Geography Reference
In-Depth Information
Box 5.2
Earthquake monitoring at Parkfield, California
In 1976, the prospects for earthquake prediction looked
rosy, especially as the Chinese had claimed a major
success at Haicheng the previous year. Yet two decades
later, little practical progress seemed to have been made,
despite vast improvements in both monitoring
technologies and the understanding of earthquake
source mechanisms.
Statistical studies of Californian earthquakes revealed
that the southern part of the state has an 86 per cent
chance of experiencing a magnitude 7 tremor by the year
2024. One locality that bears a particularly high risk level
is the town of Parkfield (population 34), which sits on a
part of the San Andreas fault that periodically
accumulates strain and releases it in earthquakes of
magnitude 6 or more that have an average recurrence
interval of about twenty-two years. The US Geological
Survey therefore selected it for intensive monitoring of
earthquake precursors. Several hundred instruments
were installed in the local area (Figure 5.4), including
130 seismometers and accelerometers, eighty
geodolite lines, nineteen alignment arrays, eighteen
water well sampling sites, thirteen creep meters, seven
dilatometers, six soil hydrogen meters and four
tiltmeters. Many of these transmit data continuously to
the USGS regional headquarters at Menlo Park near
San Francisco.
Figure 5.4
Distribution of
earthquake-monitoring
instruments on the San Andreas
Fault in the Parkfield of
California.
Source:
After Bakun
et al.
1988.
The Parkfield Earthquake Prediction Experiment relies
on two principles. First, it recognises that seismic
precursors are sufficiently complex to require
simultaneous interpretation of many geophysical
phenomena. Second, earthquake advisories require
good communication and understanding between
scientists, emergency managers and the public. In 1992
and 1993, considerable mass media interest was
stimulated when the USGS issued advisories that
warned of an impending earthquake at Parkfield, and
substantial preparations were therefore made by the
California Office of Emergency Services. However, no
earthquake occurred, which is perhaps just as well, as
the residents of the
