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FICTION: California will split apart from the rest of the United States and become an island/Oceanfront property in Barstow
FACT:
California is split by a transform
plate boundary separating the
Pacific plate from the
North American plate. This means that the
land west of the San Andreas fault
is sliding northwest past the
rest of the United States, towards San Francisco. This sliding does
not create any space between the two plates for water to fill in. Just
like if you rub your hands together, your hands are sliding past each
other, but there is no space opening up in between them.
In fact, because there is a bend in the San Andreas Fault, the land on
both sides of the fault (at the bend) are actually converging and getting
closer together. This is what caused the formation of the Transverse Ranges,
mountains to the northeast of the Los Angeles basin. The 1994 Northridge
earthquake (M 6.7) is an example of an earthquake caused by this convergence.
Independent of plate motion, Barstow is at an elevation of over 2100 feet
(640 meters). This means that the ocean water would have to travel
significantly uphill to even reach Barstow.
So there will be no oceanfront property in Barstow or anywhere else
in central California.
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FICTION: The earth will open and swallow a train, my car, my town, or me
FACT:
A popular literary device is a fault
that opens during an earthquake to
swallow up an inconvenient character. But unfortunately for principled
writers, gaping faults exist only in movies and novels. The ground moves
across a fault during an earthquake, not away from it. If the fault could
open, there would be no friction. Without friction, there would be no
earthquake. Shallow crevasses can form during earthquake-induced landslides,
lateral spreads, or other types of ground failures. Faults, however, do not
open up during an earthquake.
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FICTION: An earthquake rupture will follow train tracks
FACT:
Train tracks are located on the earth's surface. Faults are located within the earth
and are caused
by processes deep in the earth. The two features are completely unrelated. The faults do not know
where the train tracks are, nor do they care when they are rupturing.
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FICTION: Thinking an earthquake is an aftershock because they couldn't find an epicenter -
all earthquakes have epicenters
FACT: Aftershocks ARE
earthquakes!! The only
difference is that they
occur after a larger earthquake instead of by themselves. You cannot tell
from looking at a seismogram
if an earthquake is a mainshock or an aftershock,
because they look the same. Aftershocks are earthquakes that follow the
largest shock of an earthquake sequence. They are smaller than the mainshock
(the biggest earthquake) and close to the mainshock fault. Aftershocks can
continue over a period of weeks, months, or years. In general, the
larger the mainshock, the larger and more numerous the aftershocks, and
the longer they will continue. Every aftershock is an earthquake.
After a major earthquake you should expect to experience periodic shaking
from aftershocks.
Every earthquake has an epicenter!! Earthquakes begin
far below the surface of the earth. We call
the point where they start the hypocenter. The point on the
surface of the Earth directly above the hypocenter is called the epicenter. All
earthquakes have epicenters, even aftershocks, since they are just
smaller earthquakes occurring as the result of a larger earthquake, or
mainshock.
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FICTION: Earthquake weather: Subduction zone earthquakes caused by recent wet weather
FACT: Many people believe that earthquakes are more common in
certain kinds of weather. In fact, no correlation with weather has been found.
Earthquakes begin many kilometers below the region affected by surface
weather. People tend to notice earthquakes that fit the pattern and
forget the ones that don't. Also, every region of the world has a story
about earthquake weather, but the type of weather is whatever they had
for their most memorable earthquake. So no earthquakes, including subduction zone
earthquakes, can be caused by recent wet weather.
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FICTION: Get in a doorway!
FACT: An enduring earthquake
image of California is a collapsed adobe
home with the doorframe as the only standing part. From this came our belief
that a doorway is the safest place to be during an earthquake. True-- if
you live in an old, unreinforced adobe house. In modern houses,
doorways are no stronger than any other part of the house and usually
have doors that will swing and can injure you. You are safer under a
table.
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FICTION: Nuclear explosions can "seal" faults
FACT: Nuclear explosions CANNOT seal faults.
Earthquakes are part of a global tectonic process that generally
occurs well beyond the influence or control of humans. The hypocenters
(points of origin) of earthquakes are typically several miles -- even tens
to hundreds of miles -- underground
(the deepest humans have been able to drill is 12 km (7.2 mi)).
The scale and force necessary to produce
earthquakes are well beyond our daily lives. And in fact, some fault
surfaces are already melted! The friction caused by movements along a fault
can cause the rocks to get so hot they melt.
