Earth & climate
Earthquakes
When the ground shakes: how earthquakes happen, what the Richter scale means and how to stay safe.
What you need first
A glass clinks in the cupboard, the lamp swings for a moment: earthquakes happen in Germany too, but most are far too weak to feel. Every year, instruments around the world record hundreds of thousands of quakes. Almost all stay harmless, but a rare few are among the most powerful natural events there are. So what is actually happening down there?
Why the Earth shakes
At faults, which are fracture zones in the crust, blocks of rock push past each other. They often get stuck, and the strain builds up over years or decades, like a wooden slat being bent further and further. At some point the lock breaks, the blocks jerk forward, and the stored energy is released all at once. It races through the rock as waves at about 6 km per second and makes the ground tremble. The shaking is strongest at the epicentre, the point on the surface directly above the quake's origin. The largest faults lie where two of Earth's plates grind against each other, which is why most quakes happen at plate boundaries: around the Pacific, across Türkiye or along the Andes.
Effect: pictures shake, furniture slides. The Richter scale is logarithmic, each step means ten times the amplitude.
The Richter scale
The strength of a quake is given as a magnitude, in the news usually on the Richter scale. The special thing: the scale is logarithmic. One step up means ten times the on the measuring instrument and roughly 32 times the released energy. Two steps up therefore mean a hundred times the amplitude and roughly 1000 times the energy. You barely feel a magnitude 3 quake, a magnitude 6 can damage houses, and the 2011 quake off Japan reached magnitude 9. For quakes that huge, experts today use the more precise moment magnitude, calculated from the size of the ruptured area. The numbers are read just as before, and there too one step up means roughly 32 times the energy.
Preparedness and safety
Earthquakes cannot be prevented, but their consequences can. In quake regions, houses are built so they can sway without collapsing, and schools regularly practise for the real thing. If the ground shakes while you are in a building, the rule is: drop to the floor, get under a sturdy table, hold on and stay away from windows. Early warning systems detect the fast, still weak first waves and raise the alarm before the stronger waves arrive. Those few seconds are enough to slow down trains and shut off gas lines.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Where do most earthquakes happen?
The ground is shaking and you are inside a building. What is the right thing to do?
By what factor does the amplitude on the instrument grow when the magnitude rises by one step?
Put the making of an earthquake in the right order.
- 1Over years the strain in the rock builds up.
- 2Two blocks of rock get stuck at a fault.
- 3The lock breaks and the blocks suddenly jerk forward.
- 4Waves race through the rock and the ground shakes.
By roughly what factor does the released energy grow per magnitude step?
The point on the surface directly above the quake's origin is called the ….
Where this leads