Spatial and temporal variations in t s /t p at Monticello Reservoir, South Carolina

Following its impoundment in December 1977, induced seismicity was observed at Monticello reservoir in South Carolina. This shallow (< 2 km), low level (M L < 3.0) activity is still continuing and is confined to the immediate vicinity of the reservoir (in an area of about 5 × 10 km²). Spatial and temporal variations in t s /t p ratios were examined in detail for the period July‐September 1978. We observed two periods of precursory decrease of 6‐12% in t s /t p for M L 2.7 and 2.3 events on August 27 and September 15, 1978, respectively. A precursor time of about 15 days was observed for each of these events. During the anomalous periods, stations within about 8 km of the epicentral area (S‐P ∼ 1 sec) were associated with low ratio values, while the distant stations were associated with normal ratio values ( ∼ 1.75) suggesting that the size of the anomalous zone associated with 2.0 < M L < 3.0 earthquakes is about 4 km in radius.