More than 6000 aftershocks were recorded following the 1994 Arthur's Pass Mw6.7 earthquake in South Island, New Zealand. Here we relocate over 3500 of the aftershocks using double-difference tomography to map detailed fault structures, and obtain a 3-D model of P-wave velocity and Vp/Vs ratio in the volume around the aftershock region. Waveform cross-correlation methods are used to calculate high quality differential times for event pairs. Two parallel clusters in particular are very well defined after the double-difference relocation, with a dip of ~70--80¿ and ~N60E ¿10¿ strike. A third NW-dipping cluster aligns very closely with one of the preferred fault planes for the mainshock, corroborating previous interpretations of the mainshock source mechanism. Results from the double-difference tomography indicate P-wave velocities and Vp/Vs change markedly immediately above the upper bound of the seismicity, with a Vp of 4.5 to 5.5 km/s and elevated Vp/Vs found in the top 2 km.