Devastating M6.8 Earthquake Strikes Sarangani Island

The Earthquake 

A M6.8 earthquake struck Davao Occidental on the afternoon of November 17. The epicenter of this earthquake was identified near Sarangani Island, sending tremors throughout the region. 

Glan, a municipality situated on Sarangani Island, bore the brunt of the quake, experiencing the highest intensity recorded at Intensity VII. This level of intensity indicates significant damage and poses a considerable threat to structures, explaining the subsequent challenges faced by the community. Tragically, the municipality of Glan also witnessed a devastating landslide, primarily impacting the mountainous terrain of Brgy. Mudan. This incident resulted in the loss of two lives—a mother and her daughter—who were discovered buried beneath their collapsed residence. Furthermore, at least 10 houses within the immediate vicinity succumbed to the destructive force of the landslide. 

As of November 23, according to the National Disaster Risk Reduction and Management Council (NDRRMC) Sitrep No. 5, the earthquake’s aftermath has claimed 9 lives and left 30 individuals injured. These numbers underscore the profound impact of the seismic activity on the local population, highlighting the urgent need for relief and recovery efforts. 

Landslide Analysis 

Based on the Planet satellite images (Fig. 1), a total of 74284 m2 area failed and moved downward during this landslide event. The landslide affected a large area but did not have a massive runoff. Rather, it behaved like a lateral spread with slumping. This type of landslide is usually triggered by rapid ground movement – in this case, the Sarangani Earthquake. It is caused by the liquefaction of loose, less cohesive, or more saturated material. This material is usually overlain by a more coherent layer which subsequently experiences fracturing, extension, and subsidence once the material below it liquifies. The structures formed by this fracturing and extension are known as horst and graben complexes (Fig. 2B). 

Figure 1. Before and after satellite images showing terrain change caused by the landslide. The purple circle is a rooftop that demonstrates the movement of the landslide.
Figure 2 (A) Physical model of a lateral spread (Image from Philip S. Prince) (B) Actual drone footage of the event, showing the horst and graben features of the spread (Screenshot from Youtube Video).

During this type of landslide, tilting and sinking of structures are quite common (Fig. 2A). Unfortunately, in the case of Glan landslide, this had caused deaths as well as the destruction of at least 10 houses within the vicinity (Fig. 2B). The residents of the area had been evacuated and advised not to return as this area is dangerous. Notably, the NOAH landslide hazard mapping team had previously identified the vicinity as having high susceptibility to landslides (Fig. 3B) and this can be found in the UP NOAH website. 

Figure 3 (A) Interpretation of the landslide area with the horst and graben complexes (blue line dash) that accommodated the downward extension of the event. (B) NOAH landslide hazard map showing that the area had been identified as a high-hazard area.

Strong earthquakes cause ground shaking and increase the possibilities of occurrences of landslides. In the case of Glan landslide, the intensity of ground shaking induced landslides in hilly and mountainous areas. To check and know about the hazards in your area, please visit https://noah.up.edu.ph. These hazard maps capacitate and empower Filipinos to prepare against the adverse impacts and effects of extreme weather conditions.