Figure 1.9 Continental-continental convergenceEARTHQUAKES
Earthquakes occur during abrupt movement on an existing fault, along tectonic
plate boundary zones, or along mid-oceanic ridges. A massive amount of stored
energy, held in place by friction, is released in a very short period of time as
stored energy is suddenly and rapidly released when plates slide past each other.
The area where the energy is released is called the focus. From the focus, seismic
waves travel outward in all directions. Directly above the focus, on Earth’s
surface, is the epicenter.
Richter Scale
The strength or magnitude of an earthquake is commonly measured by the
logarithmic Richter scale and is recorded by a seismograph onto a seismogram.
The Richter magnitude scale really compares amplitudes of waves on a
seismogram, not the strength (energy) of the quakes. Because of the logarithmic
basis of the scale, each whole-number increase in magnitude on the Richter scale
represents a tenfold increase in measured amplitude. In terms of energy,
however, each whole-number increase corresponds to an increase of about 32
times the amount of energy released. It is the energy or strength of the
earthquake that knocks down buildings and causes damage.
The intensity of an earthquake is variable over the area affected by the
earthquake, with high intensities near the epicenter and lower values further
away. These effects are allocated a value that depends on the effects of the
shaking according to the Mercalli Intensity Scale.