 A View By De-Fault
A cache by SAMO-NPS Hidden: 7/6/2009
Size:  (Not chosen) Difficulty:  Terrain:  (1 is easiest, 5 is hardest)
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Welcome to the Santa Monica Mountains National Recreation Area
EarthCaching Program. This earthcache site is available year round.
This cache is on trail. Please do not travel off trail for your
safety and preservation of resources.
Located at Ranch Sierra Vista / Satwiwa this cache provides
views of the mountains, canyons, the Pacific Ocean and, on clear
days, the Channel Islands can be seen from this trail.
Information regarding Rancho Sierra Vista and Satwiwa
Pets are permitted on a leash no longer than 6 feet (2 meters).
Bicycles and horses are not permitted to this cache
• Rancho Sierra Vista / Satwiwa is open daily from sunrise to
sunset.
• All plant material, rocks, animals, and historical features
are protected by law and may not be collected or disturbed.
• Safety information:
- Poison Oak can be found in this area. It is identified by three
leaves ranging in color from green to crimson. The plant is
deciduous, so it does lose its leaves in the winter.
- Watch out for mountains lions, rattlesnakes and ticks.
Satwiwa Native American Culture Center Information:
Open weekends from 9AM – 5PM
Phone number: 805-375-1930
National Park Service Visitor Center Information:
Open daily from 9 AM – 5PM.
Address: 401 West Hillcrest Drive, Thousand Oaks, CA, 91360
Phone number: 805-370-2301
www.nps.gov/samo
In emergency: dial 911
Vocabulary:
Richter Scale: Created in 1935 by Charles Richter at Cal
Tech, this logarithmic scale was devised to measure the energy
output of an earthquake. The scale ranges from 0.0 to 10.0. Each
increase in an earthquake magnitude (4.0 to 5.0) is equal to a 10
times increase in wave amplitude. Because of the math associated,
this also means that a 5.0 has 31 times more energy than a
4.0!
Modified Mercalli Intensity Scale (MMI): A scale developed
for urban areas to represent the amount of destruction and damage
following an earthquake. These numbers are always written in Roman
numerals. The scale ranges from I (not felt) to a X+ (extreme
shaking). An MMI of IV is about the threshold in which most people
start to feel an earthquake.
Quaternary: A geologic period that includes from the last
1.65 million years to present.
Holocene: A geologic epoch that includes from the last
10,000 years to present.
Fault: A fracture in the Earth’s crust where both
sides of the split have moved in opposite directions.
Surface Rupture: A visible surface feature of a fault formed
during an earthquake.
Active Fault: Having had significant movement and / or an
earthquake in the last 10,000 years.
Sycamore Canyon Fault: An approximately 13-mile long fault
that runs northeast to southwest on the western edge of the Santa
Monica Mountains range.
Boney Mountain Fault: An approximately 9-mile long fault
that runs northeast to southwest on the western edge of the Santa
Monica Mountains range.
Santa Monica Mountains Blind Thrust Fault: A 32-mile long
fault that runs east – west under the Santa Monica Mountains
that has no visible surface rupture.
Geological information:
California, especially the Los Angeles Area is home to movie stars,
famous beaches, traffic, and earthquakes. Earthquakes are as much
as part of this landscape as oak trees and coastal sage scrub. It
can even be reasoned that without earthquakes, that the
Mediterranean ecosystem, which the Los Angeles Area and the Santa
Monica Mountains are a part of, would never have evolved.
Earthquakes, volcanoes, and even plate tectonics created this Mecca
of sun that millions of people call home.
An earthquake. The ground is shifting. The land is shaking. Rocks
are splitting. Objects and buildings are swaying. The Earth is in
fact, moving. Earthquakes are the only natural disaster whereby we
have no idea where the next one is going to be. Kind of.
”When is the “Big One” going to hit Los
Angeles?” is a question for the ages, but we can study the
past as a way of looking to the present and future.
Let’s propose it this way.” Is there going to be an
earthquake in California today?” Yes, anywhere from 75
– 125 earthquakes occur in California a day. However, most of
them are too small to feel. “Is there going to be an
earthquake in the Los Angeles Area today?” Yes, there are 1
– 10 earthquakes each day in the Los Angeles Area. Yet again,
most are too small to feel. But it is the ones that are felt that
we become concerned about. “So, when is going to
happen?” “How big will it be?”
The site you are visiting is looking out at two major fault
lines with another under your feet. Do you see them? Most
likely not. Two of them are hidden beneath vegetation and the other
is underground. So how do we know they are there and how does this
relate to the “Big One?”
The two faults that you are looking towards that have surface
ruptures are the Sycamore Canyon Fault and the Boney
Mountain Fault. The road you can see leads into Sycamore
Canyon. The mountain face to your left, at the base, is the Boney
Mountain Fault. It’s not exactly what we think of when we
think “fault”. Instead, the image that comes to mind is
the San Andreas Fault. Yet, with these two fault lines, they are
not considered active faults. These two faults are potentially
active faults in that they last had a significant earthquake in the
Quaternary period, between 11,000 and 1.65 million years ago
(Jennings, 1994). This means, the faults could have an earthquake,
but they haven’t in a while.
