Rupture process of the 1990 Luzon, Philippines (M_W = 7.7), earthquake

The rupture process of the destructive July 16, 1990, Luzon, Philippines, earthquake is analyzed using a long-period surface wave spectral inversion technique to estimate the average source properties and a broadband body wave and surface wave empirical Green function technique to investigate the slip distribution. The average source model has a seismic moment of 4.2 ± 0.1 × 10²⁰ N m (M_W = 7.7), a total duration of 30 to 35 s, a left lateral strike-slip focal mechanism with strike, Φ = 153°, dip, δ = 89°, and rake, λ = 16°, and a north-northwestward directivity. These results are consistent with previous work on the Luzon earthquake. However, the broadband analysis indicates that, for periods longer than about 20 s, the teleseismic source time function is characterized by a single, relatively smooth pulse of energy release with azimuthal variations in duration of 24 to 60 s. This contrasts with previous body wave results that inferred two discrete pulses of energy release. The simpler source process is inferred as a resulrof using empirical Green functions rather than standard theoretical Green functions for a layered crustal model, and by considering only the seismic energy with periods longer than 20 s. The Luzon earthquake began rupturing bilaterally, but evolved into a predominantly northwest directed rupture that extended for 75 to 100 km. The largest slip (10-15 m) occurred about 25 km northwest of the epicenter, beyond which the slip decreased gradually as the rupture propagated into a restraining bend. The peak slip at depth appears to exceed the surface rupture by a factor of 2 to 3. There is little evidence for significant energy release later than 50 s after the rupture initiated.