Results of ongoing paleoseismicity studies along the Wasatch fault zone, Utah, and Lost River fault zone, Idaho, suggest that spatial and temporal clustering of large magnitude surface-faulting earthquakes occur on each. The timing of individual past events is based primarily on radiocarbon and thermoluminescence dating of faulted deposits and on identification of displaced volcanic ashes, all observed in trenches.
Figure 1 shows the faulting history of the Wasatch for the past 6 ka. Even though there is variability, distinct patterns emerge. Five of the seven active segments ruptured between 0.3 and 1 ka; if the Salt Lake segment is included, six segments have ruptured between 0.3 and 1.5 ka. From Salt Lake south, a similar sequence of events occurred between about 4.5 and 6 ka. Figure 2 shows timing of events along the Lost River during the past 12 ka. No events occurred from at least 12 ka until middle Holocene. Between 5.5 and 8 ka the northern three segments ruptured; actual timing of events could have been considerably closer. The Thousand Springs segment produced the 1983, MS 7.3, Borah Peak earthquake.
These observations suggest thousands of years of quiescence can occur between clusters of activity during which adjacent segments fail in short periods of time (hours, months, decades, a few hundred years?). Segments adjacent to historical ruptures may have a higher potential to produce the next earthquakes, especially if they have elapsed times similar to the historical rupture segment. Similar clustering with even longer periods of quiescence could characterize seismic sources in the East.