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FICTION: You can prevent large earthquakes by making lots of small ones,
or by "lubricating" the fault with water
FACT: Seismologists have observed that for every magnitude 6
earthquake there are 10 of
magnitude 5, 100 of magnitude 4, 1,000 of
magnitude 3, and so forth as the events get smaller and smaller. This sounds
like a lot of small earthquakes, but there are never enough small ones
to eliminate the occasional large event. It would take 32 magnitude 5's,
1000 magnitude 4's, or 32,000 magnitude 3's to equal the energy of one
magnitude 6 event. So, even though we always record many more small events
than large ones, there are never enough to eliminate the need for the
occasional large earthquake. As for "lubricating" faults with water or some
other substance, injecting high- pressure fluids deep into the ground is
known to be able to trigger earthquakes to occur sooner than would have
been the case without the injection. However this would be a dangerous
pursuit in any populated area, as one might trigger a damaging earthquake.
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FICTION: California can have larger earthquakes than Washington
FACT:
The largest earthquakes
occur at subduction
zones, not along transform
boundary faults like the San Andreas. While earthquakes that occur on strike-slip
faults can cause great damage,
they rarely generate earthquakes larger
than a magnitude 7, and
almost never have earthquakes larger than a
magnitude 8. Subduction zones, on the other hand, can produce earthquakes
over magnitude 9, such as the 1960 Chile earthquake, which had a magnitude
of 9.5. This is because subduction zone earthquakes occur along
faults, which are much longer and go deeper into the earth than transform
boundary faults. This results in more motion during an earthquake along
a subduction zone fault. Washington is located above a subduction zone
and therefore has the potential to have much larger earthquakes than the
transform boundary in California.
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FICTION: The Space Needle and Golden Gate Bridge will collapse
FACT: Both the Space Needle and the Golden Gate Bridge are
highly engineered buildings designed to withstand large earthquakes. It
is interesting to note that in the movie, the Space Needle falls on a
completely intact unreinforced brick building. Unlike the Space Needle,
brick buildings do very poorly in earthquakes. A brick building
would have collapsed long before the Space Needle.
The Golden Gate Bridge:
Structures the size of the Golden Gate, such as other large bridges or
off-shore platforms, typically have a very long natural period of vibration. This tends to reduce the
response the structure experiences from the more rapid motions of an earthquake. Nonetheless, major
seismic challenges remain. In the case of the Golden Gate Bridge, one of these challenges is to
connect its various large-scale components together, such as the approach bridge structures at the
north and south ends to the main bridge structure.
Necessary retrofits have been
completed or are underway in Phases 1 and 2 of the overall seismic
upgrade project for the approaches at the north and south ends. The third phase will tackle some weak
points in the suspension bridge itself, especially in the manner in which the deck is connected to the
towers. The complete seismic retrofit program is budgeted to cost approximately $400 million.
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FICTION: Knowing the magnitude of an earthquake as it happens, and calling out increasing
magnitudes while the earthquake is happening
FACT:
The magnitude of an event is
determined from the strength of the seismic waves detected at
each
seismic recording station. We use several different formulas to determine the magnitude. Most formulas
depend on a measure of the shear, or S-waves,
which have the largest amplitude and carry most of the
seismic wave energy. S-waves travel more slowly than the P-waves used to locate the
earthquake,
at about 2 to 3 miles/second, so a particular magnitude may not be available until a few minutes after
the earthquake.
In contrast, the location of the hypocenter
is available within a minute of less after the earthquake.
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FICTION: Staying on a bicycle during a M7.9 earthquake, or walking during a M9.2 earthquake, while
buildings are collapsing all around
FACT:
If there is a M7.9 earthquake
there is no way a person could stay upright on a bicycle. In a region
of intensity VII (explanation of
Modified Mercalli Scale of Intensity) people have difficulty standing, and at VII drivers have
trouble steering. The Northridge earthquake, which was a M6.7, had intensities above VII, so a M7.9
certainly would have shaking strong enough to knock someone off of their bicycle - especially when
buildings right next to the bicyclist are collapsing. The same applies for a person trying to walk
during a M9.2 earthquake - it can't be done.
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FICTION: Earthquakes can happen below the asthenosphere, the San Andreas Fault can have an
earthquake at 700 km depth
FACT:
The earth is too hot below the asthenosphere for there to be
earthquakes.
Temperature increases as you go deeper into the earth. Most
earthquakes stop happening at depths of about 20 km because the
rock is too hot. The asthenosphere begins at about 100 km into the earth.
Why does rock temperature matter?
