ALONG-FAULT STRESS REGIME EVOLUTION AND OFF-TRACE NORMAL FAULTING DURING THE Ms=7.8 1990 LUZON EARTHQUAKE: INSIGHTS TO STRESS CONDITIONS ALONG AND AROUND RUPTURING FAULTS
M.A. Aurelio and J.A. Punongbayan
The 110-km rupture along the Digdig Fault during the July 1990 Luzon Earthquake (Ms = 7.8) evolved from a bilateral pure left-lateral shear from its epicenter into a unilaterally propagating transpressional stress regime northwards, and finally into an off-trace localization of a transtensional aftershock region northwestwards of the northernmost trace of the rupture. This aftershock region that includes the heavily damaged Baguio City area and vicinity, is located about a hundred kilometers NW of the epicenter near Cabanatuan City.
Focal mechanism solutions indicate that fault movement in this area was by normal faulting, distinctly different from the expected left-lateral strike-slip mechanism along the main rupture trace. Such normal faults may be associated with transtensional structures observed to control depressions at high altitudes in the vicinity of Baguio City and La Trinidad in Benguet province. Understanding the evolution of the rupture process and the off-fault distribution of the 1990 aftershocks and their fault mechanisms offers insights to the conditions of stress in and around rupturing faults during earthquakes.
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