Geoscience Reference
In-Depth Information
earthquake, were responsible for most of the damage.
This earthquake was not unique. There have been
earthquakes of similar size in recent geological times,
with the earthquake at Fort Tejon on 9 January 1857
being the largest yet recorded in southern California.
The Owen Valley tremor on 26 March 1872 is consid-
ered the third greatest earthquake to strike California
in recent times. It produced surface faulting extending
up to 150 km, with a maximum vertical displacement of
4 m. Other noteworthy earthquakes occurred in 1769
south-east of Los Angeles; on 10 June 1836 in the East
Bay area of San Francisco; in June 1838 at Santa Clara;
on 8 October 1865 in the Santa Cruz Mountains;
on 21 October 1868 south of San Francisco; and on
24 April 1890 at Chittenden Pass.
The San Andreas Fault has been active since the
Jurassic
, 135 million years ago, and has accumulated
550 km of displacement. During the
Pleistocene
(last
2 million years) horizontal displacement has amounted
to 16 km. Dating of charcoal deposited over the past few
thousand years over various fault lines indicates that
major earthquakes occur at intervals of 50-300 years.
The Pacific Plate is moving slowly north-west relative to
the North American Plate at the rate of 5 cm yr
-1
. Most
of the movement is sideways; but in a few places, there
is a vertical component on the North American Plate
side. In most places, this movement is continual;
however, in some places, the plates become locked
together and the movement is released in one sudden
jerk. The San Francisco earthquake gave rise to the
greatest lateral displacement - 6 m - yet measured along
the San Andreas Fault. Even where slow creep takes
place, earthquakes can occur.
In southern California around Los Angeles, there
has been no major movement since the 1857 earth-
quake. The most recent earthquake, at Northbridge
on 17 January 1994, measured only 6.7 on the M
s
scale. Based upon geological evidence, this area
together with the San Francisco area is overdue for a
major earthquake greater than 8 on the M
s
scale.
Both cities exist in seismic gap areas, surrounded by
zones that have experienced recent tremors in the
past 30 years. At present, the probability of an earth-
quake occurring in these gaps is 50 per cent. One of
the more overwhelming responses to the earthquake
hazard in California is neglect. In the Parkfield area
1873
San Andreas
N
Big PIne
1980
1922
Santa Ynez
San Cayelano
San Gabriel
Red Mt.
Oak Ridge
Santa
Susana
San
Fernando
Simi
1892
1906
Hayward
San
Francisco
1868
1838
Calaveras
Malibu
Coast
Pilarcitos
Santa Cruz Island
1836
Newport
Inglewood
1865
1989
0 10 20 30 km
1872
Los Angeles Faults
1983
1857
San Andreas
2003
Garlock
1952
1971
1925
San Gabriel
1927
1812
1918
1992
Big Pine
1994
Mission Creek
1812
1769
1890
Los Angeles
Banning
Elsinore
M
s
Magnitude
> 8.0
7.0-7.9
6.0-6.9
1968
1987
1933
1940
Imperial
1892
San Diego
1979
0 50 100 150 200 250 km
San Jacinto
Historically active fault systems of southern California and noteworthy earthquake epicenters for the last two centuries (adapted from
Oakeshott, 1983 and Robinson, 2002).
Fig. 10.6
