No. 77: Sep-Oct 1991
Yellowstone's Old Faithful has a namesake in Calistoga, California. This notso-well-known geyser is usually very dependable, erupting every 90 minutes, shooting 350°F water 60 feet into the air. However, some 60 hours before the October 1989, 7.1-magnitude quake in the San Francisco Bay area, the geyser's period suddenly lengthened to more than 100 minutes. After the quake, it settled back into its usual routine. Prior to two other earthquakes, in 1975 and 1984, the clockwork of Calistoga's Old Faithful also ran slow. (Anonymous; "Unfaithful Geyser," Discover, 12:8, July 1991.)
Comment. Since the quake epicenters were many miles distant from the geyser, how is the geyser's clockwork altered? Somehow, small earth movements must have changed the size of the geyser's water reservoir or, possibly, pressure changes in the surrounding rocks might have reduced the flow of water into the reservoir. Pertinent here are the often-observed changes in well levels and spring flows prior to earthquakes.
Reference. Geysers display many anomalies. These are cataloged in GHG1-GHG3 in the book Earthquakes, Tides. For details on ordering, visit: here.