In October 2003 we slightly revised the national maps in limited portions of California, Oregon and Washington. The changes only affect a limited area along the hanging wall of thrust or reverse faults. The changes involve at most an increase of about 2-3% in the probabilistic ground motions in the maps. These changes are not large enough to be visible in the maps posted in the website, so we did not post new maps.
It came to our attention that we used an incomplete definition of the hanging wall for the Abrahamson and Silva (1997) attenuation relation for thrust or reverse faults, one of the four attenuation relations used in the hazard maps for non-extensional areas. In that paper they state, “the hanging wall term should be used for sites on the hanging wall and within the edge of rupture.” (p.106). We interpreted this to mean that the hanging wall term was to be used only for sites within the surface projection of the rupture zone, i.e., the fault plane. We also had a miscommunication with the lead author on this point. In fact, the authors intended that the hanging wall term should also be invoked for sites behind the surface projection of the fault plane, although a distance taper makes this a limited area. This definition was presented in Abrahamson and Somerville (1996), although the angle used for the expanding zone behind the surface projection was not specified in that paper. This paper was not referenced in Abrahamson and Silva (1997), contributing to this misinterpretation.
For the October, 2003 revision we corrected the definition of the hanging wall in our hazard calculations to make it completely consistent with the definition of the authors of these attenuation relations. This leads to a small increase in probabilistic ground motions for thrust and reverse faults in limited areas behind the surface projection of the fault planes. We used an angle of 22.5 degrees in map view, as specified by N. Abrahamson (pers. comm., 2003) to describe the widening of the hanging wall zone in map view behind the projection of the fault plane. We did not apply the hanging wall term to sites behind the surface projection of thrust faults with dips less than 30 degrees, because the geometrical justification for the term being used behind the surface projection of the fault plane is less valid for very shallowly dipping faults. We still used the hanging wall term for sites over the surface projection of the fault plane for the shallowly dipping thrust faults and all the other thrust and reverse faults, as was done in the original 2002 maps.
- Abrahamson, N.A. and P.G. Somerville (1996). Effects of the hanging wall and footwall on ground motions recorded during the Northridge, California, earthquake, Bull. Seism. Soc. Am., v. 86, no. 1B, pp. S93-99.
- Abrahamson, N.A. and W.J. Silva (1997). Empirical response spectral attenuation relations for shallow crustal earthquakes, Seis Res. Letts., v. 68, no. 1, pp. 94-127.