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About
Earthquakes
What
Causes an Earthquake?
Unlike many
other areas where a shift in tectonic plates is the sole
cause of an earthquake, 95% of earthquakes in Hawaii are
linked to volcanic activity. These earthquakes can occur
before or during eruptions, or as molten rock travels
underground.
How
is an earthquake measured?
Earthquakes
are generally measured in terms of magnitude and intensity.
The Richter
Scale measures magnitude. An earthquake of 5.0 is a moderate
event, 6.0 is a strong event, 7.0 is a major earthquake,
and a "great quake" exceeds 8.0. For each whole number
increase there is a 10-fold jump in seismic wave amplitude
(or, a 30-fold gain in energy released). For example,
a 6.0 earthquake generates 30 times more energy than a
5.0 quake and 900 times (30*30) greater than a 4.0 earthquake.
In the United
States, the Modified Mercalli Intensity Scale (MMI) measures
intensity - the effects of an earthquake felt by people.
MMI ranges from I (faintly registered by instruments)
to XII (nearly total destruction). Ratings decrease with
increasing distance away from an earthquake's source.
Seismic
Hazard Zones In Oahu
It is difficult
to assign earthquake hazard zones to Oahu because ground-shaking
during an earthquake varies within a small area, depending
on the nature of the underlying ground (e.g., lava bedrock
or soil). Local topography also affects earthquake hazards.
Steep slopes composed of loose material may produce large
landslides during an earthquake. The type of construction
also affects the risks of damages to a property. For these
reasons, earthquake hazards are highly localized and difficult
to assign regional earthquake boundaries that share the
same relative degree of hazard.
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