  SENECA is a hybrid air shower simulation written by H. Drescher that utilizes
both Monte Carlo simulation and cascade equations. By using the cascade
equations only in the high energy portion of the shower, where the shower is
inherently one-dimensional, SENECA is able to utilize the advantages in speed
from the cascade equations yet still produce complete, three dimensional
particle distributions at ground level which capture the shower to shower
variations coming from the early interactions. We present a comparison, on an
event by event basis, of SENECA and CORSIKA, a well trusted MC simulation code.
By using the same first interaction in both SENECA and CORSIKA, the effect of
the cascade equations can be studied within a single shower, rather than
averaged over many showers. Our study shows that for showers produced in this
manner, SENECA agrees with CORSIKA to a very high accuracy with respect to
densities, energies, and timing information for individual species of
ground-level particles from both iron and proton primaries with energies
between 1 EeV and 100 EeV. Used properly, SENECA produces ground particle
distributions virtually indistinguishable from those of CORSIKA in a fraction
of the time. For example, for a shower induced by a 10 EeV proton, SENECA is 10
times faster than CORSIKA, with comparable accuracy.
