Causes and Prediction of Earthquakes and Volcanoes - Natural Hazards - page 179

Causes and Prediction

of Earthquakes

and Volcanoes

CHAPTER 9

INTRODUCTION

SCALES FOR MEASURING
EARTHQUAKE INTENSITY

Of all natural hazards, earthquakes and volcanoes
release the most energy in the shortest time. In the past
40 years, scientists have realized that the distribution of
earthquakes and volcanoes is not random across the
Earth's surface, but tends to follow crustal plate bound-
aries. In the past 20 years, research has been dedicated
to monitoring these regions of crustal activity with the
intention of predicting - several days or months in
advance - major and possibly destructive events. At the
same time, planetary studies have led to speculation that
the clustering of earthquake or volcanic events over time
is not random, but tends to be cyclic. This knowledge
could lead to prediction of these hazards decades in
advance. Before examining these aspects, it is essential
to define how earthquake intensity is measured, because
earthquakes are always characterized by their magni-
tude. This aspect will be examined first, followed by a
description of the distribution of earthquakes and volca-
noes over the Earth's surface and some of the common
causes of these natural disasters. The chapter concludes
with a discussion on the long- and short-term methods
for forecasting earthquake and volcano occurrence.

(Holmes, 1965; Wood, 1986; Bolt, 1993; National
Earthquake Information Center, 2002)

Seismic studies were first undertaken as early as
132 AD in China, where crude instruments were made
to detect the occurrence and location of earthquakes.
It was not until the end of the nineteenth century that
these instruments became accurate enough to measure
the passage of individual seismic waves as they traveled
through, and along, the surface of the Earth. The
characteristics of these waves will be described in more
detail in the following chapter. The magnitude or
intensity of these waves is commonly measured using
scales based on either measurements or qualitative
assessment. The first measured seismic scale was the
M s scale established by Charles Richter in 1935.
Richter defined the magnitude of a local earthquake,
M L , as the logarithm to base ten of the maximum
seismic wave amplitude (in thousandths of a milli-
meter) recorded on a standard seismograph at a
distance of 100 km from the earthquake epicenter.
The scale applied to Californian earthquakes and to
the maximum amplitude of any type of seismic wave.
Each unit increase on this scale represented a tenfold
increase in the amplitude of ground motion. The scale
was later standardized for any earthquake using