Saturday, November 27, 2021

Special Report: Shake, rattle and roar

THE magnitude 7.2 earthquake that hit Bohol and Cebu on Oct. 15, 2013 threw a curve at residents, but data from the Philippine Institute of Volcanology and Seismology (Phivolcs) show that, in fact, sizeable earthquakes in the country aren’t unusual.

From 2011 to 2013 alone, the Philippines had 187 earthquakes at least 5.0 in magnitude, of which 18 were 6.0 to 6.9 in magnitude, and three at least 7.0 in magnitude.

Of the 21 quakes at least 6.0 in magnitude, five each occurred in Luzon and the Visayas, while 11 hit Mindanao.

Those with epicenter in the Visayas were the magnitude 6.2 earthquake west of Cauayan, Negros Occidental on July 12, 2011; the M6.9 and M6.2 temblors near Tayasan town, Negros Oriental on Feb. 6, 2012; the M7.6 temblor near Guiuan, Eastern Samar on Aug. 31, 2012; and the M7.2 temblor near Sagbayan, Bohol on Oct. 15, 2013.

The last four of these were felt strongly in Cebu. (See box of destructive earthquakes in the Philippines.)

Debris from building after Cebu-Bohol earthquake
QUAKES NOT RARE. A 28-year-old woman died in Talisay City, Cebu when the magnitude 7.2 earthquake that struck Bohol last Oct. 15, 2013 caused portions of this apartment building to collapse. The debris from the building fell on the van her group was in. From 2011 to 2013, five earthquakes at least 6.0 in magnitude have struck the Visayas region. (Sun.Star Cebu photo/Alex Badayos)


The 2012 Negros earthquake was felt at Intensity 6.0 in Cebu, where chaos ensued as residents tried to flee to higher ground amid the Phivolcs’s tsunami warning. No tsunami struck Cebu, however.

Magnitude refers to the energy released by an earthquake, while intensity refers to the strength of the shaking, which varies in different locations.

A tsunami, which is a series of sea waves, is usually generated by strong undersea earthquakes. It can be higher than five meters, said Phivolcs. If the earthquake source is near, the tsunami could hit the shore in two to five minutes.

Based on the country’s history, an earthquake would have to have a magnitude greater than 6.0 and a shallow focal point to create a tsunami, said Robinson Jorgio, seismic observer at the Phivolcs Seismic Monitoring Station in Lapu-Lapu City, Cebu.

The focus is the quake’s point of origin. The epicenter is the surface above the focus.


Phivolcs says coastal areas, particularly those facing the Pacific Ocean, South China Sea, Sulu Sea and Celebes Sea, are vulnerable to tsunamis. But this doesn’t mean that provinces not directly facing these bodies of water are safe.

“In Cebu, the risk is the same even if it is surrounded by islands. No island is exempted from (the risk of) a tsunami,” said Jorgio.

If tsunamis are caused by the movement of faults far away, land barriers might slow down tsunamis reaching Cebu, “but from locally generated tsunamis, no one is exempt,” he said.

During the Feb. 6, 2012 Negros earthquake, a five-meter tsunami occurred in Barangay Martilo in La Libertad town, Negros Oriental facing Cebu, he said.

During the M6.8 February 1990 Bohol earthquake, a tsunami also occurred in Bohol facing Cebu, he said.

Asked to confirm talk that a tsunami had already hit Cebu’s Olango Island, Jorgio said Dr. Leo Bautista, a seismologist-scientist at Phivolcs, had found historical accounts, “probably from news manuscripts, of unusual sea level change in Olango in 1922.”

According to Australia’s Bureau of Meteorology, tsunamis can travel more than 700 kilometers per hour or as fast as an airplane.

As it nears land, it will slow but its height will grow, so that a tsunami “unnoticeable at sea may grow to be several meters or more in height near the coast,” it said.

