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Earthquakes
About 8,000 mini-quakes a day, and on average, one exceptionally big one per year

by Borgna Brunner


Earthquakes in residential areas, such as this 1964 one in Seward, Alaska, can
cause unparalleled damage to life and property.
There are thousands upon thousands of earthquakes annually, ranging on average
from 18 major quakes to more than 2 million very minor ones per year
(approximately 8,000 per day). On average, we can expect one exceptionally big
one (with a magnitude of 8 or higher) each year.


Earthquakes are caused by forces deep within Earth's interior that
continuously affect its surface. When the energy from these forces is released
suddenly—usually by shearing movements along faults in the Earth's crust—an
earthquake results. See also plate tectonics.


Not "On the Richter Scale"

Although "on the Richter Scale" is still a commonly used expression, the
scale, developed by Charles F. Richter of California Institute of Technology
in 1935, is no longer the most commonly used magnitude measurement. One of its
flaws is its imprecision in measuring the biggest earthquakes, those in the
range of 8 or 9.


Different methods of measuring magnitude have superseded the Richter Scale,
including surface-wave magnitude, which measures the seismic waves crackling
around Earth's surface, and moment magnitude, the newest method, which is
based on the size of the fault on which an earthquake occurs and the amount
the Earth slips. Moment magnitude is the most uniformly applicable scale.

Intensity versus Magnitude

RELATED LINKS
Earthquakes and Mortality Figures Worldwide, 1990—2004

Estimated Deaths from Earthquakes, 2004

Estimated Earthquake Deaths in 2003

Major Earthquakes around the World, 2004

Major Earthquakes Around the World, 2003

Ten Largest Earthquakes of the Century

25 Largest Earthquakes in the United States

Chronology of Eruptions

Encyclopedia: Earthquakes

Encyclopedia: Tsunami

Severity of an Earthquake

Frequency of Earthquakes

Seismology

Floods, Avalanches, and Tidal Waves

Quiz: Disasters
These two terms are often confused: intensity is based on the observed effects
of an earthquake on its surroundings, whereas magnitude measures the amount of
seismic energy released.

The Modified Mercalli (MM) Intensity Scale measures 12 increasing levels of
intensity (each designated by a Roman numeral) from imperceptible shaking to
catastrophic destruction. More subjective and less scientific than magnitude,
intensity is nevertheless a meaningful way of capturing the terror and
destruction of an earthquake. It is the intensity of the 1906 San Francisco
earthquake that is remembered, not its magnitude.


The Twentieth Century's Largest Earthquake

Three of the ten largest earthquakes of the twentieth century occurred in
Alaska, striking in 1957, 1964, and 1965. The strongest of these (magnitude:
9.2) took place on Good Friday in 1964, hitting 80 miles east of Anchorage.
The biggest earthquake of the century struck Chile on May 22, 1960. Its
magnitude, the highest ever recorded, was 9.5.

The deadliest earthquake of all time is believed to have occurred on Jan. 24,
1556, in Shaanxi (Shensi) Province, China — 830,000 were killed.

Tsunami

The Alaskan Good Friday earthquake was followed by tsunami, or tidal waves,
that reached 50 feet high and traveled a phenomenal 8,445 miles at 450 miles
per hour. Tsunami is a Japanese term (tsu, port; nami, waves) presumably
referring to the rising height of tsunami as they approach shore.

Breaking on land, these tremendous waves can cause enormous destruction. In
addition to earthquakes, volcanoes can cause tsunami — the worst tsunami ever
is believed to have occurred after the eruption of Krakatoa on August 27,
1883. More than 36,000 people were killed by the tsunami alone.