Earth's Secret Signals

Earth's Secret Signals

The deepest hole anyone has ever drilled goes down about twelve kilometres. The centre of the Earth is more than six thousand kilometres away. Nobody has been anywhere near it. And yet we know the outer core is liquid, and we have known it for a century. Not because somebody went and looked, but because of what arrives at the far side of the planet after an earthquake, and what does not. This module follows that reasoning to its conclusion. It is one of the neatest pieces of detective work in the whole specification.

Two kinds of wave 〰️

An earthquake sends out two kinds of seismic wave at once, and they differ in every way that matters:

Two shocks

A seismograph records a small tremor, and then about forty seconds later a second, stronger shaking from the same earthquake. What is happening?

• The P-waves arrived first because they travel faster, and the S-waves followed
• There were two separate earthquakes, forty seconds apart
• The second shaking is an echo of the first, reflected off the core
• The S-waves arrived first, and the P-waves followed more slowly

Why a liquid stops one and not the other

This is worth thirty seconds of thought, because it turns four things to remember into one thing to understand. A longitudinal wave works by squeezing the material along its path and letting it spring back. A liquid resists being squeezed just as a solid does, so it carries the wave perfectly well. That is why P-waves cross the core. A transverse wave works by pushing the material sideways and relying on it to pull back into place. A solid resists being twisted sideways; a liquid does not resist at all, it simply flows. With nothing to pull it back, the wave has nothing to travel on. So it is not that liquids are too thick or too thin. It is that a liquid has no sideways springiness, and a transverse wave has nothing else to work with.

No sideways spring

Why exactly can a transverse wave not travel through a liquid?

• A liquid offers no resistance to being pushed sideways, so there is no restoring force to pull it back and keep the wave going
• Liquids are too dense for a transverse wave to move through
• Transverse waves are too slow to get through a liquid before it moves
• The liquid absorbs the wave and turns it entirely into heat

Name the waves

P-waves are _____ waves, which squeeze the rock along the direction of travel. They are the _____ of the two, and they can pass through both solids and liquids. S-waves are transverse, shaking the rock sideways, and they can only travel through _____, because a liquid gives them nothing to spring back against. This is why S-waves are missing from detectors on the _____ side of the Earth.

Inside the Earth

The Earth is layered, and each layer is a different material in a different state:

How do we know?

What the detectors record

Put a ring of seismographs around the world and set off one earthquake. Here is what comes back. Stations near the earthquake record both kinds of wave: a P-wave arrival, then an S-wave arrival behind it. Stations on the far side of the Earth record P-waves, and no S-waves at all. Not weak S-waves, not late ones. None. The region where S-waves fail to arrive is called the S-wave shadow zone, and it covers a large part of the globe opposite the quake. Every detector in it works perfectly and records the P-waves that arrive at the same site. That is the evidence. What follows from it is the next step, and it is yours to work out.

Diagnose the core

You are the seismologist reading the data. Work out what it means, one step at a time.

• Every detector in the shadow zone records P-waves but no S-waves. What is the first thing you can rule out?
• So something between the earthquake and those detectors stopped the S-waves without stopping the P-waves. What property must it have?
• The S-waves reach detectors reasonably far from the quake, and disappear beyond that. Which layer is liquid?
• A colleague says the shadow proves the whole core is liquid. Are they right?

The chain of reasoning

Put the steps of the argument into the order a scientist would make them.

• An earthquake sends P-waves and S-waves out through the Earth at the same moment
• Detectors on the far side record P-waves arriving but no S-waves
• The detectors are working, so the S-waves genuinely did not arrive
• S-waves are stopped by liquids, and P-waves are not
• Therefore a liquid layer lies between: the outer core

Suppose it were different

Imagine the data had come out the other way, and S-waves WERE detected at stations right across the far side of the Earth. What would seismologists have concluded?

• That the outer core is solid, because a liquid layer would have stopped the S-waves
• That S-waves had somehow become faster than P-waves
• That the Earth has no core at all
• That every detector in the world had failed at once

Explain the evidence

Exam practice. In about 50 words, explain how seismic waves show that the Earth's outer core is liquid. Include:

• what the detectors on the far side of the Earth record, and what they do not
• the property of S-waves that makes the missing signal significant
• the conclusion that follows, and which layer it applies to

The signal rules

P-waves are _____ waves and travel through both solids and liquids, arriving first. S-waves are transverse and travel through _____ only, because a liquid offers no sideways resistance for them to push against. Because S-waves are not detected on the far side of the Earth, the _____ core must be _____, and that conclusion was reached without anyone ever going there.