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expected recurrence of the next Parkfield earthquake is near and the historic Parkfield earthquakes have been so similar, Parkfield provides a unique opportunity to prepare an experiment to observe the final stages of the process leading up to the earthquake.

The observations and accounts available for the last characteristic Parkfield earthquake, on June 28, 1966, serve as a template for anticipating the features of the next Parkfield earthquake. In the 6 months before June 28, all earthquakes near Parkfield occurred on the San Andreas fault northwest of the rupture zone. The foreshocks, smaller earthquakes occurring near the rupture zone shortly before the main shock, occurred within a mile or so of the main shock epicenter, which was located at the northwest end of the rupture zone; this area has been named the "preparation zone." A similar foreshock and characteristic earthquake pattern occurred in 1934 within this preparation zone. Although many small earthquakes occur on the San Andreas fault northwest of the rupture zone, earthquakes within the critical preparation zone are infrequent. The U.S. Geological Survey has established a very dense network of seismograph stations near Parkfield to monitor the seismicity within and near the preparation zone; any increase in

seismic activity will be scrutinized closely.

Some geologists think that earthquakes are preceded by fault creep (movement on a fault without detectable earthquakes). An irrigation pipeline that crosses the San Andreas fault near Parkfield broke without detectable earthquakes about 9 hours before the 1966 earthquake. Unfortunately, the history of the movement that resulted in the broken pipe is unknown; although geologists infer 1 to 2 feet of displacement from the broken pipe, perhaps only a very small part of the displacement occurred in the days and weeks just before the 1966 earthquake. Also, fresh cracks of uncertain origin. were observed along the San Andreas fault 12 days before the 1966 earthquake. The broken pipeline and the fresh cracks are consistent with fault creep near Parkfield just before the characteristic earthquake in 1966.

If significant and unusual fault creep precedes the next characteristic Parkfield earthquake, the sophisticated, sensitive devices deployed near Parkfield should record it. Surface fault movement is monitored continuously by several creepmeters, 30-foot-long devices that cross the fault trace. Also, approximately 2- to 5-mile-long lines of sight which cross the fault will be measured each night on a two-color laser distance-measuring geodimeter

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Veniaminof Volcano in eruption, January 1984. The lava flow down the volcano's south flank has melted a large pit in the surrounding glacier ice. (U.S. Geological Survey photograph by T. P. Miller.)

The 1984 Eruption of Mauna Loa Volcano

March 30, 1984, was an historic day for the United States and a busy one for the U.S. Geological Survey-four volcanoes were in simultaneous eruption in the United States! About 50 volcanoes are active in the United States (those known to have erupted in historic times), but, until this date, no more than two of them had been known to erupt on the same day.

The circumstances leading up to this historic day began in June 1983, when Veniaminov Volcano, on the Alaskan Peninsula, began a series of eruptions, which continued at a low level into spring 1984. Then, on March 25, Mauna Loa Volcano, on the island of Hawaii, began a long-expected major eruption. This was followed on March 29 by the beginning of another eruptive phase at Washington State's Mount St. Helens Volcano, which has been active intermittently since March 1980. The final actor in this volcanic drama burst into action early on the morning of

March 30 as Kilauea Volcano, also on the island of Hawaii and frequently active since January 1983, began a short episode of high fountaining. So, on this date, U.S. Geological Survey field crews were at work at three volcanoes-in the tropical rain forest of Kilauea, high above the timberline on Mauna Loa, and within the snow-covered crater of Mount St. Helens. Poor weather on that day prevented access to the remote, largely ice-covered Veniaminov, although villagers 20 miles away reported that a steam cloud rose to 7,500 feet above the volcano.

The largest of these four eruptions, and the one of greatest concern to public safety, was that of Mauna Loa, the largest volcano on Earth. The eruption, which began suddenly at 1:30 a.m. on March 25, climaxed 10 years of increasing seismicity and accompanying inflation of the volcano. This long period of premonitory activity

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