The alert showed a magnitude-6.7 quake again rocking Abra, with an epicenter near the town of Lacub, or only about 15 kilometers northeast of that in the July 27 tremor. It was a case of a geological event of that nature coming too soon. Based on current scientific understanding, earthquakes do not strike the same place at very short intervals, except for aftershocks of the main one. 

My research associate and co-author Sandra Donna Catugas and I decided to include the latest ground shaker in our presentation. For the next two nights, she burned the midnight oil figuring out numerous models to understand any connection between the two events.

Why quakes occur

Sandra’s iterations showed that the magnitude-6.7 quake (later downgraded to 6.4), fell on the same stress decrease shadow zone in her Coulomb Stress Transfer or CST model. When tectonic plates forming the outer layer of the earth are moving constantly, these cause stresses to each other. Over time, these stresses accumulate along the boundaries of the plates, manifested as deep cracks called “faults.” 

When stresses exceed the strength of a fault, the latter moves (or ruptures) and releases a certain amount of energy in the form of seismic waves that cause the earth to shake, thus the term “earthquake.” 

CST theory states that the section of the fault that ruptures is weakened as the accumulated stresses are drastically released; it is said to experience a stress decrease. At this point, this section is not expected to release any more energy soon; thus, no earthquakes are likewise expected soon. It would take some geologic time to allow stresses to accumulate on the fault again for it to be ripe for the next tremor.

By current indications, the Oct. 25 quake seems to defy the CST theory. For one, its epicenter is too close to that of the July 27 event. For another, its hypocentral location falls within the stress decrease zone that resulted from the earlier quake. 

Further, earthquake theory suggests that although the magnitude-6.4 quake released energy that was almost 15 times weaker, it was too strong to qualify as an aftershock of the magnitude-7.0 event. The first level of aftershock is theoretically one order of magnitude weaker than that of the main shock, which means a maximum of magnitude-6.0 for the July 27 event.

Alternatively, the previous quake may not have been able to release all the energy accumulated, necessitating rupturing in October on the same fault to release the residual energy. This scenario seems likely as the plotted epicenters and hypocenters of both quakes lie on the same fault plane. 

More number crunching, however, is needed to further find the best-fitting models. 

Magnitude and intensity

The amount of energy released by a fault during an earthquake defines its strength or “magnitude”—an absolute value that does not change, regardless of location. Magnitude is often a function of the size of the fault that ruptures: Larger fault rupture means larger magnitude. 

On the other hand, intensity is the amount of shaking experienced at the surface of the earth, or “ground shaking,” often expressed as acceleration defining how fast the ground moves to a certain velocity from a zero-motion state. Think of a world-class sprinter running from standstill to about 35 km per hour at the end of 100 meters within 10 seconds. This translates to an acceleration a bit slower than that due to gravity of 980 cm/s², or “g,” the unit normally used to quantify ground shaking. A 1.0 g means that the ground accelerates as fast as a free-falling object (a 0.5 g means half as fast).

Intensity is generally a function of several factors, including distance from the epicenter and integrity of the subsurface. Unlike magnitude, intensity values can vary from place to place. Given the same magnitude, a site located farther away from the epicenter will experience weaker intensity. 

The difference between magnitude and intensity is sometimes explained to school children through an analogy with a light bulb. The brilliance of a light bulb with a certain wattage (magnitude) appears dimmer (intensity) from a distance.

A site underlain by poor subsurface material, such as in sandy beaches or in river floodplains, is expected to experience stronger ground shaking than that located the same distance away but underlain by solid rock. For instance, an earthquake generated by the West Valley Fault would produce more intense ground shaking in the Marikina River valley than in a location of the same distance in Diliman, which is underlain by adobe (consolidated volcanic ash).

Some residents of Ilocos Norte, particularly in the towns of Marcos, Banna and Batac, and the city of Laoag, claim that the latest magnitude-6.4 quake generated stronger ground shaking (higher intensity) than the July magnitude-7.0 event. This may be valid because, for one, the epicenter of the October quake is closer to Ilocos Norte. For another, these towns are located within the vast floodplains of the mighty Laoag River system, which may also explain the more intense damage to infrastructure in these towns despite the weaker magnitude.

Ingredients of disaster

In July, it was difficult to identify the culprit fault of the magnitude-7.0 quake because no surface rupture was produced (or none was observed, at least). Most indications then did not point to the Abra River Fault but instead favored another structure, the Vigan-Aggao Fault. 

Both faults belong to the Philippine Fault system, an active fault more than 1,000 km long that has been the source of devastating quakes in the recent past. The strongest of those quakes so far was the magnitude-7.8 earthquake that ruptured the Digdig, Nueva Ecija, segment on July 16, 1990, causing numerous deaths and extensive infrastructure damage in northern and central Luzon, to as far north as Baguio and Dagupan, and as far south as Cabanatuan and Dingalan. 

This earthquake was at least 50 times stronger than the October temblor!

The latest quake, although unexpected, provides an important insight in tracing the culprit fault that generated it and the earlier event.

