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The
largest recorded earthquake in the
United States was a magnitude 9.2 that struck Prince
William Sound, Alaska on Good Friday, March 28, 1964.
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The
largest recorded earthquake in the
world was a magnitude 9.5 (Mw) in Chile on May 22,
1960.
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The
earliest reported earthquake in California
was felt in 1769 by the exploring expedition of Gaspar de Portola
while the group was camping about 48 kilometers (30 miles) southeast
of Los Angeles.
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Before
electronics allowed recordings of large earthquakes, scientists
built large spring-pendulum seismometers
in an attempt to record the long-period motion produced by such
quakes. The largest one weighed about 15 tons. There is a medium-sized
one three stories high in Mexico City that is still in operation.
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The
average rate of motion across the San
Andreas Fault Zone during the past 3 million years
is 56 mm/yr (2 in/yr). This is about the same rate at which
your fingernails grow. Assuming this rate continues, scientists
project that Los Angeles and San Francisco will be adjacent
to one another in approximately 15 million years.
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The
East African Rift System
is a 50-60 km (31-37 miles) wide zone of active volcanics and
faulting that extends north-south in eastern Africa for more
than 3000 km (1864 miles) from Ethiopia in the north to Zambezi
in the south. It is a rare example of an active continental
rift zone, where a continental plate is attempting to split
into two plates which are moving away from one another.
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The
first "pendulum seismoscope"
to measure the shaking of the ground during an earthquake was
developed in 1751, and it wasn't until 1855 that faults were
recognized as the source of earthquakes.
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Moonquakes
("earthquakes" on the moon) do occur, but they happen less frequently
and have smaller magnitudes than earthquakes on the Earth. It
appears they are related to the tidal stresses associated with
the varying distance between the Earth and Moon. They also occur
at great depth, about halfway between the surface and the center
of the moon.
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Although
both are sea waves, a tsunami and a
tidal wave are two different unrelated phenomenona.
A tidal wave is a large sea wave produced by high winds, and
a tsunami is a sea wave caused by an underwater earthquake or
landslide (usually triggered by an earthquake) displacing the
ocean water.
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The
hypocenter of an earthquake
is the location beneath the earth's surface where the rupture
of the fault begins. The epicenter
of an earthquake is the location directly above the hypocenter
on the surface of the earth.
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The
greatest mountain range
is the Mid-Ocean Ridge, extending 64,374 km (40,000 mi) from
the Arctic Ocean to the Atlantic Ocean, around Africa, Asia,
and Australia, and under the Pacific Ocean to the west coast
of North America. It has a greatest height of 4207 m (13,800
ft) above the base ocean depth.
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The
world's greatest land mountain range
is the Himalaya-Karakoram. It countains 96 of the world's 109
peaks of over 7317 m (24,000 ft). The longest range is the Andes
of South America which is 7564 km (4700 mi) in length. Both
were created bythe movement of tectonic plates.
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It
is estimated that there are 500,000
detectable earthquakes in the world each year. 100,000
of those can be felt, and 100 of them cause damage.
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It
is thought that more damage was done by the resulting fire after
the 1906 San Francisco earthquake
than by the earthquake itself.
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A
seiche (pronounced SAYSH)
is what happens in the swimming pools of Californians during
and after an earthquake. It is "an internal wave oscillating
in a body of water" or, in other words, it is the sloshing of
the water in your swimming pool, or any body of water, caused
by the ground shaking in an earthquake. It may continue for
a few moments or hours, long after the generating force is gone.
A seiche can also be caused by wind or tides.
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Each
year the southern California area has
about 10,000 earthquakes. Most of them are so small
that they are not felt. Only several hundred are greater than
magnitude 3.0, and only about 15-20 are greater than magnitude
4.0. If there is a large earthquake, however, the aftershock
sequence will produce many more earthquakes of all magnitudes
for many months.
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The
magnitude of an earthquake
is a measured value of the earthquake size. The magnitude is
the same no matter where you are, or how strong or weak the
shaking was in various locations.
The intensity of an earthquake
is a measure of the shaking created by the earthquake, and this
value does vary with location.
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The
Wasatch Range, with its
outstanding ski areas, runs North-South through Utah, and like
all mountain ranges it was produced by a series of earthquakes.
The 386 km (240-mile)-long Wasatch Fault is made up of several
segments, each capable of producing up to a M7.5 earthquake.
