Discrimination of a seismic source doublet in the Northridge, California earthquake of 17 January 1994

The January 17, 1994 Northridge earthquake (Mw = 6.7, 34.213° N, 118.537° W, depth = 18.4 km) was recorded extensively in the immediate source region by strong, ground motion accelerometers. The resulting seismograms show complex S wave patterns. Nevertheless, visual correlations of the strong-ground-motion velocity and displacement time-histories clearly identify two significant wave pulses: a secondary S pulse (called S2) arriving 3–5 seconds after the initial S wave pulse (called S1). A plausible assumption is that these phases are generated at areas on the rupturing thrust fault that experienced especially large slip. Conventional travel-time computations, relating the relative arrival times between the onsets of the primary S1 and secondary S2 phases, yield a hypocenter of the initiation point, constrained to a independently etimated fault plane, of the secondary wave source (called H2) at 34.26°N, 118.54° W, with a depth of 14.1 km; the 68% confidence error in depth is 1.3 km. This location is about 6 km up-dip and north from the estimated hypocenter, on the fault plane of the initial principal seismic source (called H1). The seismic moment for both the initial H1 and secondary source H2 was estimated from the SH displacement pulse. Values averaged over eight stations were 8.61 ± 9.56 × 1024 dyne-cm and 2.49 ± 2.31 × 1025 dyne-cm respectively. Reasons why the sum of the two seismic moments is smaller than the total estimated seismic moment of 1.2 × 1026 dyne-cm for the Northridge earthquake are discussed. The location of the initiation point of a second source H2 in the Northridge thrust faulting is consistent with independent computations of the fault slip pattern. The estimated stress drop for the initial and secondary sources are Δ σ1 = 150 ± 15 bars and Δ σ2 = 110 ± 11 bars, respectively.