Minimal damage despite quake’s power of ‘32 atomic bombs’

Friday night’s earthquake off the eastern Philippine coast packed energy “equivalent to 32 Hiroshima atomic bombs” experts said.

However, a combination of luck and other factors spared Filipinos from catastrophe.

The country’s top seismologist, Renato Solidum, said the 7.6 magnitude quake centrered in the Philippine Trench off Eastern Samar was powerful enough but the tremor would have been more intensely felt had its epicenter been on land.

Also, if there was more vertical displacement of ocean water in the Philippine Trench, the undersea quake could have spawned a destructive tsunami, as in what happened in the 1976 Moro Gulf earthquake that killed thousands, the Philippine Institute of Volcanology and Seismology (Phivolcs) director said.

“We were lucky,” said University of the Philippines geologist Alfredo Mahar Lagmay, executive director of the government’s Project Noah (National Operational Assessment of Hazards).

He said the Aug. 31 earthquake did not meet all the conditions of a larger scale disaster: power, proximity and the type of structures in the affected places.

The quake was certainly strong enough, Solidum said, pointing out that “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 VS INTENSITY

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 based on its effects on people, human structures and the environment, according to the US Geological Survey.

On Friday, 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 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 to the ground, and “most people are frightened and run outdoors.”

This could have been the scenario if Friday’s earthquake had originated on land, Solidum told the Inquirer. Instead, “what happened was it hit offshore, 112 kilometers east of Guiuan, Eastern Samar, so the shaking was not felt very much,” he said. Still, people in such places as Cebu City and Iloilo City spilled out onto the streets.

On the other hand, an undersea quake also entails its own set of dangers, especially of tsunamis, Solidum said.

Over fears of giant waves rushing to shore, Phivolcs advised the immediate evacuation of residents in affected areas in the Visayas for three hours after the earthquake struck at 8:37 p.m.

LIFTED WARNING

It lifted the warning at past midnight Saturday, about an hour after the US Pacific Tsunami Warning Center cancelled its own alert.

The quake generated tsunami waves of only less than half a meter off Siargao Island, and 19 cm waves off Surigao, heights which Solidum said he considered “non-threatening,” or, at the 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 he could think of, of an earthquake approximating the qualities of Friday’s temblor, was the Moro Gulf earthquake of 1976. But that was much more destructive.

The magnitude 7.9 quake, of tectonic origin, struck in August 1976 and originated at sea, with the epicenter in the Celebes Sea near the islands of Mindanao and Sulu. The quake spawned a powerful tsunami and killed more than 5,000 people.

UP’s Lagmay said the country escaped, nearly unscathed, from seven types of earthquake  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 6-7, which is already strong, there wasn’t much shaking on the ground because the epicenter was too far away,” he told the Inquirer.

NO TALL BUILDINGS

Lagmay noted that the heavily hit areas, in Samar, Leyte and Surigao, were not highly populated and did not have clusters of tall buildings and other structures, unlike urban centers.

“If this happened in the Manila Trench, there might have been a much bigger effect,” he said.

Echoing Solidum’s explanation, he said the threat of a tsunami did not materialize because of the apparent “sideways” movement of the quake. Thus, there was not enough displacement of water that could send walls of water crashing into the shore, as in the 2011 Japan earthquake and tsunami events.

But Lagmay said the decision of Phivolcs to issue a tsunami warning and advise the immediate evacuation of coastal residents in affected areas was justified, as there was no sure 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 product 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 in the Pacific Ring of Fire, remains a place where big earthquakes can strike at any time, he said.

BIG ONES

Among the last “big ones” to hit the country were the 7.1 magnitude earthquake in Mindoro in November 1994 and the 7.9 magnitude earthquake in Baguio in July 1990, Phivolcs’ Solidum said.

In the meantime, Solidum said residents of coastal villages affected by the quake should brace for aftershocks, which could be felt “for weeks, or even months.”

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 9:14 p.m. and 9:27 p.m. 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./INQUIRER