Manila (Philippine Daily Inquirer/ANN) - 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.