Phivolcs research specialists
NO ISLAND EXEMPT. Dr. Rhommel Grutas (left), supervising science research specialist at the Philippine Institute of Volcanology and Seismology (Phivolcs), and Robinson Jorgio, seismic observer at the Phivolcs Seismic Monitoring Station in Lapu-Lapu City, Cebu, say all islands are vulnerable to a tsunami, even Cebu which is surrounded by other islands. They say tsunamis have already hit Central Visayas, even if it is fl anked by other islands that more directly face the larger bodies of water like the Pacific Ocean. (Sun.Star Cebu photo/Allan Cuizon)

Significant hits

Dr. Rhommel Grutas, supervising science research specialist at Phivolcs, said 40 “significant” tsunamis had hit the Philippines in the past 400 years.

Tsunamis one to three meters high have hit Negros Oriental, particularly the southern part near Dumaguete City, as well as southeastern Bohol and the northeastern coast of Leyte. In the eastern coast of Samar, tsunamis have reached three to five meters high, he said.

The tsunamis were highest in Mindanao, reaching five to eight meters high, Grutas said.

He revealed that after the M9.0 earthquake struck Japan in March 2011, the resulting tsunami reached 20-30 meters high in some parts of Japan’s coastline.

“The velocity and height of a tsunami are a function of the depth of the water. If the water is deep, it’s possible the tsunami will be high,” said Jorgio.

Not all undersea earthquakes bring tsunamis. It depends on the topography of the sea bed.

“A tsunami is created if there’s a rupture in the sea bed,” Grutas said.

Finding faults

The Philippines is earthquake-prone because it is located in the Ring of Fire, the 40,000-kilometer horseshoe-shaped region in the Pacific Ocean stretching from New Zealand up to Indonesia, China, Japan, Alaska, down to the coasts of North America and South America.

The ring is home to 75 percent of the world’s volcanoes.

It is also where many tectonic plates meet and where nearly 80 percent of the world’s largest earthquakes occur. The collision of the plates creates earthquakes.

Phivolcs said earthquakes occur in the Visayas because parts of the 1,250-kilometer-long Philippine fault zone (PFZ)—which cuts across the Philippines from northwestern Luzon to southeastern Mindanao—traverse Masbate in the Bicol Region, Leyte in Eastern Visayas, and Eastern Mindanao.

A fault is a break in the earth’s crust where movement or displacement has occurred or may occur again. Earthquakes occur along faults.

The PFZ has caused more than half of the large (at least magnitude 7) crustal earthquakes in the country, said Hiroyuki Tsutsumi and Jeffrey Perez in their 2013 paper on the PFZ.

The Visayas also has its own faults—the West Panay Fault, the Central Negros Fault, the East Bohol Fault, the newly discovered North Bohol Fault, the Cebu Lineaments, the Northern Samar Lineament and the Eastern Samar Lineament.

The Visayas is also sandwiched between the Negros Trench and Sulu Trench on the left and the Philippine Trench on the right.

A trench is a long, steep valley on the ocean floor formed when one tectonic plate slides beneath another in a subduction zone. Movements in the trenches produce earthquakes. The Negros Trench generated a magnitude 5.8 earthquake in November 2012.

Cebu faults

Based on satellite images, four lineaments exist in Cebu, two each in central and southern Cebu.

“A lineament is an approximate trace of a fault,” Jorgio said.

“This year, we will do high-resolution mapping of the faults. The team will go to the area and find where the faults are,” said Grutas.

One of the faults is in a mountain barangay, Jorgio said.

This year, Grutas said, the multi-hazard mapping team from the Phivolcs, National Mapping and Resource Information Authority, Mines and Geosciences Bureau, and Philippine Atmospheric, Geophysical & Astronomical Services Administration will finish five projects: fault mapping, ground shaking hazard mapping, liquefaction hazard mapping, tsunami mapping and earthquake-induced landslide mapping.

The fault mapping will include the sea.

“Ships will go around Cebu to check if there are active faults,” Grutas said.

