Recent crustal deformation in the Luzon Island (Philippines): Insights from strain rate patterns

The Luzon Island is situated in the complex plate boundary between the colliding Eurasian Plate (EUP) and Philippine Sea Plate (PSP). As a consequence of this convergence, intense deformation is currently undergoing in the Philippine Island Arc System. Oblique collision between the EUP and the PSP is partitioned between the oppositely dipping subduction systems as well as the strike-slip fault zone, flanking and traversing the Philippine Mobile Belt, respectively. To understand the present crustal motions and strain in the region, we utilized the earthquake moment tensor summation. For these calculations, the Luzon Island area was divided into 50km x 50km x 30km crustal blocks. Strain rates as well as principal contractional and extensional axes directions for each block were determined. The contractional axes for most of the blocks typically reflect the NW convergence of the PSP with the EUP. Contraction rates range from about 10^-6 yr-1 to 10^-11 yr-1. Contractional strain axes in the vicinity of the southern Central Cordillera area are almost N-S. This area coincides with the location of the Digdig Fault, the rupture area of the 1990 Luzon Earthquake. This reflects the pure strike-slip motion along this structure within the Philippine Fault Zone. Results also show that there is minimal or absent crustal motion in the fault traces south of the Digdig Fault as observed from the Global CMT catalogue of significant seismic events. The absence of relatively large earthquake events (Mw>4.5) in the Macolod Corridor also poses an interesting scenario. The absence of significant seismic energy release in this area may have implications on the possible aseismic deformation; or an accumulation of significant levels of strain that will eventually be released by future earthquakes in the southern Luzon Island.