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Chances of a Major Earthquake

FULL STORY AT http://www.livescience.com/forcesofn...0913_slip.html

An important seismic event imperceptible to humans has begun in the Pacific Northwest as predicted, according to the government agency Geological Survey of Canada.

The chance of a major earthquake is 30 times higher now for a roughly two-week period, but the odds are still remote, scientists say.

The event is called episodic tremor and slip (ETS). It involves a slow movement of the Juan de Fuca and North America tectonic plates along the Cascadia margin of southern British Columbia. Faults associated with the plates have been the sites of major earthquakes -- akin to the colossal tsumani-causing quake last December in Indonesia -- every 500 years or so, the geologic record shows. The last such temblor in the area struck on Jan. 26 in the year 1700.

The movement is slower than a traditional earthquake but more rapid than the normal creep associated with the fault. It runs in the reverse direction of the normal creep.

The movement was predicted. Scientists recently learned that these ETS events recur about every 14 months. It has been detected by Global Positioning System instrument.

The event does not mean an earthquake is imminent, but geologists are eager to study it and learn more and they say sooner or later an ETS event is likely to trigger a major quake.

"Compared to the steady year-round stress accumulation, this more rapid stress increase implies that a large subduction earthquake is more likely to happen during the time of an ETS event," the Canadian geologists write.

The slippage and associated minor tremors "are directly related to megathrust (Sumatra-like) earthquake potential," lead geologist John Cassidy and a colleague said in Tuesday's statement. "Neither the tremor nor the slip can be felt."

The slip began Sept. 3 on the Olympic Peninsula of Washington State and has migrated north to the Vancouver Island area, Cassidy wrote. Victoria moved 0.12 inches (3 millimeters) to the West over the course of two days. The events are thought to last six to 15 days.

Cassidy's colleague, Stephane Mazzotti, has done some calculations on the odds of a large temblor.

"The probability of occurrence of a megathrust earthquake is about 30 times higher during this approximately two-week window, than during the rest of the 14.5 month cycle," Cassidy told LiveScience. "Having said that, 30 times a small number is still a small number."

Geologists simply don't know when one of these events will trigger a major quake, Cassidy said.

The immediate importance of the event is that it occurred as predicted and can now be used to improve understanding of the region's seismology.

"By better understanding these events, we will be able to better predict the effects (and perhaps timing) of future magnitude 9 earthquakes along the West Coast," Cassidy and his colleague write.

A separate study recently concluded that a major earthquake along the fault could be overdue, given clusters of the events seen in the geologic record. Because the fault is offshore, scientists say its rupture could create a devastating tsunami.

Scientists: Sumatra Quake Longest Ever Recorded

Temblor big enough to 'vibrate the whole planet
Courtesy of Marsha Walton

(CNN) -- Dramatic new data from the December 26, 2004, Sumatran-Andaman earthquake that generated deadly tsunamis show the event created the longest fault rupture and the longest duration of faulting ever observed, according to three reports by an international group of seismologists published Thursday in the journal "Science."

"Normally, a small earthquake might last less than a second; a moderate sized earthquake might last a few seconds. This earthquake lasted between 500 and 600 seconds," said Charles Ammon, associate professor of geosciences at Penn State University.

The quake released an amount of energy equal to a 100 gigaton bomb, according to Roger Bilham, professor of geological sciences at the University of Colorado. And that power lasted longer than any quake ever recorded.

The quake, centered in the Indian Ocean, also created the biggest gash in the Earth's seabed ever observed, nearly 800 miles. That's as long as a drive from Los Angeles, California, to Portland, Oregon.

Scientists estimated the average slippage (ground movement up and down) along the entire length of the fault was at least 5 meters (16.5 feet) -- with some places being moved nearly 20 meters (50 feet).

Scientists have also upgraded the magnitude of the quake from 9.0 to between 9.1 and 9.3, a dramatically more powerful event. As a comparison: the ground shook 100 times harder during December's earthquake than what was felt in the 1989 Loma Prieta quake in California. That 6.9 magnitude quake caused extensive damage from Santa Cruz to San Francisco.

Monster quake

The stunning power of Asia's earthquake and tsunamis last December has left even veteran scientists in awe. "I think it was humbling for everyone that analyzed the earthquake," said Thorne Lay, professor of earth sciences and director of the Institute of Geophysics and Planetary Physics at the University of California, Santa Cruz.

"We're sitting in our laboratories working on the signals from this earthquake, trying to understand what happened scientifically, and then watching TV at night and seeing the death toll rising for weeks," he said.

