Among wireless sensor networks applications, event monitoring is an application that has attracted a lot of attention in the recent years. As a physical event may be mobile, communication protocols should be designed to support mobility and allow the user to track the mobile event continuously, in energy-efficient way. In this paper, we present a new mobile event monitoring protocol for wireless sensor networks. Once the tracked event detected, the protocol builds a cluster that contains all nodes that detect this event with a certain threshold. In order to save energy, we propose to use a cluster membership update mechanism to update the cluster status when the event changes its position, instead of forming a new cluster each time the event moves. We evaluate the proposed protocol by simulations and show that our protocol results in much more energy saving than other monitoring protocols for average and high speed mobile events.