Geology of Southern California.pdf - Grossmont College

36 Geology of Southern California
Southern California.9
Southern California Earthquakes
Earthquakes in California are an inevitable consequence of
the interactions between tectonic plates along the forward
edge of a west-moving continent. The forces generated
along the transform boundary between the Pacific and
North American plates result in thousands of earthquakes
every year. The ground literally shakes continuously in
California, but most of these tremors go unfelt by people
and rarely cause damage. Nonetheless, the seismic history
of southern California includes some devastating events
that serve as reminders of the serious hazards facing those
who live near such as active plate boundary.
Historic and Ancient Earthquakes in Southern
California: Though earthquakes occur in many parts of
California, exceptionally powerful temblors have repeatedly
shaken the southern part of the state. The 1857 Fort Tejon
earthquake and the Lone Pine earthquake of 1873 rank
among the greatest seismic events in California history.
The Fort Tejon earthquake (estimated magnitude 7.9)
resulted from a sudden slip along more than 300 kilometers
(180 miles) of the San Andreas fault from the Transverse
Ranges area to the central Coast Ranges. Effects of this
earthquake were widespread and included uprooted trees,
damaged building, and disrupted stream drainages, and
liquefaction. Only the sparse human population at the
time limited the death toll to one. The Lone Pine earthquake
was more tragic. At least 23 people died when the
magnitude 7.8 quake toppled most of the adobe brick
buildings the eastern Sierra Nevada town. The Lone Pine
event resulted from slip along the normal fault zone at the
base of the Sierra Nevada, and thus occurred in a completely
different plate tectonic setting as the Fort Tejon shock.
The scarp, or surface exposure, of this normal fault is still
visible in the hills just west of Lone Pine (Figure SC.19).
More recently, the Long Beach earthquake of 1933
(magnitude 6.4) resulted in 120 deaths and more than
$50 million in damage (Figure SC.71). In 1971, the
San Fernando earthquake just north of Los Angeles (magnitude
6.6) caused ten times more damage as the Long
Beach temblor, even though it was of comparable magnitude
(Figure SC.72). This illustrates how the population
density and level of development can influence the impact
of an earthquake on people. Even more catastrophic was
the 1994 Northridge earthquake in the Los Angeles Basin
(Figure SC.73). This magnitude 6.7 event claimed 61 lives,
injured thousands of people, and caused damages exceeding
$40 billion. All three of these examples of southern
California earthquakes had tragic consequences at magnitude
values in the 6 to 7 range. There are indications that
even more powerful events have occurred in the past, and
will again at some point in the future.
In recent years, geologists have developed several techniques
for recognizing ancient earthquakes and estimating
their magnitude even without an instrumental record of the
� Figure SC.71 Building damaged during the 1933 Long Beach
earthquake.
� Figure SC.72 Many freeway overpasses collapsed during the
1971 San Fernando earthquake.
waves they produced. Disrupted soil layers and landslide
deposits, exposed in trenches dug across active faults, can
be indications of sudden and violent movement that
occurred during an ancient earthquake. Organic material
and some minerals in the soils can be dated to determine
the approximate timing of the prehistoric earthquakes.
These investigations have revealed that southern California
has been repeatedly shaken by earthquakes more powerful
than any experienced in the region historically. From information
collected at many sites along the San Andreas fault
system geologists have determined that no fewer than
14 major earthquakes (M7.7 to 8.0) have occurred in southern
California over the past 1,400 years. Coupled with the
Courtesy of the U.S. Geological Survey
Courtesy of the U.S. Geological Survey