A segment of the Philippine Fault system is the main suspect for the recent magnitude-7.0 earthquake. But surface ground rupture has not yet been found. Is that the whole story?
By Mario Aurelio, Sandra Donna Catugas, Structural Geology and Tectonics Laboratory at the University of Philippines National Institute of Geological Sciences, and Alfredo Mahar Francisco Lagmay, Executive Director, University of the Philippines Resilience Institute-Nationwide Operational Assessment of Hazards Center (@nababaha)
Citation: Aurelio, M., Catugas, S.D., and Lagmay, A.M.F., Fault that caused a July quake in the Philippines still in question, Temblor, http://doi.org/10.32858/temblor.268
On July 27, 2022, at 8:43 a.m. local time, (12:43 a.m. GMT), a 15-kilometer-deep magnitude-7.0 earthquake struck the province of Abra on the northwestern sector of the main island of Luzon in the Philippines. The temblor inflicted widespread infrastructure damage in at least four provinces over an area of 10,000 square kilometers (about 3,800 square miles). As of Aug. 11, 2022, 11 deaths and an estimated 2 billion Philippine pesos (approximately USD 40,000,000) in infrastructure and agricultural damage have been reported. The tremor was felt as far away as the capital city of Manila, more than 300 kilometers away from the quake’s epicenter, prompting several establishments to implement earthquake evacuation procedures. In downtown Manila, underlain by river and estuarine deposits of the Marikina -Pasig River system, people reported ground shaking as high as Intensity V on the Philippine Institute of Volcanology and Seismology (PHIVOLCS) Earthquake Intensity Scale. Places closer to the epicenter experienced shaking intensities as high as Intensity VII — considered destructive.
Setting of seismic activity
As of this writing, PHIVOLCS shows the epicenter of the mainshock at 17.64° N latitude, 120.63° E longitude, or around 5 kilometers northeast of the town of Bangued, the capital of the province of Abra (Figure 1). Since the mainshock, more than 3,000 aftershocks have been recorded, with felt aftershocks averaging about six per day (based on onsite experience by M. Aurelio while conducting field surveys). The strongest recorded aftershock so far is magnitude-5.1.
The mainshock’s focal mechanism solution, also called a beach ball diagram that shows scientists the possible motion and orientation of faults during an earthquake, consists of two reverse faults. One of them strikes 233°, or northeast-southwest, with a relatively steep dip of 69°, and the other strikes 8° — approximately north-south — with a shallower dip of 28° (PHIVOLCS Earthquake Data, 2022). A fault oriented in either of these directions must have caused the earthquake. PHIVOLCS has identified the Abra River Fault as the responsible structure (PHIVOLCS Poster, 2022). Given the relatively strong magnitude and shallow depth, the responsible fault should have reached the surface, rupturing the ground. However, two weeks after the earthquake, a surface ground rupture has yet to be located.
Ground deformation and infrastructure damage
Manifestations of ground deformation due to the earthquake are mostly in the form of landslides and liquefaction. Landslides are most prominent along roadcuts in mountainous areas, such as in the towns of Licuan-Baay and Malibcong (Figure 2a). PHIVOLCS has reported a major landslide in Dolores, Abra, and extensive liquefaction in the coastal towns of Ilocos Sur and in the floodplains of the Abra River system (PHIVOLCS Poster, 2022).
Infrastructure damage includes collapsed buildings, fallen perimeter walls (Figure 2b), cracked floors, cracked and fallen walls, and damaged beams of houses. In the urbanized center of Bangued, notable damage includes three- to four-story buildings that have collapsed (Figure 2c). Several of these collapsed multistory commercial and residential buildings are located at or close to rice fields near the floodplains of the Abra River system. Buildings atop rice fields are often constructed over a back-filled foundation, which means that these foundations are more susceptible to liquefaction if sandy, or subsidence of the backfill material is finer grained (e.g., silt or clay). Houses of more well-to-do families incurred losses related to broken ceramic furniture, china and chandeliers.
Centuries-old churches, such as those in Tayum, Abra, and in Bantay and in Caoayan, Ilocos Sur, sustained significant damage. For example, rubble that fell from high facades skirts some of these structures (Figure 2d). In more remote areas, such as in mountain villages of Tayum, Lagangilang, Sallapadan and Licuan-Baay, poorly built houses — those with insufficient structural components, often lacking posts, beams, rebar and shear walls — were also significantly damaged (Figure 2e). About two weeks after the earthquake, some residents are still camping out (Figure 2f) for fear of collapse of their already weakened houses as aftershocks continued.
