A surprising increase in seismicity started in and around Monticello Reservoir, South Carolina in December 1996, and by the end of 1999, over 700 earthquakes with –0.4 ≤ ML ≤ 2.5 had been located. This seismicity occurred in a new hypocentral region and filled the gaps in earlier seismicity at depths shallower than 2 km. The seismicity occurred in four episodes each with at least one earthquake of magnitude ML ∼ 2.0. The new seismicity started at depths of 0.8–2 km within a previously a seismic granofel body and in its surrounding volume (episodes I and II). Episode III began more than a year later and also occurred in granofels. It was located to the east of the first two episodes and at shallower depths (from the surface to ∼1.4 km deep). The seismicity then migrated less than 1 km to the north and south and occurred in granodiorites (episode IV). We speculate that the rocks in the new hypocentral regions were isolated from the regions of earlier seismicity by fractures filled with zeolites. Twenty years of reaction with water led to the reopening and weakening of the zeolite-filled fractures, allowing fluids to enter the previously aseismic regions and triggering these episodes of intense seismicity. We suggest that pore pressure migration was associated with these episodes of seismicity. Each of the four episodes was associated with two stages with different temporal and spatial patterns. In the first stage, there was a rapid increase in seismicity in a small volume. We interpret this to be associated with a rapid build-up of pore pressure. In the second stage the seismicity spread, and its activity rate decayed. We interpret this stage to be associated with the equilibration of pore pressure.

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