We examine and model the arrival time of a large secondary seismic arrival recorded in the Long Valley/Mono Craters region of east central California. Zucca et al. (1987) and Peppin (1987) both previously reported on different features of this same arrival. Using both arrays of sources and receivers, we demonstrate that the arrival in an S-to-P converted phase as first suggested by Lewis and Peppin (1988). Back projection of the observed travel times allows us to constrain the location of the converting material to somewhere within a southeast dipping zone between 7 and 16 km depth and within 5 km on either side of the northwest topographic margin of the caldera. The analysis demonstrates the power of source and receiver array combinations when analyzing seismic arrivals in complicated environments. ¿ American Geophysical Union 1989