Seismic gaps occur along fault lines and plate boundaries.
The term seismic gap is applied to areas along fault lines where no earthquakes have occurred for some time. Because fault lines move at an average pace every year, earthquakes are usually distributed evenly along them. When there is a gap in seismic activity, pressure builds up and a seismic gap forms. Seismic gaps are places where larger earthquakes are expected to occur, depending on how much pressure has built up over time.
Stillwater, Nevada
According to the Seismological Society of America, there is a 40-kilometer seismic gap in Nevada along the central Nevada/eastern California seismic belt. This gap was recorded during 1915 and 1949 earthquakes that were aligned except for the gap. This gap is now referred to as the Stillwater seismic gap. Because there is no history of severe seismic activity in this area and other gaps of similar size have been found nearby, the Stillwater gap is not considered a big threat. However, should an earthquake of large magnitude occur, the Stillwater gap would likely cause significant damage.
Yakataga Bay, Alaska
In 1979, a 7.1 magnitude earthquake hit St. Elias, Alaska and drew attention to a gap that was mostly left undisturbed. According to the American Geophysical Union, this 40- to 60-kilometer gap was thoroughly surveyed during the 1980s. The gap occurs at the meeting of the North American plate and the Yakutat Block. Since its discovery, it has been named the Yakataga seismic gap. According to John Lahr and George Plafker from the science journal "Geology," if a 3.5 earthquake were to hit the region the Yakataga gap, it would likely produce shifts of a magnitude 8.
Hawaii
There are two notable seismic gaps in the Hawaiian Islands. One is called the south Kuna gap and the other is called the east Kuna gap. Earthquakes that occurred in 1951 and 1968 defined these seismic gaps. Geologists of the Seismological Society of America expect more earthquakes to fill in these gaps. The south Kuna gap may see rumbles of up to 7.9 in magnitude, while the eastern gap may produce thrusts of up to 6.9. There have been no main shocks in this area since 1932, which makes these gaps a significant threat.
Loma Prieta, California
The San Andreas Fault is one of the most famous areas for earthquakes in the United States. Loma Prieta, on the San Francisco peninsula, sits on top of a seismic gap that runs from Parkfield to San Francisco along the fault line. This gap was surveyed throughout the 1970s and defined by the Loma Prieta earthquake of 1989. According to researchers at Stanford University, this gap has a history of breaking every 22 years on the nose. It was expected to happen during the mid 1980s and since that time it has been building pressure.
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