occurs but also in advance, what time and how strong the earthquake will be.
father
That's right. It seems that research on earthquake prediction has made quite a bit of progress. If that can be achieved, it would likely help to mitigate the damage caused by earthquakes to some extent.