The major Hebgen Lake earthquake on August 17, 1959, profoundly affected the hot springs and geysers of Yellowstone National Park. The epicenter of this earthquake was about 48 km northwest of Upper Geyser Basin, and its magnitude was 7.1 on the Richter scale. No earthquake of closely comparable intensity had previously jarred the geyser basins in historic time. By the day after the earthquake, at least 289 springs in the geyser basins of the Firehole River had erupted as geysers; of these, 160 were springs with no previous record of eruption.
New hot ground soon developed in some places or became apparent by the following spring as new fractures in hot spring sinter or as linear zones of dead or dying trees. Some new fractures developed locally into fumaroles, and a few of these evolved into hot springs or geysers.
One fracture was inspected frequently as it evolved into a fumarole and, in about 2½ yr, into a small geyser. During the next few years, it became a very vigorous geyser, now named “Seismic,” that erupted to heights of 12 to 15 m and explosively excavated a jagged-walled vent more than 12 m in maximum diameter and more than 6 m deep. Major eruptions ceased in 1971 when a small satellite crater formed and then assumed dominance.
The formation and evolution of Seismic Geyser provide the keys for understanding the origin of the craters and vents of other geysers and probably also the large smooth-walled nongeysering pools and springs of the morning glory type that provide no direct evidence of their origin.
Earthquakes, largely localized just outside the Yellowstone caldera, result in violent shaking of the large high-temperature convection systems of the geyser basins. New fractures form in the self-sealed shallow parts of these systems where high-temperature water is confined at pressures much above hydrostatic. As old fractures and permeable channels become sealed by precipitation of hydrothermal minerals, new channels are provided by the periodic seismic activity.
Our explanations for the origin of geyser and hot spring vents apply specifically to the geyser basins of Yellowstone Park, where near-surface fluid pressure gradients are commonly 10 to 50 percent above the hydrostatic gradient, and temperature gradients and thermal energy available for explosive eruption are correspondingly high. The same general explanations seem likely to account for the origin of geyser tubes and hot spring vents in other less favored areas where pressures and maximum temperatures are limited by hydrostatic pressures, probably with little or no overpressure being involved.