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Common
Myths About Earthquakes
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Q: Can you predict earthquakes?
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A: No.
Neither the USGS nor Caltech nor any other scientists have
ever predicted a major earthquake. They do not know how,
and they do not expect to know how any time in the foreseeable
future. However based on scientific data, probabilities
can be calculated for potential future earthquakes. For
example, scientists estimate that over the next 30 years
the probability of a major EQ occurring in the San Francisco
Bay area is 67% and 60% in Southern California.
The USGS focuses their efforts on the long-term mitigation
of earthquake hazards by helping to improve the safety of
structures, rather than by trying to accomplish short-term
predictions.
For further information, see:
Earthquake Prediction
discussions |
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Q: Can animals predict earthquakes?
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| A: Changes in animal behavior
can not be used to predict earthquakes. Even though there
have been documented cases of unusual animal behavior prior
to earthquakes, a reproducible connection between a specific
behavior and and the occurrence of an earthquake has not
been made. Animals change their behavior for many reasons
and given that an earthquake can shake millions of people,
it is likely that a few of their pets will, by chance, be
acting strangely before an earthquake.
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Q: Can you prevent large
earthquakes by making lots of small ones, or by "lubricating"
the fault with water?
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A:
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, 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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Q: Can some people sense
that an earthquake is about to happen? (earthquake sensitives)?
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| A: There is
no scientific explanation for the symptoms some people claim
to have preceding an earthquake, and more often than not there
is no earthquake following the symptoms. |
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Q:Is there earthquake weather?
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A: In the 4th Century B.C.,
Aristotle proposed that earthquakes were caused by winds
trapped in subterranean caves. Small tremors were thought
to have been caused by air pushing on the cavern roofs,
and large ones by the air breaking the surface. This theory
lead to a belief in earthquake weather, that because a large
amount of air was trapped underground, the weather would
be hot and calm before an earthquake. A later theory stated
that earthquakes occurred in calm, cloudy conditions, and
were usually preceded by strong winds, fireballs, and meteors.
However, there is no connection between
weather and earthquakes. They are the result of geologic
processes within the earth and can happen in any weather
and at any time during the year. Earthquakes
originate miles underground. Wind, precipitation, temperature,
and barometric pressure changes affect only the surface
and shallow subsurface of the Earth. Earthquakes are focused
at depths well out of the reach of weather, and the forces
that cause earthquakes are much larger than the weather
forces. Earthquakes occur in all types of weather, in all
climate zones, in all seasons of the year, and at any time
of day. Sometimes, we are asked: "Do earthquakes change
the weather in any way? Earthquakes themselves do not cause
weather to change. Earthquakes, however, are a part of global
tectonics, a process that often changes the elevation of
the land and its morphology. Tectonics can cause inland
areas to become coastal or vice versa. Changes significant
to alter the climate occur over millions of years, however,
and after many earthquakes.
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| Q: Are there more earthquakes
in the morning/in the evening/at a certain time of the month?
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| A: Earthquakes
are equally as likely to occur at any time of the day or month
or year. The factors that vary between the time of the day,
month, or year do not affect the forces in the earth that
cause earthquakes. See answer below also. |
Q: Can the
position of the moon or the planets affect seismicity?
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A: The moon, sun, and other
planets have an influence on the earth in the form of perturbations
to the gravitational field. The relative amount of influence
is proportional to the objects mass, and inversely proportional
to the square of its distance from the earth. No significant
correlations have been identified between the rate of earthquake
occurrence and the semi-diurnal tides when using large earthquake
catalogs. There have, however, been some small but significant
correlations reported between the semi-diurnal tides and
the rate of occurrence of aftershocks in some volcanic regions,
such as Mammoth Lakes. (UC Berkeley)
For further information, see: "Gravitational
Forces"
University
of California, Berkeley, Seismological Laboratory
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Q: Can the ground open up
during an earthquake?
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| A: 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. Movement occurs along
the plane of a fault, not perpendicular to it. If faults opened
up, no earthquake would occur because there would be no friction
to lock them together. |
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Q:Will California eventually
fall off into the ocean?
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| A: No. The
San Andreas Fault System, which crosses California from the
Salton Sea in the south to Cape Mendocino in the north, is
the boundary between the Pacific Plate and North American
Plate. The Pacific Plate is moving in northwest with respect
to the North American Plate at approximately 46 millimeters
per year (the rate your fingernails grow). The strike-slip
earthquakes on the San Andreas Fault are a result of this
plate motion. The plates are moving horizontally past one
another, so California is not going to fall into the ocean.
However, Los Angeles and San Francisco will one day be adjacent
to one another! For further information, see: "Earthquakes, Megaquakes, and the Movies" |
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Q: Why are we having so
many earthquakes? Has earthquake activity been increasing?
Does this mean a big one is going to hit? OR
We haven't had any earthquakes in a long time; does this
mean that the pressure is building up and there will be
a big one?
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A: Although it may seem that
we are having more earthquakes, earthquakes of magnitude
7.0 or greater have remained fairly constant throughout
this century and, according to our records, have actually
seemed to decrease in recent years. A partial explanation
may lie in the fact that in the last twenty years, we have
definitely had an increase in the number of earthquakes
we have been able to locate each year. This is because of
the tremendous increase in the number of seismograph stations
in the world and the many improvements in global communications.
In 1931, there were about 350 stations operating in the
world; today, there are more that 4,000 stations and the
data now comes in rapidly from these stations by telex,
computer and satellite. This increase in the number of stations
and the more timely receipt of data has allowed us and other
seismological centers to locate many small earthquakes which
were undetected in earlier years, and we are able to locate
earthquakes more rapidly. The NEIC now locates about 12,000
to 14,000 earthquakes each year or approximately 35 per
day. Also, because of the improvements in communications
and the increased interest in natural disasters, the public
now learns about more earthquakes. According to long-term
records (since about 1900), we expect about 18 major earthquakes
(7.0 - 7.9) and one great earthquake (8.0 or above) in any
given year. However, let's take a look at what has happened
in the past 32 years, from 1969 through 2001, so far. Our
records show that 1992, and 1995-1997 were the only years
that we have reached or exceeded the long-term average number
of major earthquakes since 1971. In 1970 and in 1971 we
had 20 and 19 major earthquakes, respectively, but in other
years the total was in many cases well below the 18 per
year which we may expect based on the long-term average.
A temporal increase in earthquake activity does not mean
that a large earthquake is about to happen. Similarly, quiescence,
or the lack of seismicity, does not mean a large earthquake
is going to happen.
See NEIC's
Earthquake Statistics webpage for the tables of earthquake
counts by magnitude and year.
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Q: Do earthquakes cause
volcanoes?
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| A: No, there are different earth
processes responsible for volcanoes. Earthquakes may occur
in an area before, during, and after a volcanic eruption,
but they are the result of the active forces connected with
the eruption, and not the cause of volcanic activity. |
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