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Philippine earthquake 'equivalent to 32 Hiroshima atomic bombs'

Publication Date : 02-09-2012

 

The earthquake that struck off the east coast of the Philippines on Friday night packed energy “equivalent to 32 Hiroshima atomic bombs”, but a combination of factors spared the Filipinos destruction from a catastrophe, scientists said yesterday.

Director Renato Solidum of the Philippine Institute of Volcanology and Seismology (Phivolcs) said the 7.6-magnitude quake would have been more strongly felt had its epicentre been on land or if there was more vertical displacement of ocean water, triggering a destructive tsunami, as what happened in the Moro Gulf quake that killed thousands of people in southern Mindanao and Sulu in 1976.

Asked if he considered it a miracle, Solidum replied: “It’s always a blessing when damage from an earthquake is minimal. I believe in God … but there are scientific explanations for what happened.”

“We were lucky,” said University of the Philippines (UP) geologist Alfredo Mahar Lagmay. He said the Philippines was fortunate that the earthquake did not meet the conditions of a larger-scale disaster: power, proximity and the kind of structures in the affected places.

The 7.6-magnitude earthquake struck 106 kilometres east of Guiuan town, Eastern Samar, at a depth of 34.9 km, the US Geological Survey (USGS) said. The Phivolcs placed the epicentre a bit farther at 112 km east of Guiuan, in the Philippine Trench.

The Tsunami Warning centre in Hawaii raised a Pacific-wide tsunami alert, but cancelled the warning shortly after the temblor generated only small waves.

The temblor killed one person in Cagayan de Oro City, knocked out power in several towns, and spurred panic about a tsunami that ended up generating only tiny waves.

Only minor damage

Executive Director Benito Ramos of the National Disaster Risk Reduction and Management Council (NDRRMC) identified the dead as Emelita Ubalde of Barangay Lapasan, Cagayan de Oro City, whose house was buried in a landslide when the quake, felt at intensity 3 in the city, struck at 8:47pm on Friday.

Thousands of villagers who fled their coastal homes after Friday night’s quake returned home on Saturday, but hundreds more still jittery from the temblor remained in evacuation centres, disaster officials said.

Ramos said the quake generated no large tsunamis and caused only minor damage, including cracks on buildings and several bridges.

Certainly strong

The temblor was certainly strong enough, Solidum told the Inquirer. He said “a magnitude 7 quake has energy equivalent to 32 Hiroshima atomic bombs, while magnitude 8 would be equivalent to 1,024 Hiroshima atomic bombs.”

Magnitude is a measure of the energy released at the source of an earthquake. It is different from intensity, which gauges the strength of tremors in specific places, and is determined according to its effects on people, structures and the environment, according to the USGS.

“Typically, a large-magnitude earthquake should generate a large-scale tsunami,” Solidum said. “That was also the prediction of the US Pacific Tsunami Warning centre. So when [it did not happen], we did a review and analysed the earthquake, we found that there was also a horizontal movement, so the tsunami it generated was not that much,” he said.

Varying intensities

On Friday, the Phivolcs reported earthquake intensities ranging from 5 to 7 mostly in coastal areas facing the Pacific Ocean. Intensity 7 was registered in the towns of Guiuan, Oras, Sulat, Gen. MacArthur and Llorente, and Borongan City, in Eastern Samar; and Tacloban City in Leyte.

According to the Phivolcs, people would feel an intensity 7 quake strongly, with considerable damage to poorly built structures, cracks on roads and dikes, heavy objects and furniture falling, and “most people are frightened and run outdoors.”

This could have been the scenario if Friday night’s earthquake had originated on land, Solidum said. Instead, “what happened was it hit off shore, 112 km east of Guiuan, Eastern Samar, so the shaking was not felt very much,” he said.

On the other hand, an undersea quake also entailed its own set of considerable dangers, especially that of a tsunami, Solidum said.

Immediate evacuation

Expecting a giant tsunami, Phivolcs urged immediate evacuation of residents in coastal towns in the Visayas for three hours after the earthquake struck, but lifted the warning at past midnight, about an hour after the US Pacific Tsunami Warning centre cancelled its own alert.

The quake generated tsunami waves of less than half a meter off Siargao Island, and 19-centimetre waves off Surigao, heights that Solidum said he considered “nonthreatening”, or, at most, only posed some danger to the beach.

From their seismic readings of the quake, which was tectonic in origin, Solidum said Phivolcs scientists could explain why.

“The motion of the quake was not fully vertical. There were some horizontal elements to the motion,” he said. “This means there was not much rising of the seabed, so the vertical displacement of the water was not significant,” Solidum said.

Solidum said the closest example of an earthquake approximating the characteristics of Friday’s temblor he could think of was the Moro Gulf earthquake of 1976. But that was much more destructive.

The 7.9-magnitude quake, of tectonic origin, struck in August 1976, with the epicentre in the Celebes Sea near the islands of Mindanao and Sulu. The quake generated a powerful tsunami that killed more than 5,000 people.

Nearly unscathed

UP’s Lagmay said the Philippines escaped, nearly unscathed, from seven types of earthquake-wrought hazards: tsunami, ground shaking, liquefaction (of soil), ground rupture, ground subsidence (sinking), landslides and fires.

“But as you can see, even though we had intensity 6 to 7, which is already strong, there wasn’t much shaking of the ground because the epicentre was too far away,” Lagmay, also executive director of the government’s Project Noah (Nationwide Operational Assessment of Hazards), told the Inquirer.

Lagmay said the heavily hit areas—in Samar, Leyte and Surigao—were not densely populated and did not have clusters of tall buildings and other structures, unlike urban centres. “If this happened in the Manila Trench, there would have been a much bigger effect,” he said.

Echoing Solidum’s explanation, he said a giant tsunami did not materialise because of the “sideways” movement of the quake. Thus, there was not enough displacement of water that could send walls of water crashing on the shores, as in the earthquake and tsunami disasters in Japan in March 2011.

But Lagmay said the decision of the Phivolcs to issue a tsunami warning and advise the immediate evacuation of coastal residents in affected areas was justified, as there was no surefire way of predicting the impact of an earthquake.

“While the event is occurring, it is just right to issue a tsunami alert, because at that point, you still don’t know what’s going to be the [effects] of the earthquake,” he said.

“Just because we were lucky this time does not mean we should be complacent,” Lagmay said. The Philippines, one of the countries sitting on the Pacific Ring of Fire, remains a place where big earthquakes can strike at any time, he said.

Other big quakes

The last big quakes to hit the Philippines were the 7.1-magnitude earthquake in Mindoro in November 1994 and the 7.9-magnitude earthquake in Baguio in July 1990, the Phivolcs’ Solidum said.

In the meantime, Solidum said residents of the coastal villages affected by Friday night’s quake should prepare for aftershocks, which could be felt “for weeks, or even months after the earthquake.”

He said the strongest aftershocks, so far, were two that immediately followed the main seismic event, one 6.4 in magnitude and the other 6.8 in magnitude, at 9:14pm and 9:27pm on Friday, respectively.

As of Saturday morning, more than 150 aftershocks have been felt in the quake-affected places, most of them mild, Solidum said. “In general, most seismic events will be followed by smaller events,” he said, meaning weaker aftershocks.

With reports from Philip C. Tubeza and AP

 

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