Geoscience Reference
In-Depth Information
Chapter 7
When the ground shakes
A supertanker crossing the ocean under full steam carries a lot of momentum. Its stopping
distance will be many kilometres. An entire continent will stop for nothing. We've already
heard about the 55-million-year slow-motion collision of India with Asia. The other tectonic
plates are also moving with respect to one another. Where they grate against each other they
make earthquakes. A map of major earthquakes shows up the tectonic boundaries even more
clearly than volcanoes do.
GPS measurements show how the tectonic plates are slowly and steadily gliding past one
another at several centimetres per year. But as you approach the plate margins, the motions
become less smooth. There are places where the movement is steady, without major earth-
quakes, as if the rocks were lubricated or so soft that they can move by the mechanism
known as creep. But many of the plate boundaries get stuck. The continents keep moving,
however, and strain builds up until the rocks can take it no longer and suddenly crack, pro-
ducing an earthquake.
Some earthquakes occur at great depths as ocean crust subducts into the mantle. But most
quakes happen in the top 15 to 20 kilometres, where the crust is hot and brittle. The rocks
break along what are known as fault lines, sending out seismic waves. The waves appear to
radiate out from a focus or hypocentre underground along the fault. The point on the ground
surface above the hypocentre is called the epicentre.
