Earthquakes

Little known facts about temblors:

The epicenter of an earthquake is not the same as its hypocenter (focus, point of origin within a fault-line). The epicenter is the point on the surface of the Earth directly above the focus. Dangerous, shallow-focus quakes originate 0-70 kilometers below the surface. Less damaging deep-focus tremors occur between 70-700 kilometers down. Subduction zone earthquakes (like the one that gave rise to the lethal tsunami on December 26, 2004) occur when one tectonic plate moves under another (subducts). There are interplate and intraplate quakes, which take place along plate boundaries or within the fracturing crust of a single plate, respectively.

Earthquakes are not rare at all - several hundred earthquakes occur every day. There are about 1 million of them annually - of which 50,000 can be felt without the aid of instruments. Tremors of the magnitude of Kobe in 1995 (which caused an estimated damage of $100 billion ) are measured 20 times in an average year.

The Encyclopedia Britannica (2005 edition) describes a "swarm" of such events thus:

"In the Matsushiro region of  Japan, for instance, there occurred between August 1965 and 1967 a series of hundreds of thousands of earthquakes, some sufficiently strong (up to local magnitude 5) to cause property damage but no casualties. The maximum frequency was 6,780 small earthquakes on April 17, 1966."

The Pacific ocean is the unhappy recipient of well over 80 percent of all the energy released by earthquakes worldwide. Japan alone suffers from 1500 tremors annually (of which two thirds are greater than 3.5 in magnitude). Fault lines abound and new ones are discovered frequently. One fault line runs under 125th street in Manhattan, New-York.

Still, in the last 5 centuries, all earthquakes combined killed less than one tenth the victims of World War II - and this includes the 240,000 who died in the 1976 Tang-Shan, China event.

Earthquakes are composites of:

I. Primary (or compression) and secondary (or shearing) body waves (that travel in the rocks under the surface of the Earth at speeds of up to 7 kilometers per second and frequencies of between 20 Hertz and one vibration per 54 minutes)

and

II. Two types of surface waves, named after British physicist Lord Rayleigh and British geophysicist A. E. H. Love (with frequencies of 1-0.005 Hertz).

Some earthquakes are caused by human activities (such as the filling of water reservoirs behind dams, injecting water into deep wells, and underground nuclear tests). More than 600 tremors were recorded in the decade following the filling of Lake Mead behind Hoover Dam on the Nevada-Arizona state border.

Some earthquakes produce low-pitch sounds and light effects (flashes, streamers, and balls). Water in lakes and reservoirs oscillate causing flooding (a phenomenon called seiche). Seiches were observed in Scotland and Sweden following the Lisbon quake of 1755. Similarly, the Alaskan tremor in 1964 produced seiches in Texas and throughout the southwestern parts of the United States.

Measuring the magnitude of earthquakes is more a fine art than an exact science.

Charles Richter developed his eponymous logarithmic scale in 1935. It measures the amplitude (the height) of seismic surface waves. Each unit represents a tenfold increase in the energy released by the tremor. An earthquake of magnitude 9 is, therefore, 1000 stronger than a tremor of magnitude 6. The Kobe earthquake measured 6.8 on the Richter scale, the San Francisco tremor of 1906 was 8.3 (as was the earthquake in the Mississippi Valley in 1811), and both the Alaskan quake of 1964 and the South Asian underwater temblor of 2004 were around 9 (9.2 in Alaska to be precise)

The Richter scale is used mainly by the media. Professional seismologists use the Moment Magnitude Scale (MMS) which takes into account the properties of the area and the amount of slippage (displacement). It captures the total energy of the tremor. The Kobe earthquake measured 7 on the MMS, the San Francisco tremor of 1906 was 7.6, and the Alaskan quake of 1964 was 9.

Then there is the still-used 12-grade Modified Mercalli Scale (adapted in 1931 by American seismologists H. O. Wood and Frank Neumann from the original Mercalli scale, proposed in 1902 Italian seismologist Giuseppe Mercalli). It measures the impact that an earthquake has on the natural and man-made environment to gauge its magnitude. The Europeans have a similar 12-grade scale, called MSK.

Seaquakes are earthquakes that start on land and then travel into the sea at the speed of sound (about 1.5 kilometers per second).

Quakes occur even on the moon which has no plates, volcanic activities, or ocean trenches. The five seismograph stations of the Passive Seismic Experiment set up between 1969 and 1977 as part of the United States Apollo Program detected up to 3,000 moonquakes every year. Mars, on the other hand, seems not to have quakes at all!

Some notable earthquakes in history:

Lisbon, November 1, 1755, 09:40 AM (All Saints Day)

Property damage: 12,000 houses, fire raged for 6 days

Casualties: 60,000 dead

Felt as far as: Algiers (1100 kilometers to the east)

Side effects: tsunami 20 meters high (at Cadiz) to 6 meters high (at Lisbon). Traveled to Martinique (6100 kilometers) in 10 hours and rose to 4 meters when it struck the shore.

New Madrid, Missouri, USA - December 16, 1811, January 23 and February 7, 1812

Felt as far as: Louisville, Kentucky (300 kilometers away); Cincinnati, Ohio (600 km. away); Canada; Gulf Coast.

Side effects: 1874 aftershocks; The tremor affected 100,000 square kilometers. An area of 240X60 kilometers sank by 1-3 meters and was flooded as a nearby river rushed in.

San Francisco, April 18, 1906, 05:12 AM

Property damage: Fire destroyed the business district of San Francisco. Cities along the fault (e.g., San Jose, Salinas, and Santa Rosa) obliterated.

Casualties: 700 dead

Felt as far as: Los Angeles in the south and Coos Bay, Oregon, to the north

Side effects: At least a 430 kilometers fault slippage (break).

Tokyo–Yokohama, September 1, 1923

Property damage: Fifty four percent of brick buildings and one tenth of other, reinforced, structures collapsed. Hundreds of thousands of houses crumbled or burned.

Casualties: 140,000 dead

Side effects: Twelve-meter high tsunami crashed against Atami on the Sagami Gulf, destroyed 155 houses and killed 60 people.

Chile, 1960

Property damage: Pegged at millions of US dollars.

Casualties: 5700 killed and 3000 injured.

Side effects: Seismic sea waves (tsunamis) struck Hawaii, Japan, and the Pacific coast of the United States.

Alaska, March 27, 1964

Casualties: 131 dead

Side effects: Felt over an area of 1,300,000 square kilometers and tilted an area of more than 120,000 square kilometers. Land was thrust up by as much as 25 meters and sank by up to 2.5 meters. Numerous tsunamis affected locales as far as Crescent City, California. The fault extended for 1000 kilometers and there were tens of thousands of aftershocks.

Tang-shan, China, July 28, 1976

Property damage: Entire city razed to the ground.

Casualties: 240,000 killed and half a million injured.

Mexico, September 19, 1985, 07:18 AM

Property damage: Most buildings in Mexico City - 400 kilometers from the epicenter - damaged extensively.

Casualties: 10,000 killed.

Side effects: Seismic sea waves (tsunamis) struck Hawaii, Japan, and the Pacific coast of the United States.

http://pubs.usgs.gov/gip/earthq1/

http://www.crustal.ucsb.edu/ics/understanding/

http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html

Cyclopedia of Factoids

http://samvak.tripod.com/factoidsindex.html