Philippine E-Journals

Movement of the Upper Philippine Fault: Its Geological Repercussions in Northern Luzon

Alfonso De La Cruz Jr.

Discipline: Engineering

 

Abstract:

A total of 125 kms length from the Upper Philippine fault moved during the destructive earthquake that struck the country on July 16, 1990. This segment is part of the 1300 km-long Philippine Fault Zone (PFZ). The movement has been classified as left lateral strike-slip, with an accompanying vertical displacement. The movement, recorded from Gabaldon, Nueva Ecija to Kayapa, Nueva Viscaya, was part of the Digdig Fault splay from onee of the upper three (3) braches of the PFZ. The horizontal movement ranges from 1.0 - 6.2 m., while the vertical slips from 0.1 - 2.0 m. Sympathetic en-echelon ground fracturing and displacements were seen in Pangasinan and Nueva Ecija. Associated with the strong earthquake brought about by this rupturing, several geohazards of various kinds were also noted: liquefaction phenomena where ground-shaking of water saturated coastal areas of Dagupan, Aringay and Agoo and the fresh-water environs of Tarlac and Pangasinan. Consequently, loose materials along barren mountainous areas in the fault zone were mobilized, bausing shallow landslides triggered by monsoonal rains and typhoons. Log jams due to excessive influx of stream load in rivers draining the affected areas caused sudden floods in the flat lands nearby.