A publication of the Archaeological Institute of America
Volcanoes can be deadly in a number of ways. Mount St. Helens demonstrated many of them in 1980: an explosive eruption with velocities near 700 mph, extensive ash fall, 90 mph lahars or mudflows, gases, associated lightning, etc. Particularly deadly are pyroclastic flows, sometimes called a nuées ardentes ("glowing clouds"), incandescent mixtures of volcanic fragments and gases that sweep along close to the ground at velocities as great as 450 miles an hour. Their high temperature and speed make them extremely destructive. During the 1902 eruption of Mont Pelée, on Martinique, a nuée ardente demolished the coastal city of St. Pierre and killed nearly 30,000.
The Volcanic Explosivity Index (VEI) has been proposed as a means of comparing eruptions, with 0 as the lowest and 8 as the highest. A 0 denotes a nonexplosive. Eruptions designated 5 or higher, considered "very large" explosive events, occur only on an average of about once every two decades. Mount St. Helens eruption was barely a 5. The A.D. 79 eruption of Vesuvius was another 5.
Deadliest eruptions since A.D. 1500, their VEI (volcanic explosivity index), and victims
The Richter Scale
Devised by Charles Richter in 1935, the scale measures the energy released by the earthquake (other factors are involved in how damaging a quake might be, such as its depth). The scale is logarithmic, that is each successive whole number represents a ten-fold increase in power. A magnitude 7 earthquake, equivalent to 179,100 metric tons of TNT, is ten times more powerful than a magnitude 6. For comparison, a one-megaton nuclear bomb can measure 7.5 on the Richter scale.
There are about 35 earthquakes worldwide each day, but only 18 major and one great earthquake per year. Recent U.S. quakes include Northridge (1994), a 6.7, and Loma Prieta (1989), a 7.1. The 1906 San Francisco quake is estimated to have been an 8.25.
The Ten Deadliest Earthquakes, their magnitudes and victims
The Turkey quake
On August 16, at 3:02 AM local time, a magnitude 7.4 earthquake struck near Izmit 55 miles east-southeast of Istanbul, killing some 15,000. The problem in the earthquake-prone eastern Mediterranean is basically congestion. The Arabian plate is pushing north against Eurasian plate, squeezing the small Turkish microplate westward. The Turkish microplate is bounded on the east by the East Anatolian Fault, on the north by the North Anatolian Fault (NAF), on the south by the Hellenic and Cyprus arcs (beneath which the northward-moving African plate plunges), and on the west by a diffuse deformation zone in the Aegean (hence the September 10 quake just north of Athens that killed more than 100). The 900-km long NAF, running east to west across northern Turkey is similar to the San Andreas Fault, a right-lateral strike-slip fault (if you look across it, the movement of opposite side is to the right; strike-slip meaning two crustal blocks are sliding past one another). From 1939 through 1999 the NAF has produced seven magnitude 7 earthquakes, rupturing the fault progressively from west to east.
Tsunamis (the name is from Japanese tsu, harbor, and nami, wave) are generated by earthquakes. Because of this they occur mostly in the Pacific (80 percent are related to the circum-Pacific seismic zone). On the high seas tsunami wave crests may be only a foot or so in height (the amplitude) but 600 miles (965 km) apart (the wavelength). As the wave comes into shallower water near the coast, its wavelength decreases and amplitude increases--up to 200 feet (61 m). Tsunamis move very quickly, often at speeds of 150 mph (241 kph) and up to 180 mph (300 kmph).
Recent tsunamis occurred in 1992 Nicaragua (1992) and Peru (1996), but the most devastating was the one that struck Papua New Guinea on July 17, 1998. In a magnitude 7.1 earthquake, the seabed off West Sepik Province on northern coast of Papua New Guinea suddenly dropped about seven feet (two meters) along a 24-mile (40 km) fault line. This spawned a tsunami estimated at 50 feet (15 m high) when it hit the low-lying coast, killing about 8,000. (See "Tsunami Destroys Villages in Papua New Guinea," November/December 1998.)
According to the USGS, Hurricane Mitch (category 5) was the most destructive hurricane in history of the Western Hemisphere. From October 27 through November 1, 1998, it hammered parts of Honduras, Nicaragua, El Salvador, and Guatemala with 180+ mph winds, dumping from one to six feet of rain. Widespread flooding and landslides left 11,000 dead and destroyed 50+ percent of the infrastructure in Honduras and Nicaragua. More than 2 million were left homeless.
The Saffir-Simpson Hurricane Damage Potential Scale
Concerned by frightening reports in popular media about possible celestial roadwreck including the earth and comets, asteroids, and the like, MIT's Richard Binzel presented a scale for conveying to the public the gravity of any possible threat, like that used for hurricanes, to a meeting of colleagues at Turin (Torino), Italy, this June. The "Torino scale" was adopted by the International Astronomical Union in July. It measures the kinetic energy of an asteroid approaching earth against probability of a collision, giving a rating--from 0 to 1--of how serious the thing really is.
If you want to risk it, the best meteor showers are the Quadrantids (January 3-4, 100 per hour), Perseids (August 12, 80 per hour), and Geminids (December 13, 100 per hour). Be certain to wear a hard hat or carry an umbrella, just in case.
No scale has yet been suggested for rating the danger posed by bizarre human behavior inspired by astronomical phenomena, for example the mass suicide of Heaven's Gate cult members when comet Hale-Bopp was visible in 1997 or, more recently, the death of a baby, born during the solar eclipse in August in a remote village in northern Romania, whose mother killed it, apparently fearing an ancient superstition that monsters are born during eclipses. Such behavior may be a more immediate threat than catching an asteroid.