Two offshore earthquakes in the Philippines: What do coastal communities need to know?

Magnitude 6.8 and 7.4 earthquakes struck Mindanao Island in the last couple of weeks. The strong ground shaking caused significant structural damage, liquefaction, landslides, sea-level disturbance and tsunamis. Experts examine what happened and advise how to stay safe.
 

By Jeffrey S. Perez (jeffrey.perez@phivolcs.dost.gov.ph), Department of Science and Technology, Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS) and Ma. Mylene Martinez-Villegas, Department of Science and Technology, Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS)
 

Citation: Perez, J. S. and Martinez-Villegas, M. M., 2023, Two offshore earthquakes in the Philippines: What do coastal communities need to know?, Temblor, http://doi.org/10.32858/temblor.329
 

This article is also available in Filipino.
 

Within 15 days, two significant offshore earthquakes were felt on the Philippine island of Mindanao. The first event occurred on Nov. 17, 2023, at 4:14 p.m., Philippine Standard Time (PST), with a magnitude of 6.8. The epicenter — its source — was located offshore, near Sarangani Island, in the province of Davao Occidental, at a focal depth of 63 kilometers (39 miles).

The second major earthquake, a much stronger magnitude-7.4 event, jolted Mindanao Island and was felt as far as the Visayas region on Dec 2, 2023, at 10:37 p.m. local time. The hypocenter was located offshore of the municipality of Hinatuan, Surigao del Sur, and ruptured at a depth of 25 kilometers (15.5 miles). This event triggered tsunami alerts in the Philippines and southern Japan. Both of these earthquakes reached a maximum intensity of VII (destructive) on the PHIVOLCS Earthquake Intensity Scale (PEIS). The strong ground shaking caused significant structural damage, liquefaction, landslides, sea-level disturbance and tsunamis. Social media posts and media reports provided a comprehensive overview of what happened during and after the earthquake, including people taking ill-advised risks to photograph the tsunamis. We examine what happened and what you should do when the ground starts shaking.
 

 

What happened after the initial strong shake?

Shortly after the Nov. 17 event, the Department of Science and Technology – Philippine Institute of Volcanology and Seismology (DOST-PHIVOLCS) released a tsunami advisory, stating that while no destructive tsunami was expected, coastal communities were cautioned about potential sea-level disturbance. For the Dec. 2 event, DOST-PHIVOLCS issued a tsunami warning and informed coastal communities in the provinces of Surigao del Sur and Davao Oriental to immediately evacuate to higher ground or move farther inland.

The tsunamis that occurred were not destructive. However, in defiance of the advisory, social media posts showed individuals approaching the shoreline after shaking calmed, filming as the sea water receded and then returned to the shoreline. These actions are ill-advised after a strong earthquake, particularly near shorelines.
 

The science of tsunamis

The word “tsunami” is a Japanese word that translates to “harbor waves.” They are commonly generated by offshore earthquakes with shallow depths (less than about 30 kilometers or 19 miles) and magnitudes greater than 6.5. They can also be generated by submarine landslides, underwater or submarine volcanic eruptions and meteor impacts.

Tsunamis can be more destructive than strong, wind-generated storm surges and gravitationally induced tidal waves. For example, in 2011, a destructive tsunami generated by the magnitude-9 Great Tohoku earthquake hit northern Japan. It reached a maximum tsunami height of 39 meters (128 feet), traveled inland about 10 kilometers (about 6 miles), and killed around 20,000 people.
 

Revisiting historical tsunamis in the Philippines

Surrounded by several trenches and offshore active faults, all of the coastal areas in the Philippines are susceptible to tsunamis. More than 40 tsunamigenic earthquakes have occurred here in the past 450 years (Bautista et al., 2012). Among these, the deadliest was the 1976 Aug. 17 magnitude 8.1 Moro Gulf earthquake and tsunami, resulting in more than 6,000 fatalities. In southeastern Mindanao, the 1992 Manay, Davao Oriental earthquake and tsunami involved two earthquakes separated by 26 minutes, each with magnitude greater than 7. They caused strong ground shaking and destructive tsunamis in the province of Davao Oriental and in the coastal municipalities of Manay, Cateel, Baganga, Boston, Caraga and Tarragona (Besana et al., 2004).

