Detecting the region of emergent events is an important application of wireless sensor networks (WSN). One of the key challenges in detecting event in a WSN is how to detect it accurately while transmitting minimum information to provide sufficient details about the event. In this paper, an aggregation algorithm based on splay tree is proposed to achieve the following goals: monitoring data of any portion of the region can be obtained at one time by querying the root instead of flooding those regions, thus incurring significant energy savings. The performance and cost of the algorithm are analyzed and evaluated. The results show the proposed algorithm is efficient and effective in dealing with data aggregation.