REEFS, TRENCH PROTECT GUAM FROM TSUNAMIS

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By Steve Limtiaco

HAGATNA, Guam (Pacific Daily News, Dec. 27) - A tsunami of large magnitude may have a hard time finding its way to Guam shores thanks to the island's coral reefs and the Marianas Trench.

The only damaging tidal wave recorded on Guam happened January 25, 1849, at about 2:49 pm, following a strong earthquake. The quake lasted about one and a half minutes and opened fissures in the ground that emitted vapors, according to records kept by the Catholic Church.

Spanish Governor Pablo Perez gave the following report of the damage caused by the tidal wave or waves:

Agat: The sea swept through the streets of the pueblo, which is situated more than 500 brazas (3,000 feet) above the high tide mark, up a considerable slope.

Umatac: The sea swept in nearly as far as the pueblo, which is situated on elevated ground. It came in along the river for more than one league and destroyed many plantings.

Two whaling frigates anchored in the harbor in 16 brazas (96 feet) broke their chains, lost their anchors, and rode the three tidal waves. Their captains asserted that, after the sea receded, the place where they came to rest remained high and dry for more than five minutes. On the following day, the casks and the barrels in which they were collecting water were found at a great distance in the interior of the jungle.

Inarajan: The ocean entered along the river and swept three houses away, depositing them at a location 200 brazas (1,200 feet) distant. The pueblo was flooded. About 15 plots of planted rice and seven of sweet potatoes were lost, and the soil was washed away; the two near the pueblo and the one over the Acfallan River. It also washed away the raft used to cross the Talofofo River.

In the vicinity of this river, it washed away a rancho and a woman was drowned. The water swept her little niece away and deposited her among some rocks on the hillside, causing small bruises on her face.

(Editor's note: The Catholic Church reported that Josefa Cruz, an Agaña resident on her way to Inarajan, was caught in the undertow of a giant wave that washed over the road as she approached the Talofofo River. Her body was not recovered).

Pago: The water from the sea rose as far as the church patio, flooding the entire pueblo, which lies approximately 1,000 brazas (6,000 feet) from the beach, on a sloping rise. When the water receded, the streets were covered with fish, among them a very large one that the people of the village did not recognize.

"Reports Concerning the Mariana Islands: The Memorias of 1844-1852;" and "Chronicle of the Mariana Islands: Recorded in the Agaña Parish Church 1846-1899."

About Tsunamis

A tsunami is a system of gravity waves formed in the sea as a result of a large-scale disturbance of sea level over a short duration of time. A tsunami can be generated by submarine volcanic eruptions, by displacement of submarine sediments, by coastal landslides into a bay or harbor, by meteor impact, or by vertical displacement of the earth's crust along a zone of fracture, which underlies or borders the ocean floor.

A tsunami is not one wave, but a series of waves. The time that elapses between passage of successive wave crests at a given point usually is from 10 to 45 minutes. Several days may pass before the sea returns to its normal state.

At present, there is no method to determine if a tsunami has been generated except to note the occurrence and epicenter of the earthquake and then detect the arrival of the characteristic waves at a network of tide stations.

Tsunamis travel outward in all directions from the generating area. Their speed depends on the depth of water. In the deep and open ocean, they travel at speeds of 300 to 600 miles per hour. The distance between successive crests can be as much as 300 to 400 miles. However, the height of the waves may be no more than 1 or 2 feet, and the waves pass unnoticed.

Upon reaching shallower water, the speed of the advancing wave diminishes, its wave length decreases, and its height may increase greatly, owing to the piling up of water. Configuration of the coastline, shape of the ocean floor, and character of the advancing waves play an important role in the destruction wrought by tsunamis along any coast.

The first visible indication of an approaching tsunami is often a recession of water caused by the trough preceding an advancing wave. Any withdrawal of the sea, therefore, should be considered a warning of an approaching wave. A rise in water level also may be the first event.

During the 101-year period from 1900 to 2001, 796 tsunamis were observed or recorded in the Pacific Ocean, according to the Tsunami Laboratory in Novosibirsk, Russia.

Guam's coral reefs, and even the Marianas Trench, help protect the island from the flooding and destruction caused by tsunamis.

The island is most vulnerable in some of its bays, according to a report by government scientists, and several bayside villages were flooded and a woman was swept out to sea and killed in January 1849 -- the only damaging tsunami in the island's recorded history.

The powerful August 8, 1993, quake, which had an epicenter just north of the island and registered 8.1 on the Richter scale, did not claim any lives, but it created a tsunami in Pago Bay that swept fisherman Tony Guerrero, 51, and his truck into the bay.

"After the earthquake I was kind of figuring that a big wave might come," Guerrero said yesterday, recalling his 1993 ordeal. "All I saw was Pago Bay empty -- no water at all -- the whole bay drained out and all of a sudden the rolling of a big wave coming in. It was bigger than the truck, and I was in the truck."

The truck was dragged about 30 feet into the bay, and Guerrero climbed out a window and waded back to shore in chest-high water.

The Office of Civil Defense receives tsunami warnings from Hawaii's Pacific Tsunami warning center, which operates a series of sensors in the Pacific, said Frank Blas Jr., homeland security adviser for Guam's Office of Homeland Security.

Civil Defense can broadcast emergency warnings over radio and television if necessary, alerting residents to get to higher ground. Village mayor's offices, Guam police and Guam firefighters also will assist in alerting residents, he said.

If an earthquake happens to the east of the island, villagers may be evacuated to Agana Heights or Nimitz Hill, Blas said. If an earthquake happens to the west, villagers may be evacuated to the Umatac foothills or Talofofo, he said.

There may be little warning or no warning, he said.

"If we had a tsunami that is generated from a quake that is less than 500 miles away, you're looking at inside of 5 minutes, so there may not be that much of a warning. There isn't any system right now that is quick enough to be able to generate as quick an alert as possible."

A Geological Survey professional paper published in 1964 states that a destructive tsunami is possible in any of Guam's open bays if the wave is moving at a similar frequency as the existing waves in the bay.

"The possibility of a large tsunami causing considerable damage, however, appears remote in as much as most of the lowland on the island is protected by a band of coral reefs that acts as a filter or a baffle for long-period waves," the paper states. "Open bays, unprotected by reefs, such as Pago, Talofofo, and Inarajan, are most likely to be flooded if a tsunami should strike Guam."

The paper notes that, aside from the 1849 tsunami, a non-damaging tsunami was recorded on Guam on November 5, 1954, caused by an earthquake northeast of Japan, near Russia. It generated waves of less than a foot in Apra Harbor and waves as large as 5 feet in Ylig Bay.

Paul Hattori, a geophysicist with the U.S. Geological Survey station on Guam, said a tsunami approaching Guam's reefs would not cause the large waves typically associated with a tsunami. Instead, the water level on the reef would suddenly rise a couple of feet. That surge would be dangerous to people on the reef who are caught by surprise and swept off their feet, he said. It also would be more dangerous during high tide as opposed to low tide, he said.

Guam's eastern shore is most vulnerable to tsunamis, he said, because that is where most of the unprotected bays and rivers are -- places where the water can surge in unimpeded.

But it also depends on the direction the wave is traveling, he said. "If it's not lined up right, it's not going to happen," he said.

Hattori said the Marianas Trench would help deflect the wave energy if an earthquake happens east of the trench.

"It'll tend to disperse it. What happens is when it hits the trench; all of the submarine canyons and mountains tend to redirect the tsunami wave. The 1993 earthquake generated a tsunami in Japan because it went up the trench," he said.

December 28, 2004

Pacific Daily News: www.guampdn.com

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