Active faults are those that have moved in the last 10,000 years. Carbon dating will
determine this.

They’re active

Jorgio said Cebu has active fault systems because there have been earthquakes coming from Cebu.

Last Dec. 14, he said, a magnitude 3.0 earthquake struck the mountain barangay of Taptap, Cebu City.

The Phivolcs website listed 11 other earthquakes from Cebu from 2012-2013.

Last year, magnitude 1.8 to 2.9 earthquakes occurred northwest of Liloan town in January, southeast of Alcantara in October, and southwest of Alcoy in December.

In 2012, M2.4 to M3.4 earthquakes occurred southwest of Bantayan in January, southwest of Badian in May, east of Bogo City in June and July (four quakes), and southeast of Poro in October (two quakes).

Sun.Star Cebu reported that M2.1 to M3.6 quakes also struck near Barangay Talamban in 2003, in Bantayan Island in 2007 and southwest of Cebu City in 2008.


The Philippines gets 20 earthquakes a day, “of which only four or five are felt per week,” said Grutas.

Jorgio said Phivolcs has tsunami sensors in Negros and Samar but none in Cebu because they prioritized the installation of the sensors in places that historically had greater seismicity.

But local government units (LGU) may request a tsunami sensor, and Phivolcs will train their personnel on its use and maintenance.

“LGUs can hear the alarm systems ring. There is a wet and dry sensor. If the sensor gets dry, that means the water receded, so the alarm will ring,” Jorgio said.

For distant tsunamis, like those resulting from earthquakes generated in Japan, Hawaii or Chile, Filipinos will have one to 24 hours to evacuate, said Grutas.

“But for local tsunamis, local residents will have only two to five minutes—or one hour in some places—to evacuate because the trench or fault lines in the Philippines are usually less than 200 kilometers from the shoreline,” he said.


Since there is no way to predict when an earthquake will occur, the community must be educated on the three natural signs of a tsunami, he said.

These are Shake, which refers to strong ground shaking; Drop, which refers to unusual sea level fall or rapid sea retreat; and Roar, which refers to the tsunami’s unusual strong sound.

When this happens, residents should go inland to predetermined elevated ground about 10 meters or higher or to an upper floor of a strong, tall building.

“Go to the third or fourth floor. Wait there until the threat is over,” Grutas said.

He urged communities to make hazard assessments and evacuation maps, install tsunami
signs, and conduct coastal village education campaigns and tsunami drills.

“If you have a budget, you can put a breakwater,” he said.

Jorgio said topography makes some places prone to water encroachment, so in Japan, they just built tsunami towers that residents could run to in case of storm surges or tsunamis.

For liquefaction, the areas under threat are those with shallow water tables. In liquefaction, wet sediments start to behave like liquids.

Grutas said liquefaction-prone areas are those with loose, sandy or silty deposits; riverbanks, abandoned rivers, flood plains; coastlines; swamps and reclaimed land.

Asked to comment on the stability of structures built on reclaimed land, Grutas said if the new materials used for the reclamation are soft and the earthquake shakes that part, then the structures on it will be shaken.

He advised developers to use “hard materials for reclamation.”

For those building structures on reclaimed land, he said, “The foundation should be fixed on the rock. Structures should use pile driving to reach the old ground. Or use rigid rock or adobe rock for the foundation for the building structure.”

He advised private owners to consult with geotechnical engineers for site investigation.


As for sinkholes, though some appear after earthquakes, they are not caused by earthquakes.

“It’s a chemical process,” Grutas said.

“Rain plus limestone will create carbonic acid that will dissolve the limestone,” Jorgio explained.

Cebu is 70-80 percent limestone, they said.

Jorgio said that in Boljoon town after last October’s earthquake, a sinkhole swallowed its Department of Social Welfare and Development building because the town had used anapog or limestone as filling material.

“Gikaon ilang DSWD,” he said. (Tomorrow: Ruthless wave)
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