|
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.
|