Wireless Sensor Networks (WSNs) can be typically used to achieve Continuous Monitoring (CM) or Event-Detection inside the supervised area. In CM applications each sensor node transmits periodically its sensed data to the sink node while in Event-Detection Driven (EDD) applications, once an event occurs, it is reported to the sink node by the sensors within the event area. Applications using both continuous monitoring and event driven reporting can also be considered. In this paper, we investigate such hybrid WSNs. Specifically, we propose to reduce the energy consumption by grouping the nodes that sense an event into a separate cluster. By doing so, as it is shown in this paper, the efficiency of the data compression scheme is indeed improved. This event-aware clustering scheme is compared to the case where the event data is transmitted through already established clusters. Finally, the case where sensor nodes only transmit information whenever they sense relevant data is introduced in order to achieve further energy savings.