Last Oct. 25 at about 11 p.m., my earthquake alert app sounded while I was finalizing the visual material for my presentation at NIGSCON 2022 in two days. I would be discussing the magnitude-7 temblor that struck the province of Abra three months ago at the annual conference of the University of the Philippines’ National Institute for Geological Sciences (UP-NIGS), which serves as a venue for faculty members to present results of their research grants.

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.