Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 143

seismograph stations can be used to determine the location and the magnitude
of an earthquake.
L-waves are long surface waves that are trapped between the surface of the
Earth and the crustal layers further down. These waves are not transmitted
through either the mantle or the core, but spread relatively slowly outwards
from the epicentre along the surface of the Earth at a speed of 5000 km in
20 minutes. The energy of these waves dissipates progressively with distance
from the epicentre.

Earthquake magnitude and intensity

Earthquakes, along with volcanoes, release the most energy in the short-
est time of all the natural hazards. Magnitude and intensity of earthquakes are
measured using the Richter and Mercalli scales, respectively (Robinson, 1993).
The Richter scale measures the total amount of elastic energy released by each
shock wave during an earthquake. With this scale, magnitude ( M )isafunction
of energy ( E )and is described using the following equation,

log 10 E = a + bM (6.1)

The values of a and b have been modified several times. In 1991 their values
were approximately 5.8 and 2.4 (Bryant, 2005). These values define the released
energy in ergs. One unit increase in energy on the Richter scale describes
2.4 orders of magnitude change, which can also be described as a 240 times
increase in energy. Equation (6.1)only describes magnitude as a function of
energy. It is not strictly applicable to changes in the amplitude of the shock wave
although wave amplitude is often taken as a guide to the amount of energy. Each
unit increase in magnitude on the Richter scale describes one order of magni-
tude increase in wave amplitude. One order of magnitude can also be described
as a tenfold increase in wave amplitude.
Different estimates of the magnitude of an earthquake can be recorded on the
Richter scale because of the different pathways that seismic waves take travel-
ling to recording stations, differences in seismograph equipment and variations
that still exist in the parameters used to describe magnitude by equation (6.1).
The Moment scale was developed, in conjunction with the Richter scale, to help
overcome these difficulties. It measures the exact energy released by an earth-
quake and is a function of the surface expression of faulting associated with the
earthquake. At times, use of the Moment scale is limited because often surface
expression only occurs in earthquakes of magnitude 7.0 or greater on the Richter
scale (Bryant, 2005).