The enormous human toll from the natural disasters spurred Lay to organize dozens of scientists from all over the world to share their data and analysis of the quake. The long-term goal is to try to get more, and more accurate tsunami warning systems in place.

Whole planet vibrated

A wide array of instruments were used for the first time to study the earthquake, and its many aftershocks. Global broadband seismometers recorded the ground in Sri Lanka, a thousand miles from the epicenter, moved up and down by more than 9 centimeters (3.6 inches), according to the report. But no place on Earth escaped movement.

"Globally, this earthquake was large enough to basically vibrate the whole planet as much as half an inch, or a centimeter. Everywhere we had instruments, we could see motions," Ammon said.

Much of that information came from digital broadband seismometers, a new era of instruments that the National Science Foundation and the U.S. Geological Survey began deploying around the world several years ago.

Lay says the equipment is sensitive enough to pick up the motion of wind blowing through trees, or cows walking in a field, or the massive motions produced by this earthquake. "We'd never seen signals from an earthquake of this size, and the availability of this instrumentation was a real breakthrough in being able to see the complete rupture process of one of these truly monstrous events," Lay said.

Other tools added to the scientists' understanding. Underwater cameras documented the huge crack in the ocean floor. Tsunami buoys, and sonar from the British Navy helped with the analysis. And a fortunate bit of timing enabled researchers to get a view of the tsunami they have never seen before.

"Two hours after the earthquake has occurred, the wave is spreading out from the Bay of Bengal," Lay said. "Two satellites went over, with the capability of measuring the elevation of the ocean surface. The satellites saw the south-going wave and the north-going part of the wave. "It was just good luck that the passage of the satellites caught the tsunami in motion," he said.

Crunching numbers, and creating maps and models is taking on a new urgency for some of the scientists involved in this research. "There will be more earthquakes of this type, and with more humans exposed to the hazard there will be more devastating losses of life. What we hope to do is develop technologies that can minimize that loss," Lay said.

