A Systematic Overview of Hazard Interrelationships in the Philippines

Natural hazards interrelationships include one natural hazard process triggering or influencing another (e.g., an earthquake triggering a landslide) and often contribute to the severity of disasters. This study focuses on a methodology for systematically providing an overview of natural hazard interrelationships in the Philippines. We first explore the breadth of single hazards that might occur in the Philippines, subdividing them into 22 different natural hazard types (with groupings of geophysical, hydrological, shallow-earth, atmospheric, biophysical, space). A 22x22 natural hazard interaction matrix is subsequently developed to identify primary natural hazards that could potentially trigger or increase the probability of secondary natural hazards. Then, for each potential interrelationship (e.g., earthquake-flood) we critically review the literature to find evidence whether that interrelationship might occur, based on past case histories or theory. In total, we use 250 sources, consisting of local scientific and grey literature, and civil defence bulletins. The detailed information is synthesized into a database of 12 types of information (e.g., process, primary and secondary hazards, date and period). Interactions without existing records in the Philippines, but plausible based on global literature, are also incorporated. A total of 76 interrelationships out of a possible 484 were identified. High-impact interrelationships between natural hazards commonly observed in the Philippines include earthquake-triggered landslides, rainfall-induced landslides, and subsidence. Less common hazard interrelationship examples are the 2022 Abuyog landslide-tsunami and the 2008 Panay landslide-flood. The majority (34 out of 76) of the primary hazards involved geophysical hazards such as earthquakes, volcanic eruptions, and landslides, triggering or increasing the likelihood of other geophysical hazards, flooding, and shall earth hazards. Tropical storms, having a very high frequency in the Philippines, also trigger or increase the likelihood of several secondary hazards. The number of unique interactions (76) identified in the matrix continues to grow as more literature, both in the Philippines and globally, are collected. This matrix would serve to inform scientists, policymakers, and first-responders on possible secondary hazards in anticipation of an impending natural hazard impact. The Philippines’ geologic and meteorological setting exposes it to a large variety of high frequency and magnitude natural hazards necessitating the identification of historical and hypothetical hazard interactions for the purpose of preparedness and mitigation.