| Earthquake
Fact File
2261/ Earthquakes release
a tremendous amount of energy, which is why they can be so
destructive. The figures below show magnitudes with the approximate
amount of TNT needed to release the same amount of energy.
| Magnitude |
Approximate
TNT Energy |
| 4.0 |
6 tons |
| 5.0 |
199 tons |
| 6.0 |
6,270 tons |
| 7.0 |
199,000 tons |
| 8.0 |
6,270,000 tons |
| 9.0 |
99,000,000 tons
|
2262/ Any earthquake
above 6.0 magnitude on the Richter Scale can cause considerable
damage. A 7.0 magnitude is very strong and can be very dangerous.
If you can feel an earthquake it is at least 2.5 magnitude.
2263/ The Richter Scale
is not an actual instrument. It is a measure of the amplitude
of seismic waves and is related to the amount of energy released.
This can be estimated from the recordings of an earthquake
on a seismograph. The scale is logarithmic, which means that
each whole number on the scale increases by 10. A magnitude
6.0 earthquake is 10 times greater than a 5.0, a 7.0 is 100
times greater, and a magnitude 8.0 is 1,000 times greater.
2264/ The largest earthquake
in Canada (one of the worlds great quakes) occurred in 1949
- an 8.1 magnitude earthquake off the Queen Charlotte Islands.
2265/ 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.
2266/ The great Alaska
earthquake of March 1964, is the largest earthquake in the
United States. It had a magnitude of 9.2. 115 people died,
with most of the deaths due to the tsunami it generated. Shaking
was felt for an estimated 7 minutes, and raised or lowered
the ground surface as much as 2 meters (6.5 feet) in some
areas and 17 meters (approx. 56 feet) in others. The length
of the ruptured fault was between 500 and 1,000 kilometers
(310.5 and 621 miles). The amount of energy released was equal
to 12,000 Hiroshima-type blasts, or 240 million tons of TNT.
2267/ The largest recorded
earthquake in the world was a magnitude 9.5 (Mw) in Chile
on May 22, 1960.
2268/ 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.
2269/ 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.
2270/ 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 centre of the moon.
2271/ 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.
2272/ 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.
2273/ 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.
2274/ 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.
2275/ Florida and North
Dakota have the smallest number of earthquakes in the United
States.
2276/ 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.
2277/ 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.
2278/ It was recognized
as early as 350 BC by the Greek scientist Aristotle that soft
ground shakes more than hard rock in an earthquake.
2279/ 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.
2280/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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