Recorded Earthquakes and Volcanic Eruptions Within the Last 2000 Years

A.D. 79 - Aug. 24, Italy: eruption of Mt. Vesuvius buried cities of Pompeii and Herculaneum, killing thousands.
856 - Dec. 22, Damghan, Iran: earthquake killed 200,000.
893 - March 23, Ardabil, Iran: earthquake killed about 150,000 people.
1138 - Aug. 9, Aleppo, Syria: deadly earthquake claimed lives of 230,000 people.
1290 - Sept., Chihli, China: earthquake killed about 100,000 people.
1556 - Jan. 23, Shaanxi (Shensi) province, China: most deadly earthquake in history; 830,000 killed.
1667 - Nov., Shemakha, Caucasia: earthquake killed about 80,000 people.
1693 - Jan. 11, Sicily, Italy: earthquake killed about 60,000 people.
1707 - Oct. 28, Japan: tsunami caused by an earthquake drowned 30,000.
1727 - Nov. 18, Tabriz, Iran: about 77,000 victims killed in deadly earthquake.
1755 - Nov. 1, Portugal: earthquake, fires, and Atlantic tsunami leveled Lisbon and was felt as far away as southern France and North Africa; 70,000 killed.
1782 - South Sea, China: tsunami killed 40,000.
1783 - Feb. 4, Calabria, Italy: series of 6 earthquakes over two-month period caused massive destruction, killing 50,000. One of first scientifically investigated earthquakes.
June 8, Iceland: eruption of Laki volcano lasted until Feb. 1784. Haze from eruption resulted in loss of island's livestock and widespread crop failure; 9,350 deaths, mostly due to starvation.
1792 -
May 21, Kyushu Island, Japan: collapse of old lava dome during eruption of Unzen volcano caused avalanche and tsunami that killed an estimated 14,300 people. (Most were killed by the tsunami.) Japan's greatest volcano disaster.
1811 - Dec. 16, Mississippi Valley, nr. New Madrid, Mo.: earthquake reversed the course of the Mississippi River. Fatalities unknown due to sparse population in area. Aftershocks and tremors continued into 1812. It has been estimated that three of the series of earthquakes had surface-wave magnitudes of 8.6, 8.4, and 8.8 on the Richter scale. It is the largest series of earthquakes known to have occurred in North America.
1815 - April 5, 10–11, Netherlands Indies (Sumbawa, Indonesia): eruption of Tambora largest in historic times. An estimated 92,000 people were killed, about 10,000 directly as a result of explosions and ash fall and about 82,000 indirectly by starvation and disease.
1877 - June 26, north-central Ecuador: eruption of Mt. Cotopaxi caused severe mudflows that wiped out surrounding cities and valleys; 1,000 deaths.
1883 - Aug. 26–28, Netherlands Indies (Krakatau, Indonesia): eruption of Krakatau; violent explosions destroyed two-thirds of island and caused a tsunami on Java and Sumatra, killing more than 36,000. It was felt as far away as Cape Horn and possibly England.
1886 - Aug. 31, Charleston, S.C.: magnitude 7.3 quake, killed 60 people and caused extensive damage.
1896 - June 15, Sanriku, Japan: earthquake and tidal wave killed 27,000.
1902 - May 7, St. Vincent, West Indies: Soufrière volcano erupted, devastating one-third of the island and killing some 1,680 people.
May 8, Martinique, West Indies: Mt. Pelée erupted and wiped out city of St. Pierre; 40,000 dead.
1906 - April 18, San Francisco: earthquake accompanied by fire razed more than 4 sq mi; more than 500 dead or missing.
1908 - Dec. 28, Messina, Sicily: city totally destroyed by earthquake. Death toll, from quake and tsunami, 70,000–100,000 in Sicily and southern Italy.
1915 - Jan. 13, Avezzano, Italy: earthquake left 29,980 dead.
1920 - Dec. 16, Gansu province, China: magnitude 8.6 earthquake killed 200,000 in northwest China.
1923 - Sept. 1, Japan: magnitude 8.3 earthquake destroyed one-third of Tokyo and most of Yokohama. More than 140,000 killed.
1927 - May 22, nr. Xining, China: magnitude 8.3 earthquake claimed approximately 200,000 victims.
1932 - Dec. 25, Gansu, China: magnitude 7.6 earthquake killed approximately 70,000.
1933 - March 10, Long Beach, Calif.: 117 left dead by earthquake.
1935 - May 30, Pakistan: earthquake at Quetta killed 30,000–60,000.
1939 - Jan. 24, Chile: earthquake razed 50,000 sq mi; about 30,000 killed.
Dec. 27, northern Turkey: severe quakes destroyed city of Erzingan; about 100,000 casualties.
1948 - Oct. 5, Ashgabat, Turkmenistan: magnitude 7.3 earthquake destroyed the city, killing 110,000.
1950 - Aug. 15, India: earthquake affected 30,000 sq mi in Assam; 20,000–30,000 believed killed.
1960 - Feb. 29, Agadir, Morocco: 10,000–12,000 dead as earthquake set off tidal wave and fire, destroying most of city.
May 22, Chile: strongest earthquake ever recorded (9.5 magnitude) struck near the coast, causing a tsunami that traveled as far as Hawaii, Japan, and New Zealand, killing more than 2,000, wounding 3,000.
1964 - March 28, Alaska (03:36:14 UT; March 27, 5:36 P.M. local time): strongest earthquake ever to strike North America (9.2 magnitude) hit 80 mi east of Anchorage; followed by seismic wave (tsunami) 50 ft high that traveled 8,445 mi at 450 mph; 117 killed..
1970 - Jan. 5, Yunnan province, China: magnitude 7.7 quake killed 15,621.
May 31, Peru: earthquake left more than 66,000 dead.