During the past 6000 years, there has been a M6.5+ about once
every 350 years, and it has been 150 years since the last powerful
earthquake.
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There
is no such thing as "earthquake weather".
Statistically, there is an equal distribution of earthquakes
in cold weather, hot weather, rainy weather, etc. Furthermore,
there is no physical way that the weather could affect the forces
several miles beneath the surface of the earth. The changes
in barometric pressure in the atmosphere are very small compared
to the forces in the crust, and the effect of the barometric
pressure does not reach beneath the soil.
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From
1975-1995 there were only four states
that did not have any earthquakes. They were: Florida,
Iowa, North Dakota, and Wisconsin.
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The
core of the earth was the
first internal structural element to be identified. In 1906
R.D. Oldham discovered it from his studies of earthquake records.
The inner core is solid, and the outer core is liquid and so
does not transmit the shear wave energy released during an earthquake.
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The
swimming pool at the University of Arizona in Tucson lost water
from sloshing (seiche) caused by the 1985
M8.1 Michoacan, Mexico earthquake 2000 km (1240 miles)
away.
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Earthquakes
occur in the central portion of the United States too! Some
very powerful earthquakes occurred along the New Madrid fault
in the Mississippi Valley in 1811-1812.
The effects of shaking from these magnitude 8+ earthquakes caused
church bells to ring in Boston, Massachusetts, nearly 1600 km
(1000 miles) away.
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Most
earthquakes occur at depths of less
than 80 km (50 miles) from the Earth's surface.
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The
San Andreas fault is NOT a single,
continuous fault, but rather is actually a fault
zone made up of many segments. Movement may occur along any
of the many fault segments along the zone at any time. The San
Andreas fault system is more that 1300 km (800 miles) long,
and in some spots is as much as 16 km (10 miles) deep.
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The
world's deadliest recorded earthquake
occurred in 1557 in central China. It struck a region where
most people lived in caves carved from soft rock. These dwellings
collapsed during the earthquake, killing an estimated 830,000
people. In 1976 another deadly earthquake struck in Tangshan,
China, where more than 250,000 people were killed.
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Florida
and North Dakota have the smallest
number of earthquakes in the United States.
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The
deepest earthquakes typically
occur at plate boundaries where the Earth's crust is being subducted
into the Earth's mantle. These occur as deep as 750 km (400
miles) below the surface.
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Alaska
is the most earthquake-prone state
and one of the most seismically active regions in the world.
Alaska experiences a magnitude 7 earthquake almost every year,
and a magnitude 8 or greater earthquake on average every 14
years.
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The majority of the earthquakes and
volcanic eruptions occur along plate boundaries such
as the boundary between the Pacific Plate and the North American
plate. One of the most active plate boundaries where earthquakes
and eruptions are frequent, for example, is around the massive
Pacific Plate commonly referred to as the Pacific Ring of Fire.
- The earliest
recorded evidence of an earthquake has been traced back to
1831 BC in the Shandong province of China, but there is a fairly
complete record starting in 780 BC during the Zhou Dynasty in
China.
- It was recognized
as early as 350 BC by the Greek scientist Aristotle that soft
ground shakes more than hard rock in an earthquake.
- The cause of
earthquakes was stated correctly in 1760 by British engineer
John Michell, one of the first fathers of seismology, in a memoir
where he wrote that earthquakes and the waves of energy that they
make are caused by "shifting masses of rock miles below the
surface".
- In 1663 the European
settlers experienced their first earthquake in America.
- Human beings can
detect sounds in the frequency range 20-10,000 Hertz. If a P wave
refracts out of the rock surface into the air, and it has a frequency
in the audible range, it will be heard as a rumble. Most
earthquake waves have a frequency of less than 20 Hz,
so the waves themselves are usually not heard. Most of the rumbling
noise heard during an earthquake is the building and its contents
moving.
- When the Chilean
earthquake occurred in 1960, seismographs recorded seismic
waves that traveled all around the Earth. These seismic
waves shook the entire earth for many days! This phenomenon is
called the free oscillation of the Earth.
- The San
Andreas Fault was named in 1895 by geologist A.C. Lawson.
He named it after the San Andreas Lake, a sag pond through which
the fault passes about 20 miles south of San Francisco. He likely
did not realize at the time that the fault ran almost the entire
length of California!
See
also:
USGS
Science Challenge Questions & Answers
USGS
Pasadena GeoFacts
