SOURCE MECHANISM OF THE LUZON, PHILIPPINES EARTHQUAKE OF JULY 16, 1990

The Luzon, Philippines earthquake of July 16, 1990, with Ms = 7.8 was generated by left-lateral faulting in the central Luzon Island. Ground breakage was observed along a linear trace extending 110 km. The maximum horizontal offset as measured on the fault was as much as 6 m. A fault area of 120 x 20 km2 was inferred from a field study and aftershock data. This event is one of the largest strike-slip earthquakes ever recorded in the world. For long-period surface waveform analysis, we used very broadband records from 14 stations of the GDSN, GEOSCOPE, IRIS and ERIOS networks. The Centroid Moment Tensor (CMT) solution from surface waves represents almost pure left-lateral strike-slip faulting on a steep fault with strike = 155-degrees, dip = 88-degrees and rake = -7-degrees. The seismic moment is 3.9 x 10(20) Nm (x 10(27) dyn.cm), which gives Mw = 7.7. The average dislocation on the fault is 5.4 m and the stress drop is 5.2 MPa (52 bars). Teleseismic P and SH waves from 14 stations are used to model the spatio-temporal distribution of subevents. Five subevents are detected in an interval of 44 sec. Their sequence indicates a bilateral rupture. The mean velocity of rupture toward the north is 2.7 km/sec. The total seismic moment of the subevents is 3.6 x 10(20) Nm. The largest aftershock (Ms = 6.6) occurred on July 17 near the recognized northern end of the main fault. The CMT solution for the aftershock represents a reverse faulting with moment of 6.4 x 10(19) Nm (Mw = 6.5). This aftershock was probably caused by compression produced by block motion of the eastern block of the main fault.