To save power, many previous works propose to use wake-up scheduling to support data gathering for wireless sensor networks (WSNs). However, these works can only work well for regular data patterns. In this work, we further consider the scenario that the network may randomly have urgent events. Assume that time is divided into fixed-length slots, and several consecutive slots are grouped as a frame. For a node, we assign one regular slot and several event slots to it. Each node wakes up in its regular slots to sense the environmental information and to gather reports from its child nodes. In a frame, each node also wakes up in the assigned event slots for a short period to detect possible events. When detecting an event, the node wakes its ancestors up to relay event packets. In this paper, we design a two-phase approach to support quick event detection and reporting in a low-duty-cycled WSN. Simulation results show that our designs can indeed achieve our goals.