Page 58 - 4567

Historically, seismologists have employed a variety of methods to determine two
fundamentally different measures that describe the size of an earthquake—intensity and
magnitude. The first of these to be used was intensity—a measure of the degree of earthquake
shaking at a given locale based on observed effects. Later, with the development of
seismographs, it became possible to measure ground motion using instruments. This quantitative
measurement, called magnitude, relies on data gleaned from seismic records to estimate the
amount of energy released at an earthquake’s source. Intensity and magnitude provide useful,
though different, information about earthquake strength. Consequently, both measures are used
to describe earthquake severity.
Modified Mercalli Intensity Scale. Numerous intensity scales have been developed over
the last 150 years. The one widely used is the Modified Mercalli Intensity Scale—named after
Giuseppe Mercalli, who initially developed it in 1902. This intensity scale is divided into twelve
levels of severity based on observed effects such as people awakening from sleep, furniture
moving, plaster cracking and falling, and finally—total destruction.
Despite their usefulness in providing a tool to compare earthquake severity, intensity
scales have significant drawbacks. These scales are based on effects (largely destruction) that
depend not only on the severity of ground shaking but also on factors such as building design
and the nature of surface materials. Thus, the destruction wrought by an earthquake is frequently
not a good measure of the amount of energy that was unleashed.
Magnitude Scales. In order to more accurately compare earthquakes across the globe, a
measure was needed that does not rely on parameters that vary considerably from one part of the
world to another. As a consequence, several magnitude scales were developed.
Richter Magnitude. In 1935 Charles Richter of the California Institute of Technology
developed the first magnitude scale using seismic records. The Richter scale is based on the
amplitude of the largest seismic wave (P, S, or surface wave) recorded on a seismogram.
Because seismic waves weaken as the distance between the focus and the seismograph increases,
Richter developed a method that accounts for the decrease in wave amplitude with increasing
distance. Theoretically, as long as equivalent instruments are used, monitoring stations at various
locations will obtain the same Richter magnitude for each recorded earthquake. In practice,
however, different recording stations often obtain slightly different Richter magnitudes for the
same earthquake—a consequence of the variations in rock types through which the waves travel.
Earthquakes vary enormously in strength, and great earthquakes produce wave amplitudes that
are thousands of times larger than those generated by weak tremors. To accommodate this wide
variation, Richter used a logarithmic scale to express magnitude, in which a tenfold increase in
wave amplitude corresponds to an increase of 1 on the magnitude scale.
The convenience of describing the size of an earthquake by a single number that can be
calculated quickly from seismograms makes the Richter scale a powerful tool. Further, unlike
intensity scales that can only be applied to populated areas of the globe, Richter magnitudes can
be assigned to earthquakes in more remote regions and even to events that occur in the ocean
basins. In time, seismologists modified Richter’s work and developed new Richter-like
magnitude scales. Despite its usefulness, the Richter scale is not adequate for describing very
large earthquakes. As a result, the Richter scale is said to be saturated for major earthquakes
because it cannot distinguish among them.
Moment Magnitude. In recent years, seismologists have come to favour a newer
measure called moment magnitude (MW), which determines the strain energy released along
the entire fault surface. Because moment magnitude estimates the total energy released, it is
better for measuring or describing very large earthquakes. In light of this, seismologists have
recalculated the magnitudes of older, strong earthquakes using the moment magnitude scale.
Moment magnitude can be calculated from geologic fieldwork by measuring the average
amount of slip on the fault, the area of the fault surface that slipped, and the strength of the
faulted rock. The area of the fault plane can be roughly calculated by multiplying the surface-
rupture length by the depth of the aftershocks. This method is most effective for determining the