Chapter 2 · Plates that move a few centimetres a year
Earthquakes
A puzzle on the map
Look at a map and notice how the coast of South America fits the coast of Africa like two pieces of a puzzle. In 1912 the German scientist Alfred Wegener said that this is no accident: the continents were once joined and drifted apart. He pointed to matching rocks and to the same fossils on both sides of the ocean. The single supercontinent is called Pangaea. It was assembled about 335 million years ago and began to break up about 175 million years ago, though the dates are approximate. Most scientists doubted Wegener because he could not explain what pushed the continents. In the picture the land slides back together.
As fast as a fingernail
The answer arrived in the 1960s as plate tectonics. The outer shell of the Earth, about 100 km thick, is split into seven or eight major plates and many smaller ones that float on softer hot rock. They move 2 to 10 centimetres a year, about as fast as your fingernails grow. It sounds like nothing, but take 4 cm a year for 100 million years: 4 cm times 100 million is 4,000 kilometres, the width of an ocean. Plates pull apart, slide past each other or collide, and the places where they meet have the most earthquakes and volcanoes.
Locked, then slip
Where two plates touch, friction can lock them for decades while the rock bends like a stretched spring. When the stress is stronger than the friction, the fault slips and the rock snaps back. This is the elastic rebound idea, worked out after the San Francisco quake of 1906. The point underground where it starts is the focus, and the point above it on the ground is the epicentre. In the picture the two sides of the fault jerk past each other and rings of waves spread out.
P, S and the seismograph
A quake sends two kinds of waves. P waves are fast, travel about 6 km/s in the crust and push and pull the rock. S waves are slower, about 3.5 km/s, and shake it from side to side. A seismograph records both, because it hangs a heavy weight that stays still while the ground moves. The time gap between P and S gives the distance: roughly 8 km for every second of gap. Worked example: if S arrives 10 seconds after P, the quake is about 80 km away. Three stations draw three circles, and the circles cross at the epicentre. Try the S-P gap in the sandbox.
A step of 32
The magnitude scale is logarithmic. Each whole step means about 10 times larger ground movement and about 32 times more energy, because 10 raised to the power 1.5 is 31.6. Worked example: a magnitude 7 quake releases about 32 times 32, which is roughly 1,000 times, the energy of a magnitude 5. The largest earthquake ever measured happened near Valdivia in Chile on 22 May 1960 and had a magnitude of 9.5. The quake of 2011 off Japan was about 9.1. Pick a magnitude in the sandbox and compare it with the scale of step one.
A wave that crosses an ocean
When a quake lifts or drops the sea floor, the water above moves too, and a tsunami is born. In deep water its speed is the square root of g times the depth. For an ocean 4,000 metres deep that is the square root of 9.8 times 4,000, about 200 m/s or 700 km/h, as fast as a passenger jet. In the open sea the wave is less than a metre high, and it grows only near the coast. Official advice is simple: if the ground shakes strongly near the coast, move to high ground at once without waiting for a siren. On land, officials advise to drop, cover and hold on until the shaking stops.
Buildings that bend
Most deaths in an earthquake come from collapsing buildings, not from the ground opening. Engineers therefore design structures that sway instead of snapping. They add steel cross braces, set the whole building on rubber and steel bearings that let the ground move underneath, or add a giant pendulum inside. Taipei 101 has a damper ball of 660 tonnes hanging in the upper floors that swings against the movement of the tower. Nobody can predict the exact time of a quake. Some countries, however, have early warning systems that give seconds to tens of seconds before strong shaking arrives.
In numbers
- 9.5
- largest recorded quake, Chile 1960
- × 32
- more energy with each whole step of magnitude
- ≈ 8 km
- distance for each second of S-P gap