The Sycamore Canyon Fault is approximately 13 miles long (Jennings
and Strand, 1969) and has offset the surrounding rocks and geology
about 5,000 feet (Sonneman, 1956). The Boney Mountain Fault is
approximately 9 miles long (Jennings and Strand, 1969) but has
offset the surrounding rocks and geology about 9,000 feet
(Sonneman, 1956). These two faults are what define Sycamore Canyon
from here to the ocean directly south of where you are
standing.
The other significant fault in the area is underneath your feet.
The Santa Monica Mountains Blind Thrust Fault is a 32-mile
long fault that has no surface rupture (Saikai, 1999). This means
that there is fault line running almost the entire length of the
Santa Monica Mountain range that we cannot see. Should we be
concerned standing here? No, because remember earthquakes happen
all the time. But remember we can study the past to learn about the
future.
Seismology is the study of earthquakes and the structure of the
earth. Through this study we can analyze and understand how the
earth moves. The two most common scales that are used to measure an
earthquake are the Richter Scale and the Modified
Mercalli Intensity Scale (MMI). The Richter Scale is what most
people are familiar with. 5.0, 6.2, 7.1 is usually how we hear
about earthquakes through media. The scale, designed as a way to
quantitatively explain earthquake intensity using seismometers,
devices that record earthquakes, for scientific understanding. The
MMI was created as a way to qualitatively explain earthquake
intensity in urban areas, how do the people perceive the
earthquake. So what does this mean for the three faults we’re
talking about? First off the mountains are 16-24 million years old
and were created by volcanoes. (See the Inspiration Point
EarthCache to learn how.) They have since have been eroding away.
At one point these mountains were over 10,000 feet high. But in
that time, millions of years, the rate at which they have eroded
has change. Today, it is about 0.5 ± 0.3 mm per year or 1636
feet every million years for the last 100,000 years (Meigs et al.,
1999). On the other hand the Santa Monica Mountains are growing at
about 0.5 ± 0.4 mm per year, or 1636 feet every million
years for the last 100,000 years (Meigs et al., 1999). And what
accounts for this growth are earthquakes. The mountains are growing
at about the same speed in which they are eroding away.
This growth comes from faults and the earthquakes that are produced
from it. The story doesn’t end there. The largest of the
three faults we are looking at is the most recently active, the
Santa Monica Mountains Blind Thrust Fault, ruptures on average
every 740 years (Dolan et al., 1995). And when these earthquakes
occur, how big should we expect them? Most scientists agree that
the largest magnitudes produced by these faults would around a 6.0
– 6.2 for the Sycamore Canyon and Boney Mountain Faults
(Slemmons, 1977) and 7.2 for the Santa Monica Mountains Blind
Thrust (Dolan et al., 1995).
So back to the “Big One” and the area you’re
standing on. In 1995, a study was done by James Dolan and others
that shows after they analyzed 51 different sources that in the
last 200 years there should have been 17 major earthquakes in the
Los Angeles Area; there have been two. In the future there will
earthquakes. This location preserves the scenery that was created
by the forces that control our planet. We’ll have to wait and
see what happens next, because tomorrow there will still be
earthquakes.
Quiz
To earn your certificate for this cache click the link below and
take the quiz.
A View By De-Fault Quiz
Q: From This EarthCache site, point at the Sycamore Canyon Fault.
What direction are you pointing?
Q: In the narrative, it states that the mountains are neither
growing nor shrinking. Select the geological feautures you can see
from this EarthCache site that illustrate this theory.
Q: The Chumash took a traditional route to the ocean for trade.
Which direction do you think they traveled from the villages
closest to this cache site?
Links:
Santa Monica Mountains
Homepage
GPS
Activities in the Santa Monica Mountains
Recent Earthquakes in
California and Nevada
U.S. Geological Survey
Earthquake Hazards Program
Sources:
- Dolan, J.F., Seih, K., Rockwell, T.K., Yeates, R.S., Shaw, J.,
Suppe, J., Hufrile, G.J., and Gath, E.M., 1995, Prospects for
Larger or More Frequent Earthquakes in the Los Angeles Metropolitan
Region, Science, V. 269, p. 199-205
- Dolan, J.F., and Pratt, T.L., 1997, High-resolution seismic
reflection profiling of the Santa Monica Fault Zone, West Los
Angeles, California, Geophysical Research Letters, V. 24, N. 16, p.
2051-2054
- Jennings, C.W., and Strand, R.G., compilers, 1969, Geologic Map
of California, Olaf P. Jenkins edition; Los Angeles Sheet:
California Division of mines and Geology Map, scale 1:250,000
- Jennings, C.W., complier, 1994, Fault activity map of the
California and adjacent area: California Department of
conservation, Division of Mike and Geology, California Geologic
Data Map Series, map no. 8
- Meigs, A., Brozovic, N., and Johnson, M.L., 1999, Steady,
balanced rates of uplift and erosion of the Santa Monica Mountains,
California, Basin Research, V. 11, p. 59-73
- Sonneman. H. S., 1956, Geology of the Boney Mountain Area, Santa
Monica Mountains, California, University of California, Los Angeles
Master of Arts in Geology Thesis, p. 63-64
- Saikia, C. K., 1999, Generation of strong motion time histories
from postulated large earthquakes (Mw =7) on the Santa Monica
Mountain Fault System, URS Greiner Woodward Clyde, Pasadena, CA,
91101
- USGS Earthquake Hazards Program has created some of the graphics
that are included in this cache.
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