A magnitude-7 earthquake has long been associated with a nightmarish aftermath: massive devastation and hundreds of deaths. But more than two weeks after one such temblor jolted Abra province and its vicinity last July 27, disaster response officials reported only 11 fatalities so far and a little over P2 billion in damage to infrastructure and agriculture.
It was fortunately much less catastrophic than the magnitude-6.9 earthquake in Negros on Feb. 6, 2012, and the magnitude-7.2 earthquake in Bohol on Oct. 15, 2013.
“Magnitude” is a measure of the amount of energy released by an earthquake and does not change with distance from it. “Intensity” refers to the degree of shaking at a given place, which is mainly a function of the underlying materials and generally decreases with distance from the epicenter.
The latest ground shaking had an immediate impact area covering around 10,000 square kilometers in at least four provinces—Abra, Ilocos Sur, La Union and Benguet-with the epicenter in Tayum, Abra. It was felt as far as Metro Manila, more than 300 km away, with some establishments implementing evacuation measures.
According to the Philippine Institute of Volcanology and Seismology, the earthquake was triggered by the movement or rupture of the Abra River Fault. But data collected and analyzed over the last two weeks suggested that the complex seismic event could not be explained simply by the fault as it is currently known.
The first 72 hours after an earthquake is crucial in disaster management. A day after the tremor, I rushed to Abra and Ilocos Sur to conduct an initial assessment of ground deformation and infrastructure damage.
“It was a little past 8:30 in the morning and I was tending to my store here when the ground swayed hard,” said “Paula” in her native Ilocano. Beside her was a small table with fresh vegetables harvested from nearby farms in Palao, a village in Abra’s capital of Bangued.
“I started to hear rumbling sounds from underneath and the concrete pavement and walls in the house started breaking and cracking,” she said. “Then the furniture in our living room started falling.”
In the village of Bumagcat in Tayum, located only a few km from the epicenter and on a river floodplain, “Gorio” was fixing his house’s fallen roof and kitchen wall when I arrived.
“This wall fell,” said Gorio’s daughter, who had just started teaching at an elementary school in a nearby village. She was pointing to a small concrete partition between a wood-fired stove and a makeshift dishwashing area cum lavatory.
A hand-pumped well in the neighborhood reportedly dried up after the quake and was now drawing sand, not water. This indicated liquefaction, a process in which saturated sandy subsurface materials start behaving like liquid when shaken by a quake.
At the municipal disaster command center in the upland Licuan-Bay, situated east of Bangued and near the border with Kalinga, the mayor was supervising his staff and volunteers in preparing food packs for the affected families.
“We are quite lucky, that although several houses have been damaged by the earthquake, we have had no casualties [yet],” said the young mayor. “We are still awaiting engineers to inspect our government buildings and the landslides reported in our small-scale mining areas.”
The rocks underneath Licuan-Baay, like in many other areas in the Cordillera mountains, are mineralized. These hold significant deposits of gold, copper and semiprecious metals. Where there is mineralization, the ground is often loose and prone to landslides.
In the village of Sta. Rosa in Bangued, several three- and four-story residential buildings sustained damage, including the collapse of the ground floor designed for parking and often constructed with columns without walls in between for wider space.
“Gerry” said his house was built some 20 years ago with only two floors on a sprawling lawn about 500 square meters wide. “But recently,” he said, “my siblings and I decided to add a third floor and a rooftop to accommodate more occupants, so our families would be together.”
Like many other houses in the village, Gerry’s house stands on backfill materials over what were once rice fields near the floodplains of the mighty Abra River. Unconsolidated ground can settle during earthquakes, causing the foundation and footings of buildings to lose strength, which can eventually lead to the collapse of columns and upper floors.
In the historic town of Bantay in Ilocos Sur, a 10-minute ride from the capital Vigan, the parish priest, Father Louel, expressed sadness over the fate of his church and its bell tower. The church suffered a deep structural fracture in the midsection of both main walls.
“Only one horn remains of the original eight,” recounted “Melvin,” the sacristan, as he led me to the bell tower of Bantay. Built in 1591 on a hill overlooking the sea and the low-lying towns of Vigan and Caoayan, the bell tower is believed to be one of four watchtowers built by the colonizing Spaniards to guard against pirates.
“Bantay” means “guard” in Ilocano and Filipino.
The church in Tayum, among the worst-hit towns, only had a fallen nave roofing and the wall stripped at the midsection of its bell tower. In the far-flung villages, most houses suffered fallen walls and beams, mainly due to poor construction practices, such as absence of posts, rebars and shear walls.
From the accounts of people who witnessed the brunt of the quake’s devastation, infrastructure damage was mainly due to intense ground shaking in areas near the epicenter and to liquefaction and ground settlement in coastal areas situated on river floodplains and deltas.
Culprit fault ‘at large’
The most convincing evidence of a rupturing fault is what earthquake scientists call a surface ground rupture.
Earthquakes occur when faults, or large cracks that penetrate deep into the crust, slip. A fault slips when it can no longer bear the stress coming from the surrounding blocks of rock.
The sudden slip causes the fault to release enormous amounts of energy in the form of seismic waves. When these waves reach the surface, they cause the ground to shake and wobble, causing destruction to infrastructure.
When the magnitude of an earthquake is strong enough and originates from a shallow source, or “focus,” the fault that generates it breaks the surface, producing the surface ground rupture.
More than two weeks after the main shock, the surface ground rupture has yet to be seen.
The small number of casualties and less serious damage despite the magnitude-7 quake, and the shallow depth of focus (around 15 km), remain to be explained.
But then, the population and built environment are sparser in Abra than in the quake-hit areas in Negros and Bohol.
Or could it also be an indication that efforts in institutionalizing earthquake preparedness in the Philippines are paying off?
Mario A. Aurelio, PhD, is a professor at the University of the Philippines Diliman and teaches at its National Institute of Geological Sciences. He is the main author of a Commission on Higher Education-sponsored guide book for senior high school teachers on disaster risk reduction and management. He led the advance party of a UP Quick Response Team to Abra and Ilocos Sur a day after the earthquake. —Ed.