Facts of the Matter
Facts of the Matter
Stressed rock can
instigate earthquakesThe destructive earthquake in Colima, Mexico, last month serves as a reminder that the earth is physically alive, its activity driven by a difference in temperature between the interior and the surface.
Earthquakes occur when rock is broken. Rocks in the earth's upper 60 miles are solid, but elastic enough to deform under stress; deformation stores energy, as in a stretched rubber band. Like the rubber, the strength of rock is limited, and when it fractures the energy is released. The rubber band emits sound waves that we hear as a 'snap'; the earth sends out seismic waves that we feel as tremors.
The magnitude of an earthquake is measured on the Richter scale, named after seismologist Charles Richter, who formulated it in the 1930s. It provides a way to compare the energy released in an earthquake, using the amount of ground movement at a seismograph station and its distance from the earthquake.
The event in Colima on Jan. 22 measured 7.8 on the Richter scale, about the same as the 1906 San Francisco quake. The Great Alaska quake of 1964 measured 9.2 but released 125 times as much energy as the Colima quake. A 7.8 earthquake releases as much energy as seven and a half million tons of TNT, equivalent to 500 Hiroshima-sized nuclear bombs.
'Minor' and 'light' earthquakes are the most common, with nearly one million occurring annually. They occur mostly on the sea bed along the 40,000 mile-long oceanic ridge system that wraps around the earth. 'Moderate' and 'strong' earthquakes together number 600 or so per year. These can cause slight damage to structures, and may cause a lot of damage in populated areas.
'Major' earthquakes have magnitudes between 7 and 8. The 1989 Loma Prieta earthquake was a 7.1.
There have been three major earthquakes in Hawaii since 1868, all on the Big Island. The 7.9 shakeup in 1868 caused 77 deaths, destroyed 100 homes and generated a 50-foot high tsunami. It followed a 7.0 quake 4 days earlier. These temblors were associated with an eruption of Mauna Loa. The third, in November 1975, beneath Kalapana, caused two deaths and more than $4 million in damage, and was followed by a Kilauea eruption.
All the 'great' earthquakes, having magnitudes greater than 8, occur around the Pacific rim. The most powerful earthquake ever recorded, a 9.5, occurred deep under Chile in March, 1960. It killed 1,000, injured 3,000, left more than two million homeless and caused $550 million in damage. It generated a tsunami that caused 61 deaths and $75 million in damage in Hawaii, and also produced death and destruction in Japan and on the west coast of the United States.
The forces behind the energy of a major earthquake come from the earth's tectonic plates. Volcanic activity is responsible for most of the rest, and they are seldom above magnitude 7.
Tectonic plates bump against one another, or are torn apart by volcanic activity. Most earthquakes are generated at the margins of the plates. The most violent ones occur where the Pacific plate plunges under the continents that rim the Pacific. Luckily, Hawaii is in the middle of the Pacific plate, far from any of the plate's active boundaries.
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu