|Preliminary Fault-Slip Distribution from Geologic Investigations|
As of October 23, geologists have measured the amount and orientation of surface fault slip at more than 100 localities along the 1999 rupture zone. The figure above shows how this slip is distributed along the fault zone. Points represent mean measurements at each site (at some places these are derived from summing displacement vectors across multiple strands), and the bars represent the uncertainty in these measurements. The blue shading represents the cumulative slip estimate, and deviates from the point data where there are significant multiple strands (e.g between 30-35 km along the fault). The high-frequency "spikes" in the slip distribution are probably not real, but result from incomplete estimates of displacement at sites where deformation occurs across zones several tens to hundreds of meters wide. This is particularly true just to the north and south of the Bullion Mountains. These areas are now being carefully surveyed in order to capture an accurate estimate of the true displacement.
Although present data are incomplete, we see that the distribution of horizontal slip along the fault is remarkably symmetrical, with rather abrupt terminations of slip at the ends of the rupture. Although rupture and surface cracking has been observed over a length of more than 40 km, significant slip of more than a few cm occurs over a length of less than 35 km. To date (10/23/99), the largest horizontal displacement measured was 525 ± 85 cm right-lateral about 4 km south of the epicenter in the Bullion Mountains. The average slip across the entire fault zone is currently estimated at 2.5-3 m. Vertical displacements of nearly 2 m have been observed along the fault, but overall the sense of displacement is not consistent. This pattern is quite typical for strike-slip rupture. Between the Bullion Mountains and Lavic Lake, there is consistent W-side up displacement where the fault curves northward, producing a small releasing bend. E-side up displacement is observed just to the south of the epicenter where the fault bends westward (compressive bend).