Less than 18 hours after the Denali earthquake in Alaska, Smith and colleagues at the University of Utah Seismograph Stations reported the major jolt had triggered more than 200 small earthquakes in Yellowstone – something widely reported by news media in the days following the quake.
Smith now says the triggered quakes at Yellowstone numbered more than 1,000 within a week of the Denali quake – if the count includes tiny temblors that were not ''located,'' meaning their epicenters and depths were not determined. He says the quakes ranged in magnitude from minus 0.5 to just under 3.0. (Tiny quakes have negative magnitudes because modern seismic equipment can detect quakes smaller than was possible when the logarithmic magnitude scales were devised.)
Most of the triggered quakes were centered near geysers and hot springs.
Strong Earthquakes as Seismic and Geothermal Triggers
Scientists once believed that an earthquake at one location could not trigger earthquakes at distant sites. That belief was shattered in 1992 when the magnitude-7.3 Landers earthquake in California's Mojave Desert triggered a swarm of quakes more than 800 miles away at Yellowstone, as well as other temblors near Mammoth Lakes, Calif., and Yucca Mountain, Nev.
As the Denali quake's surface waves arrived at Yellowstone, changes in hydrothermal activity first were noted at the 100 Spring Plain hot spring system in Norris Geyser Basin.
''Several small hot springs, not known to have geysered before, suddenly surged into a heavy boil with eruptions as high as 1 meter [about 39 inches],'' Smith and colleagues wrote in Geology. ''The temperature at one of these springs increased rapidly from about 42 to 93 degrees Celsius [about 108 to 199 degrees Fahrenheit]'' and became much less acidic than normal. ''In the same area, another hot spring that was usually clear showed muddy, turbid water.''
Meanwhile, some geysers erupted more frequently than normal, while others erupted less frequently.
Yellowstone has more than 10,000 geysers, hot springs and fumaroles (steam vents), and scientists monitored how often 22 of the geysers erupted during the winter of 2002-2003. Eight of the 22 ''displayed notable changes in their eruption intervals'' after the Denali quake, 10 showed no significant changes and the other four were too erratic in the timing of their eruptions to determine if the quake changed them, the researchers wrote. Of the eight that changed:
-- Geysers that erupted more frequently following the Denali quake included Daisy, Depression, Plume and Riverside geysers in Upper Geyser Basin, and Pink Geyser in Lower Geyser Basin.
-- Geysers that erupted less frequently after the Denali quake included Castle and Plate geysers in Upper Geyser Basin and Lone Pine Geyser in West Thumb Geyser Basin.
Most geysers returned to their normal timing days to months after the Denali quake.
Oddly, geysers affected by earlier nearby earthquakes – most notably Old Faithful and Grand Geyser in Upper Geyser Basin – were not affected by the Denali earthquake.
Smith says the fact that the Denali quake triggered geyser and hot springs changes at Yellowstone raises an interesting question: ''Could large earthquakes closer to Yellowstone trigger hydrothermal explosions?''
Such steam-and-hot water explosions in prehistoric times blasted out a hole that now is Mary's Bay on Yellowstone Lake. One such explosion has occurred roughly every 1,000 years since the glaciers receded from Yellowstone roughly 14,000 years ago.
Smith says there is no evidence prehistoric quakes triggered those blasts. And such explosions were not triggered by the magnitude-7.5 Hebgen Lake, Mont., quake in 1959 or the magnitude-7.3 Borah Peak, Idaho, quake in 1983.