According to the United States Geologic Survey (USGS), "a seismic gap is a section of a fault that has produced earthquakes in the past but is now quiet." Seismic gaps also include faults that are believed to be capable of producing earthquakes, even if no earthquake has been recorded from that fault.
Faults
Faults, according to the USGS, are breaks in the earth's crust, particularly a break between rocks in which the "rocks on either side have moved past each other." When the rocks slide past each other abruptly, the resulting shift in the land is called an earthquake. The faults that make up seismic gaps are faults where the rocks have not moved suddenly enough to cause an earthquake for a significant amount of time.
Pressure Buildup
Many seismic gaps occur at the edges of tectonic plates. Normally, tectonic plates slide over one another gradually. Seismic gaps can occur when the plates halt their slow, steady sliding. As the plates keep pressing together, pressure builds. According to the Tsunami Research Facility at Oregon State University, eventually the plates will release their pent-up energy in "one large earthquake."
Cause of Tsunamis
In active faults, pressure in the fault is often bled off over time by a series of small earthquakes. Because this does not happen in seismic gaps, seismic gap earthquakes are often very powerful. When powerful earthquakes occur beneath the ocean, they can create a tsunami wave. For this reason, seismic gaps can be viewed as potential sources for tsunami-causing earthquakes.
Seismic Gap Hypothesis
The Seismic Gap Hypothesis, also known at the Seismic Gap Theory, is the idea that strong earthquakes are more likely to occur in areas that have not recently experienced a strong earthquake. This means that seismic gaps, rather being viewed as dormant faults, could be viewed as potential "hot spots" for future earthquakes. The Seismic Gap Hypothesis is still undergoing testing by the scientific community.
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