While located farther away from the epicenter, liquefaction likely contributed significantly to the damage of infrastructure in the coastal towns of Ilocos Sur because these areas were built on river delta deposits of the Abra River. On the other hand, in the mountainous areas closer to the epicenter, intense ground shaking most likely caused the infrastructure damage.
Compared to recent inland Philippine earthquakes with similar magnitudes and mechanisms (i.e., reverse faults caused by compression) such as the magnitude-6.7 Eastern Negros earthquake of Feb. 6, 2012 (Aurelio, 2012; Aurelio et al., 2017) and the magnitude-7.2 Bohol earthquake of Oct. 15, 2013 (Aurelio, 2013; Rimando et al., 2019), the degree of ground deformation and infrastructure damage caused by the Abra earthquake is notably less. The 2012 Eastern Negros and 2013 Bohol earthquakes inflicted extensive infrastructure damage in the form of fallen bridges, collapsed buildings, and the total destruction of century-old UN World Heritage churches. The Bohol earthquake in particular produced a ground rupture that displaced the surface by as much as 2.5 meters, vertically. Hundreds of people perished in each of these earthquakes.
Which fault is responsible?
PHIVOLCS reported that the Abra earthquake was generated by the Abra River Fault. But that might not be the case. This fault is a branch of the 1,200-kilometer-long active left-lateral (sinistral) Philippine Fault System (Aurelio, 2000; Aurelio and Peña, 2010). The left-lateral Abra River Fault trends northerly and includes at least three branches, with a vertical fault in the middle with the two other steeply dipping structures that coalesce toward the central fault. Scientists call such a system a “positive sinistral flower structure” (Pinet, 1990; Pinet and Stephan, 1990; Ringenbach, 1992; Ringenbach et al., 1990).
The distribution of the aftershocks appears to indicate a northerly trend, consistent with the known trend of the Abra River Fault (map in Figure 1), as well as the north-striking plane indicated in the mainshock’s focal mechanism (the northeast-striking, steeply dipping plane is not preferred).
However, when the mainshock is examined together with the distribution of the aftershocks in cross section, a gently inclined fault plane dipping around 30° to the east becomes apparent, with its shallower section slightly steeper than the deeper parts (yellow dashed line in the cross section in Figure 1). The earthquake-generating fault involves oblique faulting, with both left-lateral and reverse components. These fault characteristics are not consistent with the steep dip of the branches of the positive flower structure of the Abra River Fault, which are expected to be moving with a predominantly left-lateral motion. Further, because of the gentle dip of the preferred rupturing fault plane, its expression at the surface would not coincide with the Abra River Fault, but instead with a structure located farther to the west, around the vicinity of another branch of the Philippine Fault System — the Vigan-Aggao Fault (Figure 1).
Searching for surface rupture
Using Coulomb Stress Transfer modeling, in which we use the mainshock parameters as the “source fault” and define a “receiver fault” with the location, strike and an adjusted dip angle of the Vigan-Aggao Fault, we suspect that the culprit fault — if it ruptured the surface — might have produced ground rupture in the vicinity of the southern segment of the Vigan-Aggao Fault (Figure 3). The model suggests that the rupture process originated at the focal depth of 15 kilometers (lower blue, destressed plane in Figure 3) and progressed west and upward to shallower levels on a plane oriented in the direction of Vigan-Aggao Fault (upper red-yellow, stressed plane in Figure 3). At least two shallow aftershocks greater than magnitude-4.5 are located in the vicinity of the Vigan-Aggao Fault (Figure 1). To date, several weeks after the mainshock, surface ground rupture has not been observed. Either the search teams are not looking in the right place, or the earthquake did not break the surface at all.
Regardless, identifying which fault caused the earthquake is of utmost importance. If this is another case of a hidden fault that has not been previously recognized, similar to those that generated the 2012 Eastern Negros, 2013 Bohol and 2019 Castillejos earthquakes (Aurelio, 2012; Aurelio et al., 2017; Aurelio, 2013; Rimando et al., 2019; Yang et al., 2021) that caused significant casualties and wrought extensive damage, then the new fault must be identified and added to current active fault maps. Satellite interferometry can provide guidance to narrow the search area, but available satellite data remain inconclusive at this point.
Acknowledgment
Expenses incurred by M. Aurelio as the team leader of a University of the Philippines Quick Response Team (UP QRT) were shouldered by the University of the Philippines Resilience Institute (UPRI).
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