1972 - Dec. 22, Managua, Nicaragua: earthquake devastated city, leaving up to 6,000 dead.
1976 - Feb. 4, Guatemala: quake left over 23,000 dead.
July 28, Tangshan, China: worst earthquake to hit China in 20th century; devastated 20 sq mi of city, leaving 255,000 dead.
Aug. 17, Mindanao, Philippines: earthquake and tidal wave left up to 8,000 dead or missing.
1978 - Sept. 16, Tabas, Iran: earthquake destroyed city in eastern Iran, leaving 25,000 dead.
1985 - Sept. 19–20, Mexico: magnitude 8.1 earthquake devastated part of Mexico City and three coastal states; estimated 25,000 killed.
Nov. 14–16, Colombia: eruption of Nevada del Ruiz, 85 mi northwest of Bogotá. Mudslides buried most of the town of Armero and devastated Chinchiná; 21,800 killed.
1988 - Dec. 7, Armenia: earthquake measuring 6.9 in magnitude killed nearly 25,000, injured 15,000, and left at least 400,000 homeless.
1989 - Oct. 17, San Francisco Bay area: earthquake measuring 7.1 in magnitude killed 67 and injured over 3,000. Over 100,000 buildings damaged or destroyed.
1990 - June 21, northwest Iran: magnitude 7.7 earthquake destroyed cities and villages in Caspian Sea area. At least 40,000 dead, over 60,000 injured, and 400,000 homeless.
July 16, northern Philippines: magnitude 7.7 quake killed nearly 2,000.
1991 - July 15, Luzon Island, Philippines: eruption of Mt. Pinatubo buried over 300 sq mi under volcanic ash and resulted in more than 800 deaths.
1993 - Aug. 8, Guam: earthquake measuring 8.1 in magnitude caused severe damage to many structures but no fatalities.
Sept. 29, India: earthquake measuring 6.2 killed 9,748 and destroyed nearly all the buildings in Khillari.
1994 - Jan. 17, San Fernando Valley, Calif.: earthquake measuring 6.6 in magnitude killed 61 and injured over 8,000. Damage estimated at $13–20 billion.
1995 - Jan. 17, Osaka, Kyoto, and Kobe, Japan: 5,200 killed and 26,800 injured; estimated damage $100 billion. Magnitude: 6.9.
1997 - May 12, northeast Iran: severe earthquake measuring 7.3 in magnitude left more than 1,500 people dead and at least 4,460 injured.
June–Sept., southern Montserrat: ongoing eruption of Soufrière Hills volcano since July 1995; killed 20 people in major eruption on June 25, 1997, rendered southern two-thirds of Montserrat uninhabitable, and forced some 8,000 of the island's 12,000 residents to abandon the island.
1998 - May 30, northern Afghanistan: magnitude 6.6 earthquake and aftershocks killed at least 4,000. A quake on Feb. 4 in same area killed about 2,300.
July 17, Papua New Guinea: three tsunamis, possibly spurred by an undersea landslide following an earthquake, wiped out entire villages in the northwest province of Sepik. At least 2,100 killed.
1999 - Jan. 25, Armenia, Colombia: 1,185 dead and more than 4,000 injured in magnitude 6.2 earthquake. Over 200,000 left homeless.
Aug. 17, northwest Turkey: magnitude 7.6 quake centered near Izmit killed over 17,000 and injured about 44,000. Damage estimated at $8.5 billion. Another severe 7.2 temblor killed more than 700 in Ducze and nearby towns in Nov.
Sept. 21, central Taiwan: severe 7.7 earthquake and aftershocks killed 2,295 and injured 8,729.
2001 - Jan. 13, El Salvador: magnitude 7.7 earthquake set off some 185 landslides across El Salvador; at least 850 died and nearly 100,000 houses were destroyed.
Jan. 26, Bhuj, India: magnitude 7.7 earthquake rocked western Indian state of Gujarat, killing more than 20,000 people and leaving 600,000 homeless.
2002 - March 25, northeast Afghanistan: series of earthquakes—the largest measuring 6.1 in magnitude—rattled an area 100 mi north of Kabul. Estimated 1,000 people killed and 7,000 families homeless. The city of Nahrin, a densely populated district capital, was completely razed.
2003 - May 21, Northern Algeria: magnitude 6.8 earthquake caused the collapse of numerous buildings, killing 2,266 people. The epicenter was 40 mi east of Algiers, the capital city.
Dec. 26, Bam, Iran: magnitude 6.6 earthquake devastated the ancient historic city of Bam in southeast Iran, killing 26,200 people, injured 30,000, and left 75,000 homeless, as mud-brick buildings collapsed.
2004 - Dec. 26, Sumatra, Indonesia: magnitude 9.0 earthquake, off the west coast of Sumatra, caused a tremendously powerful tsunami in the Indian Ocean that hit 12 Asian countries, killing more than 225,000 and leaving millions homeless. It was the deadliest tsunami in history.
2005 - Feb. 22, Zarand, Iran: magnitude 6.4 earthquake in central Iran shook more than 40 villages, killing at least 612 people, injuring over 1,400, and destroying villages with many mud–brick houses.
Mar. 28, Sumatra, Indonesia: magnitude 8.7 earthquake, off the west coast of Sumatra, killed 1,313. Many buildings in the islands of Nias and Simeulue were destroyed and some officials feared another tsunami would occur. The same area was at the center of a huge tsunami in December that killed over 225,000 people. Officials at the U.S. Geological Survey said that yesterday's earthquake was an aftershock of December's 9.0 quake. The 9.0 magnitude earthquake was twice the power of the 8.7 magnitude quake.