The
community of La Conchita sits on a narrow coastal strip (about 800
ft wide) between the shore and a steep bluff. The top part of the
bluff is made up of marine shale, siltstone, and sandstone of the
Miocene (23.8 - 5.3 million years ago) Monterey Formation. The
bottom part is made up of siltstone, sandstone, and mudstone of the
Pliocene (5.3 - 1.8 million years ago) Pico Formation. All of these
rocks are very weakly cemented which means the individual grains of
the rock are not held together very well. (They are being nice
calling the material in the bluff a stone, most people would
probably just call it hard dirt). Between the two formations and
across the bluff face runs the active Red Mountain Fault. This
fault has thrust the older Monterey Formation on top of the younger
Pico Formation.
These bluffs have a long history of land slides. These old
slides are difficult to identify, but the photos in the references
call them out. Near the top of the bluff, stretching from the
recent slides west to the canyon, there is a narrow flat terrace a
little below the top of the bluffs. This is the top of an ancient
landslide that included material from the entire bluff. There are
additional smaller slides, below this ancient slide. The rail line
behind you has been buried by landslides at various intervals.
The two most recent slides are readily apparent. The March 1995
slide involved the material in front of the now unvegitated bluff
face. That material, approximately 1.7 million cubic yards,
apparently slumped down in a relatively coherent mass and slammed
into the homes below in just a few minutes. It was estimated that
the thickness of the slide was over one-hundred feet. Prior to this
massive side, there were some warning signs. Surface cracks were
discovered on the upper part of the slope as early as the summer of
1994, as well as some smaller between the summer of 1994 and March
1995.
The ultimate cause of the landslide appears to have been the
result of an unusually high rainfall season. In the rainy season
prior to the slide, approximately twice as much rain fell as
normal. During that time, it was noted that the open cracks in the
upper part of the slide were channeling water directly into the
ground. Due to the increased rainfall and direct infiltration of
water into the ground, groundwater levels directly beneath the
slide began to rise. The storm a couple of days before the slide
was the likely trigger that started the slide. The forces that were
keeping the bluff together were reduced by the groundwater until
the weight of the material in the bluff finally broke off a thick
mass of material and it slid down as one cohesive chunk. This
conclusion of what caused the landslide and how it slid is based on
the thickness of layer that slid, the fact that the slide occurred
days after a rainfall event, and eye witness accounts.
Ten years later, a second slide caused the southeastern portion
of the 1995 to slide again. In this case, 250,000 cubic yards of
material flowed down over the retain wall that had been built at
the base of the 1995 slide. As in 1995, the there had been
unusually high rainfall throughout the season and the slide
occurred after more than 2 weeks of almost constant rainfall. Video
of the slide shows relatively dry material flowing down into the
houses.
In this case, it is suggested that the landscape created by the
1995 slide concentrated the rainfall into the southeastern portion
of the slide. That rainfall saturated a layer deep under slide
causing the upper material to break loose and float down on top of
the saturated layer. Days after the slide, water was still coming
out of the bottom of the slide. These slides are not likely to be
the last in the area.
Logging requirements:
Send me a note with :
- The text "GCPD3K La Conchita Landslides" on the first line
- The number of people in your group.
- Using the photos and diagrams from the references, E-mail the
difference you see between the 1995 and 2005 slides. You can't just
use the photos since the area has changed since the photos were
taken.
- Look at the rest of the hillside and see if you see any
features to suggest other sections are ready to slide.
The following sources were used to generate this
cache.
- Jibson, Randall W, Landslide
Hazards at La Conchita, California Open-File Report
2005–1067, USGS,
http://pubs.usgs.gov/of/2005/1067/508of05-1067.html
- http://www.consrv.ca.gov/SMGB/special%20reports/Advis2005-02.pdf
- O'Tousa, James, 1995, La Conchita landslide,
Ventura county, California: AEG News, Association of Engineering
Geologists, v. 38, no. 4, p. 22-24.
