MAY 29, 2015
ALASKA, US (Reuters) - An earthquake with a magnitude of 7.0
struck off the southwest coast of Alaska late on Thursday, the U.S. Geological
Survey reported.
The USGS said the quake's epicenter was 104 km (64 miles)
south-southeast of Ugashik and 61.7 km deep. The agency upgraded the temblor to
a 7.0 after initially stating it was slightly weaker.
The Pacific Tsunami Warning Center said that based on its
available data, no tsunami was expected.
The USGS said the quake was unlikely to cause casualties and
notable damage, and there were no immediate reports of such.
The area near the quake, centered some 400 miles (643 km)
southwest of Anchorage, is lightly populated, according to the USGS.
What is an
earthquake?
Earthquakes with magnitude of about 2.0 or less are usually
called microearthquakes; they are not commonly felt by people and are generally
recorded only on local seismographs.
Events with magnitudes of about 4.5 or
greater - there are several thousand such shocks annually - are strong enough
to be recorded by sensitive seismographs all over the world.
Great earthquakes,
such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or
higher. On the average, one earthquake of such size occurs somewhere in the
world each year.
Magnitude measures the energy released at the source of the
earthquake as determined from measurements on seismographs. An earthquake has
one magnitude. The magnitude scale most commonly in use now is called the moment
magnitude scale.
Moment is a physical quantity proportional to the slip on the
fault times the area of the fault surface
that slips; it is related to the total energy released in the EQ. The moment
magnitude provides an estimate of earthquake size that is valid over the
complete range of magnitudes, a characteristic that was lacking in other
magnitude scales.
Intensity measures the strength of shaking produced by the
earthquake at a certain location. Intensity is determined from effects on
people, human structures, and the natural environment.
An earthquake can
produce shaking of many different intensities. The Modified
Mercalli Intensity Scale is used in the US.