Mass Wasting - The Changing Earth: Exploring Geology and Evolution - page 281

Geo-Focus

Southern California Landslides

Southern California is no stranger to
landslides. La Conchita, Point Fermin,
Pacific Palisades, and Laguna Beach
are all locations in southern California
that have suffered damaging mass
movements during the past 50 years.
Two regions in particular, La Conchita
and Laguna Beach, have been in the
news because of the landslides that
destroyed numerous homes.
La Conchita is a small community
along the coast at the base of a

100-m-high terrace, 120 km northwest
of Los Angeles, California. On March 4,
1995, following a period of heavy rains,
some residents of this beach com-
munity noticed that the steep slope
above their homes was slowly moving
and that cracks were appearing in the
walls of their houses, indicating that the
homes were also moving. Shortly there-
after, a 200,000 m 3 slide destroyed or
damaged nine homes in its path
(

Almost 10 years later, following
a week of heavy rainfall in southern
California, in which the hillside and
previous landslide deposits were satu-
rated with water, another landslide
occurred in the same area. This time
10 people were killed and 15 homes
were buried under 10 m and 400,000
tons of mud (

Figure 2).
What went wrong and why was
this situation repeated? The rocks
that make up the steep sloped

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◗ Figure 1).

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Figure 2 La Conchita, California, 10 years later (2005). Here
the view is from the east. Similar factors caused another massive
landslide in the same area. The landslide can clearly be seen in
the center of this photograph, and the scarp and the remains of the
1995 landslide are still visible on the right side, as is the avocado
orchard in the foreground.

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Figure 1 La Conchita, California, is located at the base of a steep
sloped terrace. Heavy rains and irrigation of an avocado orchard
(visible at the top of the terrace) contributed to the landslide that
destroyed nine homes in 1995.

rock slides take place because the local slopes and rock lay-
ers dip in the same direction (

California coast. At Point Fermin, seaward-dipping rocks
with interbedded slippery clay layers are undercut by
waves, causing numerous slides (see Geo-inSight on pages
284 and 285).

Figure 11.11), although
they can also occur along fractures parallel to a slope. Rock
slides are also common occurrences along the southern

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