Prepared by: Deo Carlo Llamas
On 17 November 2023, an earthquake of magnitude (MW) 6.8 struck off the coast of Davao Occidental, southern Philippines (Figure a). This earthquake, which originated at a depth of 63 kilometers, exhibited a reverse fault focal mechanism. The cross-section (Figure b) shows that the slip associated with this seismic event occurred along the slab subducting beneath the region, specifically along the Cotabato Trench.
The region in the southern Philippines exhibits complex crustal deformation, involving the interplay of the eastward subduction along the Cotabato Trench, westward subduction along the Philippine Trench, and the presence of the fully-subducted Molucca Sea Plate. This offers perspectives on the regional tectonic setting of the region which exerts influence on the occurrence of frequent large magnitude (M>6) earthquakes, at various depths (from shallow to deep), in the region.
Historically, the Cotabato Trench has been a focal point for strong to great (M6 to M8) earthquakes, including the 1976 magnitude (MW) 8.1 Moro Gulf earthquake. This event was particularly devastating, leading to a destructive tsunami that claimed over 5000 lives.
In the context of earthquake preparedness in the region, it is crucial to consider the following:
- Ground Shaking Hazard Awareness:
Seismic events in the region occur both onshore and offshore, spanning various depths from shallow to deep. Even when occurring at significant depths, an earthquake has the potential to cause extensive surface impacts and inflict considerable destruction. Given their consistent occurrence, as evidenced by historical and instrumental records, it is imperative to highlight the regular hazards associated with ground shaking. Strict adherence to the National Building Code is essential for enhancing structural resilience. Conducting earthquake drills and practicing the “Duck/Drop, Cover, Hold” reinforces preparedness.
- Tsunami Preparedness for Coastal Areas:
Coastal areas are vulnerable to tsunamis generated by earthquakes that cause a significant vertical movement on the seafloor, such as the 1976 Moro Gulf earthquake. Furthermore, the steep underwater topography suggests the possibility of submarine landslides, which in turn could lead to the generation of tsunamis. After a significant earthquake, prompt evacuation from coastal areas is crucial. Timely and decisive action can significantly impact your safety and that of your community. For your safety, it's important to refrain from taking videos of sea-level disturbances along the coast after the earthquake.
References:
Philippine Institute of Volcanology and Seismology (2023). PHIVOLCS Earthquake catalog. Available at: https://www.phivolcs.dost.gov.ph/index.php/earthquake/earthquake-information3 (Accessed November 18, 2023).
Philippine Institute of Volcanology and Seismology ((2023). PHIVOLCS SWIFT earthquake source parameters. Available at: https://swift1.phivolcs.dost.gov.ph/~pvjica/swift_top.html (Accessed November 18, 2023).
International Seismological Centre (2022), ISC-GEM Earthquake Catalogue, https://doi.org/10.31905/d808b825
GEBCO Compilation Group (2022) GEBCO_2022 Grid (doi:10.5285/e0f0bb80-ab44-2739-e053-6c86abc0289c)
Hayes, G., 2018, Slab2 - A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV
Global Volcanism Program, 2013. Volcanoes of the World, v. 4.11.2 (02 Sep 2022). Venzke, E (ed.). Smithsonian Institution. Downloaded Oct.20, 2022. https://doi.org/10.5479/si.GVP.VOTW4-2013
