The National Structural Code of the Philippines is set to be updated to align with international standards through the adoption of the Minimum Design Loads and Associated Criteria for Buildings and Other Structures of the American Society of Civil Engineers 7-05. This development introduces new key parameters in the ground motion section, specifically the transition long-period (T L ), which is essential for the long-period structure seismic design. To accurately represent ground motion at periods greater than 4 s, which are critical for the design of long-period structures, such as high-rise buildings, and to determine the T L values specific to the Philippines, data derived from the Seismic Hazard Assessment for the Design Earthquake of the Philippines Project, implemented by the Philippine Institute of Volcanology and Seismology, were utilized. Using this dataset, modal magnitude (M d ) maps were developed through the disaggregation of the 2% probability of exceedance in 50 years for spectral acceleration (S a ) at T =2 seconds, which were then correlated with the corresponding corner period (T c ) values. The seismic source models were subdivided into fault, subduction, and area sources. Results indicate that, in areas located near fault sources, magnitudes are predominantly influenced by the crustal earthquake generators. However, subduction sources tend to dominate the earthquake magnitudes in regions farther from the crustal fault systems.
Grutas et al. (Sat,) studied this question.