Opinion: what the 6.9 quake that shook the Philippines taught and the blind spots it exposed

They arrived at Tacloban airport on October 3 and started working with Mines and Geosciences Bureau (MGB) personnel on the same day, inspecting landslide areas that were a threat to communities downstream. The next day, a group of UP civil engineers arrived to assess the structural integrity of damaged infrastructure. Inspections included the Hibulangan Dam, which was recently built near Bugabuga village, known as the last stronghold of Japanese soldiers in the Philippines. I reached Mactan, Cebu, on the morning of October 5 with emergency and crisis management expert Martin Aguda, right after celebrating my birthday the night before.

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Our first stop was the Emergency Operations Centre (EOC), where we met with director Joel Erastain of the Office of Civil Defence (OCD) of Region VII. We briefed him on our work plan and shared the initial findings from our fieldwork in Leyte, where, unlike in Northern Cebu, the earthquake’s impacts had received relatively little attention. During the meeting, we also addressed questions about the potential aftershocks, where they might occur, their likely magnitudes and how many to expect.

Drawing from the global database of large earthquakes, we explained that aftershocks are a normal occurrence, as the fault responsible for the mainshock continues to adjust. Interestingly, the magnitude of the largest aftershock can be estimated by subtracting about 1.1 to 1.2 from that of the main event. In the case of the M6.9 Cebu earthquake, this corresponds to an aftershock of approximately M5.8, which indeed occurred 13 days after the so-called “night of terror,” and just four days after an official announcement had stated that there was a “low chance of another major quake in Northern Cebu anytime soon.”

In terms of earthquake classification, the statement that there is a low chance of another “major earthquake” is, in a sense, accurate. Scientists classify a magnitude 5.8 aftershock as a moderate earthquake, one level below the strong earthquake category assigned to the magnitude 6.9 mainshock of September 30, 2025.

However, from a safety standpoint, it is not only the magnitude but also the timing and proximity of an aftershock that determine its potential danger. A moderate earthquake occurring close to an already battered community can still cause significant damage, as recently destabilised structures are more likely to collapse. Conversely, if a similar earthquake occurred farther away, its impact would be far less severe.

To understand this better, it is important to distinguish between magnitude and intensity. Magnitude refers to the total energy released by an earthquake at its source, while intensity describes how strongly the shaking is felt at a specific location. The closer a community is to the earthquake’s source, the greater the intensity it experiences.

To illustrate, think about a light bulb: the total light it emits represents the magnitude, while the brightness perceived by an observer depends on distance, which is akin to intensity. The farther one is from the light source, the dimmer it appears, just as the shaking weakens with distance from the epicentre.

After registering with the OCD, we began our journey to the danger zone. We were told that it would take around 12 hours due to heavy traffic along the Cebu coastal highway. We planned to meet the UP geology team, who had taken the ferry from Leyte to Danao, at 5.30am.

During the drive, Martin and I discussed the stress transfer model, a geological concept that explains how stress may have shifted to the area of Northern Cebu when the Philippine Fault in Leyte moved in 2017 and triggered a magnitude 6.5 earthquake. According to this stress transfer model, the 2017 earthquake redistributed stress to surrounding areas, which may have pushed nearby faults closer to their breaking point. This fault ruptured eight years after and is now known as the Bogo Bay Fault. It devastated communities in Northern Cebu but had remained unmapped before the deadly quake.

Although the epicentre of the magnitude 6.9 Cebu earthquake was located beneath the Camotes Sea, Philippine Institute of Volcanology and Seismology (Phivolcs) documented clear signs of surface rupture in Barangay Nailon, Bogo City, on October 3, extending over a kilometre southwestward. When projected in the opposite direction, the fault’s orientation aligns with the offshore source of the 2025 Cebu mainshock. Based on these reports and the plot of the ongoing aftershocks, we estimated the fault’s total length to be between 20 and 60 kilometres, consistent with the dimensions required to generate a magnitude 6.9 earthquake.

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Identifying active faults requires a combination of remote sensing and field investigation. Remote sensing involves analysing aerial photographs, satellite imagery such as those available in Google Earth, and high-resolution 3D representations of the landscape from LiDAR. Geologists examine these images for linear features that resemble ground cracks and look for evidence of movement along them, such as displaced hills, offset rivers or abrupt linear changes in elevation known as scarps.

When a fault has shifted recently, signs of movement might be apparent in images, similar to how a fracture appears in an X-ray image. In many cases, high-resolution 3D LiDAR imagery can show the presence of faults the way a 3-dimensional CT or MRI scan can show a dislocated bone fracture.

Once potential faults are identified, geologists verify their observations through field investigations, searching for physical evidence of past movement such as displaced soil layers and offset rock formations. When field data support the presence of an active fault, they conduct more detailed studies, often by excavating trenches across the fault or using ground-penetrating radar. These methods expose the fault plane, allowing scientists to determine when it last moved and how many times it has shifted in the past.

