Hawaiian Earthquake in a Volcanically Active Zone
Oct. 15, 2006 — -- This is what everyone has worried about: A sizeable earthquake hitting a populated area, with no way to predict it, and no way to anticipate the amount of damage done.
Hawaii does not fit the mold of most major earthquake zones. It is considered a "hot spot," prone to earthquake activity, because of the rugged volcanoes that formed the island chain. But it is not on the juncture of two of the major tectonic plates that form the earth's crust, the seams where most earthquakes occur.
"Earthquakes and mountains were closely correlated," said Kate Hutton, who studies earthquakes for the U.S. Geological Survey. "If you had tall young mountains you're likely to have earthquakes. And the reason is that's how the mountains got there."
Active Volcanoes in Area
On Hawaii, the mountains are volcanic. The big island of Hawaii is made of five volcanoes, two of which -- Mauna Loa and Kiluaea -- remain active today. Mauna Loa erupted in 1984; Kilauea is considered one of the most active volcanoes on Earth today, having erupted 30 times in the last 40 years.
The vibrations that follow this release of energy, called seismic waves, spread for miles.
Most earthquakes happen within 40 miles of the Earth's surface. The strongest of Sunday's quakes was reported by the USGS to have been centered 24.2 miles below sea level, six miles southwest of Puako, Hawaii.
That underground point is called the focus, or hydrocenter, of the quake. The epicenter is the ground-level point right above it on a map.
"The epicenter is not where the worst damage is," said Lucy Jones of the USGS in California. "It's only where the rupture started. The worst damage and the worst shaking is often somewhere other than the epicenter, after it's really built up steam."
Magnitude 6.6 for Strongest Tremor
The severity of an earthquake is commonly called its magnitude. Scientists do not actually use the Richter scale, devised by geologist Charles Richter in the 1930s, but the numbers still mean the same thing.
Each rung on the magnitude scale is ten times stronger than the one before it. Thus a quake of magnitude six is considered "strong," and is ten times more powerful than a "moderate" quake of magnitude five.
A 3.0 may be hardly noticeable. A 6.0 may knock pictures off the wall. San Francisco's famous earthquake of 1906 was estimated, in retrospect, to have been a 7.8.
The greatest earthquake ever recorded was a 9.5 in Chile in 1960. It left two million people homeless. The Sumatran quake of December 24, 2004 -- which set off the great tsunami that killed more than 200,000 people across the Indian Ocean -- was a 9.0.
The Hawaiian earthquake of Sunday morning is currently estimated to have been a 6.6, revised upward from a 6.3. In the three hours following, there were at least 45 aftershocks of magnitude 1.9 or greater.