Hi All. The little 3.0Md near Coarsegold appears to be associated with the Foothills Fault System and is at the very southern end of the Melones fault zone. Quakes don’t occur all to often in this area. They don’t seem to indicate that they may be associated with a larger quake.

At or near the northern end of the Foothills Fault System there was a quake on August 1, 1975. It was an M 5.8. It occurred along the western foothills of the Sierra Nevada, near Oroville. The quake occurred along the little known Cleveland Hill fault, a small northern segment of the prominent Foothills Fault System.

Damage in the epicentral area included toppled chimneys, broken windows, cracked walls, and downed lighting fixtures. Property damage was estimated at $2.5 million

The tremor caused nearly 2.5 miles of discontinuous surface fault ruptures, which stopped just short of Oroville Dam. Maximum offset was about 2.5 inches, showing predominately vertical displacements, west side down.

The earthquake sequence (M > 3) consisted of five foreshocks, a main shock, and numerous aftershocks. The sequence included seven earthquakes of magnitude greater than 4.6.

Three other earthquakes of M 5.0 to 5.9 have occurred since 1900 at or near the Foothills fault system within 60 km of Oroville. The first two occurred in 1909, 60 km east of Oroville (Toppozada and others, 1978) and the third in 1940, 60 km north of Oroville (Bolt and Miller, 1975). Thus, the occurrence of the 1975 M 5.7 earthquake within 70 km of Oroville was not without precedent.

Two factors suggest that Lake Oroville (maximum depth 220 m; storage capacity 413 billion m3) may have contributed to both the location and timing of the 1975 earthquake.

The first factor is the proximity of the earthquake to the lake, and the extension of the causative fault to the lake as indicated by geologic, seismologic, and geodetic data (Department of Water Resources, 1979).

The second factor is the occurrence of the earthquake following an unprecedented seasonal fluctuation in lake levels. During the winter of 1974-1975, the lake was drawn down to its lowest level since filling to repair the intakes to the power plant. This unprecedented lowering of the water level and subsequent refilling was followed by the earthquake sequence of 1975. It doesn’t appear to have been the amount of water that was in the lake, but the way it was refilled that triggered the quakes. Going from maximum to minimum to maximum in a short period of time most likely caused a flexing of the fault. Take Care…Don in creepy town