Nowcasting earthquakes in the Philippines archipelago

Abstract

The continuous occurrence of destructive earthquakes in the Philippine Archipelago, generated by both mapped and unmapped faults, highlights the shortcomings of traditional fault-based hazard assessment techniques. The earthquakes caused by unmapped faults, in particular, emphasize the necessity of adopting area-based hazard evaluation approaches. In view of this, the present study implements an area–based earthquake nowcasting approach to statistically compute the current level of seismic hazards in 26 densely populated cities across Philippines. We utilize the concept of natural time, the inter–event counts of small earthquakes occurring between successive large earthquakes, to calculate Earthquake Potential Score (EPS) for the defined city regions. To derive the natural time statistics, we incorporate a diverse range of reference probability distributions, including heavy–tailed, time–dependent, time–independent, and exponentiated group of distributions. Statistical inference for observed natural times reveals that (1) the Weibull distribution provides the best representation; (2) as on August 15, 2024, the EPS values (%), corresponding to M 6.5 earthquakes for 26 cities range from 09% to 71%, with Tacloban (71%), Cagayan de Oro (69%), Dasmarinas (64%), Bacoor (63%), Las Pinas (63%), Manila (62%), Paranaque (61%), Taguig (60%), Valenzuela (60%), Makati (60%), Quezon City (58%), Pasig (58%), Caloocan (56%), Antipolo (55%), Marawi (55%), Zamboanga (54%), San Jose Del Monte (53%), Legazpi (44%), Cebu (39%), San Carlos (31%), Bacolod (28%), General Santos (27%), and Davao (09%), and (3) the nowcast scores are consistent despite some variations in threshold magnitude and city regions. These EPS values provide a unique measure to determine the ongoing progression of the earthquake cycle of large sized events of the target regions, enabling a consistent city ranking based on their current level of seismic progression. The nowcasting approach and emanated results offer valuable insights for informed decision–making to enhance preparedness and risk management strategies across the Philippine Archipelago.