|
When technology improved and seismographs
began to detect earthquakes fainter than those chosen to define the
magnitude zero on the scale, they began to be recorded with negative
numbers.
Although earthquakes with magnitude less than one produce jolts,
they are not felt by human beings. It is said that animals sense the
jolts. Human beings feel shocks only when the Richter reading is
more than two. Earthquakes with magnitudes between 2 and 2.9 occur
every year, producing three hundred thousand shocks, say experts.
Readings more than 5 on the Richter scale spells minor trouble.
Those with readings over 8 on the scale can cause total destruction.
Although the scale is open-ended, no earthquake higher than nine has
occurred thus far. The worst earthquake was one that occurred in
Lisbon in November 1755, which recorded 8.9 on the Richter scale.
Later seismographs that are more sensitive can detect earthquakes
even fainter than the ones that were originally chosen to define
magnitude zero; their magnitudes are accommodated on the Richter
scale by the use of negative numbers.
Before the Richter scale was devised, the intensity of an earthquake
was measured using a scale known as the Modified Mercalli. Devised
by an Italian seismologist, Giuseppe Mercalli, the scale was used to
assess the effects of a quake at a particular place, rather than the
quake’s overall power.
The scale begins with Intensity 0, which indicates that jolts were
registered by a seismograph only. Intensity 2 means that it was felt
by a few people at rest and that delicately suspended objects swung.
As the number increases, it is an indication that the severity was
that much greater. When people generally feel the shaking and
windows rattle, it reads Intensity 4. When the intensity is great
enough to be felt even in moving cars and there are reports of
moderate damage, the reading will show Intensity 7. Intensity 9
means panic situation. There are ground fissures and weak buildings
collapse. Intensity 10 and 11 too indicate panic situations, of
course of greater damage potential. The worst is Intensity 12, where
there is total destruction. |
