Detailed seismicity (M ≧ 2.5) of northern and central California for 1965-1969 is analyzed. The largest earthquakes were nine with magnitudes 5.0 < M < 6.0. Since the interval, 1962-1965, seismic activity has increased north of Corralitos on the San Andreas fault. Seven shocks were located in the western reaches of Monterey Bay. The histogram of focal depths (d) for the coast ranges shows all d < 16 km and 50 per cent with 0 < d < 5 km.
North of Cape Mendocino, the new Fickle Hill telemetry station shows that within 30 < Δ < 85 km of FHC, (a) only 15 per cent of recorded earthquakes (M ≧ 2.5) has M > 3.0 and (b) an average of 14 earthquakes (M ≧ 2.5) occurs each month. Some foci are as deep as 25 km in this region.
For northern California, the linear recurrence law gives b = 0.78 ± 0.04, slightly less than southern California. For the coast ranges, b = 1.00 ± 0.04. The water loading of Oroville reservoir in 1967-1968 has not been associated with increased seismicity in the area. On the present homogeneous data, a statistical t-test shows that the short-term occurrence rate of northern California earthquakes was not altered by large underground nuclear tests in Nevada.