Review UCERF3 zTop implementation

[pmpowers-usgs]

Migrated from usgs/nshm-cous-2014#39

Current implementation considers the aseismicity factor as a downdip reduction in width. This reduction was imposed in the UCERF3 inversion to reduce the moment release possible with creeping fault sections, but is inconsistent with the NGAW2 definition of zTop (or zTor). Review should consider fact that almost all smaller M ruptures will now come to the surface (zTop = 0.0); there is no capacity, currently, with the way the UCERF3 fault system is represented, to model down-dip floating ruptures as are modeled elsewhere in the WUS, and previously in CA. Also note that zTop for a rupture is the area-weight average of the aseis-reduced zTops of all participating fault sections.

See also the white paper and email exchanges with @njgregor on this topic.