Earthquakes need friction to happen. The rock on one side of the
fault
is being pushed, pulled or slid in the opposite direction as the rock on
the other side of the fault. Because of friction, the rock doesn't move
right away (just like when you start pushing a really heavy box it doesn't
move right away). Eventually, the force pushing the rocks is bigger than
the force of friction and the rocks move - this is an earthquake. When the
rocks are hotter, there is less friction, and the rocks are easier to move.
When the rocks get hot enough, the friction is less that the forces pushing
the rocks. When this happens, the rocks slide past each other easily,
without causing earthquakes.
An exception to really deep earthquakes
An exception to the depth limitation is in
subduction zones. In a subduction zone, a cold piece of
oceanic crust is being pushed underneath other crust. This piece of crust is relatively cold, and can
reach as deep as 700 km into the crust. At about 700 km the crust gets too hot and soft for there to
be an earthquake. This piece of ocean crust is not part of the asthenosphere. There is a subduction
zone off the coast of Washington, Oregon, and the very north most part of California. The San Andreas
fault is not part of a subduction zone, therefore it cannot have earthquakes at 700 km (in fact
earthquakes along the San Andreas fault don't happen below a depth of 20 km).
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FICTION: Being in Sacramento, and not feeling a M8.4 earthquake in Redding, CA
FACT:
Redding, CA is about 160 miles (260 km) from Sacramento - Sacramento would definitely feel
shaking from a M8.4 earthquake.
A M8.1 earthquake in Peru caused damage in regions
hundreds of miles from the epicenter.
The Hector Mine earthquake, which was a M7.1 earthquake, was felt by
people over 260 km away from the earthquake in all directions (see figure below).
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FICTION: Only two scientists going out to where the earthquake was located, and needing permission
from the director of FEMA to go out there in the first place
FACT:
Right after an earthquake,
many scientists will go out to the fault rupture so they can make
observations and collect data. Scientists will look for places where there has been obvious ground
motion and measure the displacement. They will look at the damage that has been done. GPS stations
will be set up to measure post seismic ground motion. If the earthquake is in a region without a lot
of seismic instrumentation, additional seismometers will be installed to measure the aftershocks.
Unless the earthquake epicenter
occurs in a government restricted area (or private property),
scientists do not need permission to go out to the earthquake site.
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FICTION: Faults are lines
FACT:
Faults are planes. They are
3-D features, which are represented usually shown on
maps as
lines.
These lines are the fault trace - the place where the fault plane meets the surface. The figure to
the right is an illustration of a fault plane. The figure below
shows some of Southern California's faults represented on the map as planes - they are the rectangles
that you
see.
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FICTION: Cannot determine the epicenter of an earthquake if the hypocenter is too deep
FACT:
An epicenter can be found for
any earthquake that creates
seismic waves large enough
to reach the
surface and be recorded by at least three seismometers. The epicenter of an earthquake is found using
triangulation. Using the arrival times of
the seismic waves, the distance between the seismic recording station and the earthquake can be
calculated. If this distance is calculated for three different stations, there should be one point
that is the correct distance away from each station. This point is the epicenter.
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FICTION: Can't find surface evidence of a M9.2 earthquake near the epicenter
FACT:
When there are large earthquakes
the surface deformation at the epicenter is very clear. Offset
rivers, ground on one side of the fault
several feet (even several meters) higher than ground on the
other side of the fault, and cracks in the ground (note: only cracking - not formation of bottomless
holes for people to fall into) are just a few ways that earthquakes can change the way the earth looks
near the fault. The following are some pictures (all taken from various USGS websites) showing
natural evidence of an earthquake. The first two are from the Hector Mine earthquake in California.
The third is from the Nisqually earthquake in Washington, and the fourth is from the Loma Prieta
earthquake in California. The first three pictures show fault offset. The fourth picture shows sand
volcanoes caused by liquefaction.
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ADDITIONAL MOVIE FICTION, WITH EXPLANATIONS TO COME...
- FICTION: An earthquake
creates dust clouds visible on satellite radar
- FICTION: Using the term lateral skip
- FICTION: Earthquakes
can cause trucks to sink in dirt, long after the earthquake happened
- FICTION: Scientists being able to successfully predict earthquakes over short time
intervals
- FICTION: Plates
conjoin at 324 feet below the earth's surface
- FICTION: The magnitude
of an earthquake in progress
cannot decrease/stabilize
- FICTION: Water draining into the faultline
- FICTION: Fault opening at
the ocean and water rushing up the fault
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