Geoscience Reference
In-Depth Information
Figure 4.14.
From 1968
to 1995 about 95,000
earthquakes with
magnitude greater than 1
occurred in the San
Francisco region. The
main faults are well
defined. (Reprinted with
permission from
Nature
(Amelung and King,
Nature
,
386
, 702-5).
Copyright 1997 Macmillan
Magazines Ltd.)
°
°
°
°
°
than 8. These types of calculations, while straightforward to perform, should only
be regarded as providing a broad indication of levels of earthquake activity.
Small
b
-values are usually characteristic of continental rifts and some regions
with deep-focus earthquakes, whereas large
b
-values are typical of mid-ocean
ridges and volcanic regions where volcanic swarms are common. Volcanic
swarms can have
b
-values of up to 2.5, meaning that there are many small earth-
quakes and very few large events. Laboratory experiments indicate that high stress
results in low
b
-values, and low stress results in high
b
-values. This observation
is consistent with most of the regional determinations that have been made.
4.2.6 Energy released by earthquakes
Few determinations of the total energy released as seismic waves by earthquakes
have been made, probably because the energy must be computed by an integration