The unmapped fault responsible for the fatal 2025 Cebu earthquake is not the first of its kind in the Philippines. Over the past 12 years, several strong earthquakes have been generated by previously undiscovered faults. One instance is the 2013 magnitude 7.2 Bohol earthquake, which resulted from movement along the previously unmapped North Bohol Fault. Another example is the 2019 magnitude 6.1 Zambales earthquake, the source fault of which is still not included in the nation’s active fault map because it is hidden. Similarly, the series of four North Cotabato earthquakes in 2019 was linked to unmapped faults. Notably, one of these active faults had already been identified in the 2014 master’s thesis of Tatum Mikko Herrero at Université Blaise Pascal in Clermont-Ferrand, suggesting that it could have been mapped in greater detail beforehand.

Under Republic Act No. 10121, or the Philippine Disaster Risk Reduction and Management Act of 2010, all local government units (LGUs) must prepare a Local Disaster Risk Reduction and Management (LDRRM) plan. This plan, undertaken with the participation of a technical working group composed of local disaster management offices and relevant mapping agencies, assesses risks from natural hazards. In Bogo City, comprehensive hazard mapping, including fault identification through remote sensing and field investigations, could have revealed the previously unmapped fault earlier, especially with the use of LiDAR data. Similar initiatives in other LGUs could likewise improve community preparedness by identifying active faults and recognising the potential risks they pose.

Of course, these analyses come with the benefit of hindsight and are easier to make after the fact. The Philippines covers roughly 300,000 square kilometres, leaving vast areas to study and many potential hazards that can easily be overlooked. Beyond earthquakes, the country also faces threats from volcanic eruptions, floods, storm surges and landslides, all of which must be mapped accurately using the best available science and advanced technologies.

LiDAR, comparable to a CT scan machine in the medical field, is one such powerful tool that can help detect elusive active faults and be used to produce detailed maps of flood and landslide-prone areas. Unfortunately, when the government defunded the Nationwide Operational Assessment of Hazards (NOAH) programme in 2017, following recommendations from the same critics we hear from today, the country also lost its local capacity for LiDAR mapping, as the Phil-LiDAR1 and Phil-LiDAR2 projects were two of the 19 projects funded under NOAH.

More than technology, it is our human resources that matter most. The country needs to train and sustain an army of research scientists dedicated to disaster risk reduction and climate change research. The more skilled professionals we have on the ground, the greater our chances of minimising the impacts of natural hazards. We must also mobilise a pool of interdisciplinary experts from State Universities and Colleges (SUCs) and Higher Education Institutions (HEIs), recognising that mapping hazards is only the first step. Equally important is developing data-driven interventions to reduce risks and mitigate potential hazard impacts across all sectors of society.

When we arrived at ground zero, I realised that there is still much more we can do to strengthen our disaster risk reduction efforts against earthquake hazards. Our own fieldwork validated the interpretations we made from the 2014 LiDAR imagery, which we analysed for potential fault traces on October 2, before deploying for our crisis response mission. This confirms that the fault could have been identified before the 2025 Cebu earthquake and integrated into the LDRRM plan of Bogo City. Since LDRRM plans are mandatory for all cities and municipalities, adopting advanced technologies can greatly assist technical working groups in developing more accurate and effective disaster preparedness strategies.

Throughout our journey from Cebu City to Northern Cebu, we also observed that only a small percentage of the thousands of houses and buildings we passed had totally collapsed or toppled. Most structures, though damaged, remained standing, giving their occupants a greater chance of survival. The buildings that completely failed were primarily located in previously identified hazard zones: along waterways, on steep slopes, or near coastal areas with liquefiable ground. Strict enforcement of the National Building Code and the Water Code is essential to prevent such occurrences.

Applying the insights from our trip to the rupture zone in Cebu to Metro Manila, we recommend prioritising assessments in preparation for the “Big One”, given the immensity of the metropolis with an ocean of rooftops stretching to the horizon.

The sheer density and scale of the National Capital Region necessitate that structural assessments concentrate initially on buildings located near waterways, on steep slopes, and on liquefiable ground, ensuring compliance with the Building Code and Water Code.

I have drawn criticism from some of my colleagues for pointing out deficiencies in hazard assessments related to active faults, as if doing so were an act of assigning blame for the consequences of earthquakes and other disasters. They contend that such criticism could tarnish the reputation of the scientific institution, a concern I recognise may hold some truth, but only temporarily.

By confronting and rectifying outdated practices in light of new evidence, we not only advance scientific understanding but also strengthen public trust. If we embrace this mindset, the long-term results will be overwhelmingly positive. While the truth may be uncomfortable or painful, the safety of the community is paramount and must always take precedence over protecting the reputation of any individual or institution.

Emphasising the lessons learned from disasters is not meant to assign blame to any person or organisation, but rather to enhance the workflow of hazard assessments and mitigation strategies. Only by learning from past disasters can we reduce the risks of those yet to come.

Our thoughts are with those affected as they work to rebuild their lives.

Filipino geologist Alfredo Mahar Lagmay is an Asia’s Most Influential 2024 honouree. 

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