In March 2023, DOST-PHIVOLCS, together with the University of the Philippines Visayas and Don Mariano Marcos Memorial State University launched the Project DANAS: Earthquake, Tsunami, and Volcano Disaster Narratives for an Experiential Knowledge-based Science Communication. The initiative aims to utilize the lived experiences of survivors through in-depth interviews in local languages. The project received funding from the DOST-Philippine Council for Industry, Energy, and Emerging Technology Research and Development.
 

Recent fieldwork under Project DANAS revealed firsthand accounts of the 1976 Moro Gulf tsunami survivors. Based on their stories, the tsunami may have reached a height of 15 meters — almost 50 feet, or equivalent to a 5-story building — with two to three devastating waves that traveled 700 meters (0.4 miles) inland. Additionally, a mass grave for tsunami victims was documented in Datu Odin Sinsuat, Maguindanao. Remarkably, the source of the Nov. 17, 2023, event, the Cotabato Trench, is the same as the 1976 Moro Gulf earthquake and tsunami.

 

Narrative accounts of tsunami survivors of the 1992 Manay earthquake described a tsunami that reached as high as 6 meters (20 feet, or a 2-story building) and completely washed out houses in Sitio Bonga, Manay. The source of this earthquake is the Philippine Trench, which generated the Dec. 2, 2023, event in Surigao del Sur.
 

Natural signs of an approaching tsunami: “Shake, Drop and Roar”

Tsunamis in the Philippines can be classified as far-field and near-field. Far-field tsunamis, originating from the Pacific Ocean, may reach the eastern seaboard within one hour to 24 hours, allowing enough time to evacuate coastal communities. DOST-PHIVOLCS, together with the Office of Civil Defense, issues appropriate tsunami alert information and warnings in such instances.

However, near-field tsunamis, generated from earthquake sources such as trenches and offshore active faults around the Philippine archipelago, pose a more immediate threat. Since the source is very close, these tsunamis may arrive within a few minutes to less than an hour after the shaking. Hence, DOST-PHIVOLCS encourages all, especially coastal communities, to recognize the natural signs of an approaching tsunami. The first sign is SHAKE — strong shaking from an earthquake. Next is DROP — a sudden drop (or rise) in sea level. Last comes ROAR — the rumbling sound of an approaching tsunami.

For safety, individuals along or near shorelines observing any of these natural signs should immediately move to higher ground or away from the shore; ideally, individuals will move to safety immediately after feeling strong shaking. It is important to prioritize safety and avoid taking photos and videos after the shaking and during the water recession.
 

Community tsunami preparedness

Awareness is key for tsunami preparedness. Tsunami hazard maps and other information materials are available and can be freely downloaded at the DOST-PHIVOLCS official website, and at hazardhunter.georisk.gov.ph.

Additionally, efforts like Project DANAS can utilize place-based experiences narrated in local languages to remind at-risk communities of potential hazards and encourage them to take action.

For instance, coastal communities are encouraged to conduct tsunami drills and install signs indicating areas prone to inundation, evacuation routes and areas, and safe zones. Additionally, protecting and planting mangroves may lessen the impact of tsunamis. Drawing lessons learned from past events and disseminating correct information may help in preparing communities to be safer and more resilient against such hazards.
 

 

Mr. Jeffrey S. Perez is a Supervising Science Research Specialist and a geologist from DOST-PHIVOLCS. He has done earthquake research focusing on Mindanao. Dr. Ma. Mylene Martinez-Villegas is DOST-PHIVOLCS Deputy Director, a geologist, and has conducted studies on perception and community preparedness for earthquakes, tsunamis and volcanic eruptions.

Science editor: Dr. Alka Tripathy-Lang, Ph.D.
Reviewer: Dr. Wendy Bohon, Ph.D.
 

References

Bautista, M.L.P., Bautista, B.C., Salcedo, J.C. and Narag, I.C., 2012. Philippine tsunamis and seiches, 1589-2012. Department of Science and Technology, Philippine Institute of Volcanology and Seismology.

Besana, G.M., Tanioka, Y., Ando, M., Mirabueno, M.H., Manahan, J., De Ocampo, J. Perez, J. and Bautista, B., 2004. The May 17, 1992 earthquakes in southeastern Philippines. Geophysical Research Letters, 31 (24). https://doi.org/10.1029/2004gl020917
 

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