The Multifractal Detrended Fluctuation (MF-DF) algorithm is applied to measure the complexity of two time series, the inter-event hypocentral distance Δδ(t), and the inter-event time series Δτ(t) In particular, we apply this methodology to the seismic sequences produced in the Carterbury region during 18 years (2000-2018). Results indicate a clear multifractal behavior of Δδ(t) and Δτ(t). Moreover, an increses in the complexity is observed when a large event occurs . These results suggest that the MF-DF algorithm could be useful as a seismic precursor index.