The mechanism of why the rupture terminated sharply on both sides of the seismic gap between the Wenchuan and Lushan earthquakes is not yet known along the southwestern Longmen Shan fault zone. The accommodation modes of compressional strain and stress are still not known well in the gap subregion. Using the Match and Locate method, we obtained a detailed catalog of background microearthquakes around the seismic gap from May to December 2015. The characteristics of spatiotemporal distribution of the events is highlighted with a complete magnitude of 0.2. These small events tend to concentrate at different depth ranges on two sides of the seismic gap, indicating that the gap serves as a transition zone between the Wenchuan and Lushan source regions. Strain release through microearthquakes is an unlikely candidate for accommodating modes in the gap, because of the sustained deficit of microseismicity. A strong anticorrelation between background seismicity and monthly precipitation variation rate is observed for different magnitude scales. Enhanced seismicity may be activated by the terrestrial water storage reduction next to a precipitation peak with a time lag of 1–2 months during the summer and autumn, indicating a seasonal modulation of seismicity by the water storage cycle. The background microseismicity exhibits prominent responses to precipitation variation due to a short nucleation duration of less than one quarter.