RUMBLE AND TUMBLE
When these huge masses of the Ear th’s crust shift, they
give of f waves of energy that move outwards in the same
way as a ripple after you’ve cast a rock into a pond. These
ripples of movement – or shocks – are what you would
feel during an ear thquake. They begin at the hypocentre,
which is the location where the plates shift. Above the
hypocentre is the epicentre on the Ear th’s sur face. The
waves from the shock can travel horizontally and ver tically.
The fur ther away from the epicentre, the weaker the
wave. Most of the damage will be found at the point of
origin. There may be several tremors within the first hour
after a large ear thquake, and you could still experience
“aftershocks” days – even weeks – later. This is because
the big shock alters the pressure along the fault line,
setting of f other shifts at dif ferent points.SIZING THEM UP
How do we know which ear thquakes are the most
power ful? We owe this means of measurement to the
seismograph, a piece of technology that looks similar to a
lie detector test! A pen is positioned above a role of paper
wound around a drum, which turns, dragging the pen
along the paper. When the Ear th trembles, the pen moves
from side to side, allowing the size of the shock to be
measured and recorded on the Richter scale of magnitude,
which was invented in 1935 by Charles Richter. Each whole
number on the Richter scale signifies an ear thquake 30
times larger than the number below it. We don’t usually
feel ear thquakes that measure under 3.0 on the scale, but
we will feel one above 5.0 – which translates to the same
amount of energy as the atomic bomb in Hiroshima! It is
thought that there are up to 500,000 ear thquakes a year,
but we only feel about 100,000 of them; only around 100
cause any real damage.SEISMOGRAPH
No.46 Issue 43939