Spatial, temporal and magnitude-frequency relationships for earthquakes occurring between 1976 and 1984 near Heron and El Vado reservoirs in northern New Mexico are examined for evidence of reservoir-induced seismicity (RIS). Most of the 264 located shocks, including those with the highest magnitude (maximum ML = 2.7; mb = 3.8), occurred in 1982 when Heron reservoir reached its maximum capacity of approximately 0.5 km3 and an impoundment depth of 66 m. Between 1976 and 1984, earthquake swarms followed, or were enhanced by, reservoir filling in three out of four cases where filling resulted in new maximum water volumes for Heron reservoir. Shocks generally cluster between the reservoirs in a region of north-south block faulting. A cumulative earthquake frequency versus magnitude plot for Heron-El Vado produced a b-value of 0.92 ± 0.03 (1 SD) which is slightly higher than other b-values for northern New Mexico and is consistent with b-values for reservoir induced shocks in other areas. The spatial and temporal distribution of these events, along with their b-value, suggests most of them have been triggered by reservoir loading. Practical ramifications of this work are that RIS can occur in reservoirs with impoundment depths of less than 100 m and RIS may occur if other dams are located along the historically seismic Archuleta anticline on the eastern margin of the San Juan basin.

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