The MARS (Modelling Autonomous Reasoning System) project aims to develop a collaborative intelligent system combining the processing powers and visualisation provided by machines with the interpretive skills, insight and lateral thinking provided by human analysts. There is an increasing volume of data generated by online systems, such as internet logs, transaction records, communication records, transport network monitors, sensor networks, etc. Typically, these logs contain multiple overlapping sequences of events related to different entities. Information that can be mined from these event sequences is an important resource in understanding current behaviour, predicting future behaviour and identifying non-standard patterns. In this paper, we describe a novel approach to identifying and storing sequences of related events, with scope for approximate matching. The event sequences are represented in a compact and expandable sequence pattern format, which allows the addition of new event sequences as they are identified, and subtraction of sequences that are no longer relevant. We present an algorithm enabling efficient addition of a new sequence pattern. Examination of the sequences by human experts could further refine and modify general patterns of events.