7.6 QUAKE EQUALS TO 32 HIROSHIMA ATOMIC BOMBS: IT'S A
BLESSING PH SPARED, SAYS SCIENTISTS
[PHOTO -Blast Radius Of The Hiroshima Nuclear Bomb: The atomic cloud
(mushroom cloud) produced just after the burst is one of the most intensive
characteristics of the HEROSHIMA bomb explosion.]
MANILA, SEPTEMBER 6, 2012 (INQUIRER) By DJ Yap - 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 on Saturday.
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 epicenter 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 kilometers east of Guiuan town,
Eastern Samar, at a depth of 34.9 km, the US Geological Survey (USGS) said. The
Phivolcs placed the epicenter a bit farther at 112 km east of Guiuan, in the
Philippine Trench.
The Tsunami Warning Center in Hawaii raised a Pacific-wide tsunami alert, but
canceled 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 8:47 p.m. 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 centers, 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 Center. So when [it did not happen], we did a review and analyzed 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 Center canceled its own alert.
The quake generated tsunami waves of less than half a meter off Siargao
Island, and 19-centimeter 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
epicenter 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 epicenter 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 centers. "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 materialize
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
9:14 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. With reports from Philip C. Tubeza and AP
Chief News Editor: Sol Jose Vanzi
© Copyright, 2012 by PHILIPPINE HEADLINE NEWS ONLINE
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