According to existing maps, the upper Laoag River Basin is surrounded by active faults which form the branches of the northern segment of the Philippine Fault system. It is in this river basin that the Ilocos Norte towns of Nueva Era, Banna, Marcos, Dingras, Solsona and Piddig are located. 

In August 1983, a magnitude-6.5 earthquake with epicenter in Solsona jolted this same region and caused heavy infrastructure damage in a large area, including Laoag and the town of Sarrat where the casualties included heritage sites. 

The poor integrity of the underlying subsurface material in the floodplains coupled with the presence of nearby active faults are perfect ingredients of disaster during an earthquake. Infrastructure must thus be built strictly according to current structural codes, especially focusing on implementing requirements for seismic loading.

Mario A. Aurelio, PhD, is a professor at the University of the Philippines, teaching at the National Institute of Geological Sciences in UP Diliman, and is head faculty of the Structural Geology and Tectonics Laboratory of the institute. 

His doctoral dissertation dealt with the tectonics and kinematics of the Philippine Fault. 

He recently received the 2022 Gregorio Zara award for basic research granted by the Philippine Association for the Advancement of Science and Technology, in cooperation with the Department of Science and Technology. —Ed.

The post Digging for clues to Abra’s earthquakes appeared first on CoverStory.

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. 

Survivors’ accounts 

Related: Are Mayon, Taal and Kanlaon volcanos connected?

“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.” 

Mineralized

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. 

Heritage sites

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.

The post Abra quake was less destructive but a complex seismic event appeared first on CoverStory.

Lagangilang is about a 20-minute drive from the earthquake’s epicenter in the municipality of Tayum. “We are fortunate that the houses in our neighborhood had only cracks in the walls and shattered glass and broken plates inside,” Manganteng said. “Our worry, what’s making us and our children lose sleep, is the scary aftershocks.”

At 8:43 a.m. on July 27, the mountainous province of Abra in northern Luzon was hit by a magnitude 7.0 earthquake, as measured by the Philippine Institute of Volcanology and Seismology (Phivolcs). Neighboring provinces  were also affected but in Abra alone, 1,729 families are in evacuation areas and 51 government buildings have been damaged. 

The province’s capital, Bangued, bore the brunt of the temblor, with a number of buildings and houses toppled or severely damaged.

Bangued resident Christine Valera, mother of two children aged 13 and 6, was in the kitchen with them when the earthquake occurred. “We immediately crawled under the table. The shaking was intense so we decided to stay there until the shaking subsided. And then we got out of the house,” she recalled.

Valera added that while her house was spared of damage, her family remained cautious, knowing that continuous aftershocks may weaken the structure. “We are camping out just to be sure,” she said. 

Related: Abra quake was less destructive but a complex seismic event

‘Like being uprooted’

Phivolcs reported that as of 8 a.m. on July 28, a total of 815 aftershocks had been recorded, of which 24 were strong enough to be felt. 

Said another Bangued resident, Neyla Manzano: “The feeling of the initial quake was like that of a plant being uprooted. The side-to-side shaking, coupled with the up-and-down movement, was just too nerve-wracking. My 7-year-old son and I had a hard time keeping our balance. Even if the aftershocks were less intense, they’re still worrisome.”

An earthquake’s destructive force depends not only on its strength (magnitude) and distance from the epicenter but also on its depth. Seismic waves from a deep earthquake (or deeper than 60 kilometers) have to travel farther to the surface, losing energy along the way.

On the other hand, as explained by Phivolcs director Renato Solidum in an interview, a shallow earthquake like the one that struck Abra—located 15 km deep—tends to cause more damage as more of the earthquake’s energy makes it to the surface, causing far greater destruction than if it occurred deeper underground. 

Solidum said earthquake damage to buildings and structures may be influenced by the type of soil they are sitting on: the softer the soil type, the greater the shaking or amplification of waves produced by an earthquake. As a result, building damage tends to be greater in areas of soft sediments.

Also, Abra is located in one of the regions in Luzon made seismically active by the presence of faults that include the northern segments of the Philippine Fault, Abra River Fault, West Ilocos Fault System, and Naglibacan Fault. 

Wednesday’s earthquake was caused by a movement along the Abra River Fault, Solidum said.

Clean water needed

The main concern of the affected families at the moment is the supply of clean water, said Valera. 

“The one supplying our area had to temporarily stop operations as the water turned muddy. Good thing we still have containers with purified water, although there are also deep wells here with water we can use for cleaning and bathing,” she said.

Manzano and her neighbors are counting themselves lucky because grocery stores have remained open, and electricity and phone signals were interrupted for only a few hours after the initial quake. 

“Even the main roads were immediately cleared of debris, although some in the inner parts of the province remain partially or completely closed because of the many landslides,” she said.

Globe Telecom Inc. (Globe), Smart Communications Inc., and its parent company PLDT (PLDT-Smart) have deployed response teams to address the damage caused by the earthquake to their broadband infrastructure and to assist residents through free services. 

There are also free calls, free charging, and a free Wi-Fi station at Bangued Plaza in Abra, adjacent to the municipality of Lagangilang.

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