Along the strike of subduction zones, tectonic tremor activity is segmented on a geologic scale, indicating local variations of the tremor-generating process. Here, we study how strong temporal clustering and long-term recurrence of activity can emerge from the synchronization of elementary tremor sources, as they interact through fluid pressure transients. We model tremor sources as rapid openings of low-permeability valves in the permeable fault zone channeling the upward flow of deep metamorphic fluids. Valve openings trigger fast pressure transients that generate seismic waves. In such a system, tremor activity is thus shaped by unsteady fluid circulation. Using numerical simulations of fluid flow for a large number of different valve populations, we show that the synchronized, collective activity of sources generates episodic activity, and that along-strike variations of fluid flux and fluid transport properties can lead to the segmentation of tremor activity. Strong tremor bursts that coherently activate wide parts of the fault and recur with a long period are associated with patches densely populated with valves and characterized by below-average permeability. Long-term tremor episodicity emerges from the synchronous activity of valves in such patches and is responsible for fluid-pressure cycling at the subduction scale. In the tremor zone of the Shikoku, Japan, subduction interface, the most temporally clustered segment coincides with a downgoing seamount chain, suggesting that the segmentation of the fault zone permeability, and hence of tremor activity, could be inherited from the topography of the subducting oceanic plate.