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Cool Earthquake Facts
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.
The
largest recorded earthquake in the
world was a magnitude 9.5 (Mw) in Chile on May 22,
1960.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
It is thought
that more damage was done by the resulting fire after the
1906 San Francisco earthquake
than by the earthquake itself.
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.
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.
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.
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.
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.
From 1975-1995
there were only
four states that did
not have any earthquakes. They were: Florida, Iowa,
North Dakota, and Wisconsin.
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.
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.
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.
Most
earthquakes occur at depths of
less than 80 km (50 miles) from the Earth's
surface.
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.
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.
Florida and
North Dakota have the
smallest
number of earthquakes in the United States.
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.
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.
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!
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