Cross sections along the San Andreas fault from north of San
Francisco to Parkfield showing three gaps in seismicity: between
San Francisco and Portola Valley, near Loma Prieta Mountain, and
southeast of Parkfield. The upper section shows the location of

150

200

earthquakes for the period from January 1969 through July
1989. The lower section shows the southern Santa Cruz Moun-
tains gap after it was filled by the October 17 Loma Prieta
earthquake (open circle) and its aftershocks.

THE LOMA PRIETA EARTHQUAKE HAD BEEN FORECASTED

The San Andreas fault is divided into many fault segments.
Earthquakes tend to recur on these segments at semi-regular intervals,
and seismic gaps delimit the segments most likely to slip in the near
future. If we know the time since the last major earthquake on a given
segment, we can determine the probability that the next major event will
occur within a specific period of time. In 1988, the Working Group on
California Earthquake Probabilities, convened by the U.S. Geological
Survey, determined the recurrence times and probabilities of earth-
quakes along the San Andreas and related faults (see table at right).
The Loma Prieta earthquake occurred along one of the six fault
segments in California identified as most likely to sustain a magnitude
6.5 or larger event within the 30-year interval from 1988 to 2018. Work
is underway to improve the resolution and reliability of such forecasts.

BUT PREDICTION WAS NOT
POSSIBLE.

Scientists are studying ways to predict reliably the time of occur-
rence of specific earthquakes. Resources available for prediction have
been concentrated in the Parkfield gap of central California, which has
been identified as more than 95 percent likely to sustain an earthquake
of magnitude 6 by the year 1993. An intense effort is being made to
predict this event on a short-term basis.

Although the Loma Prieta region had not been densely instru-
mented for purposes of short-term earthquake prediction, two suspi-
cious events were observed. Two earthquakes of magnitude 5.1 and 5.2
shook the same region 2 and 15 months, respectively, before the
mainshock. A public advisory was issued through the California Office
of Emergency Services immediately following each event stating that
because of their magnitude and location, these events could be fore-
shocks to a larger earthquake within 5 days. However, neither event
proved to be an immediate foreshock, and specific precursors that might
have allowed short-term prediction of this event were not recognized in
advance.

Probabilities of major earthquakes on California's primary fault segments
(From US Geological Survey Open-File Report 88-398, 1988, see bibliography at end of this pamphlet]

Earthquakes posing a potentially greater hazard than the Loma Prieta shock are likely to occur within California during the lifetime of most residents.

88

C

Damaging earthquakes in California and Nevada and surface ruptures of associated faults are shown from 1769 to the present. Because instruments for measuring magnitudes were not available until the late 1800's or because early instrumental coverage was not widespread, all earthquakes before 1898 and some before 1910 are rated very large to outstanding according to historical descriptions; those ratings are comparable to the three groupings of instrumental magnitudes.