The rift at the border of the states of Tennessee and Alabama, previously considered the source of weak earthquakes, turned out to be one of the most seismically disturbing points in North America in the last 25 thousand years, say scientists in an article published in the journal Bulletin of the Seismological Society of America.
“It is possible that powerful earthquakes here occurred only at the end of the glacial period and that they ceased and will no longer occur.As long as we do not have enough data to make such statements it is possible that periodic outbreaks of geological activity will occur here in the future And for other reasons, “- said Randel Cox (Randel Cox) from the University of Memphis (USA).
This fault, located at the southern spurs of the Appalachians, today is one of the most active hotbeds of seismic activity in the territory of the east of the United States. Relatively weak earthquakes occur periodically, whose strength never exceeded the magnitude of 4.8 points on the MMS scale. In other words, such tremors will always remain invisible to residents of the cities of Tennessee and Alabama, located next to this zone.
Cox and his colleagues accidentally found out that this was not always so, and that this region in a relatively recent past was one of the most troubled corners of North America, studying traces of unusual “landslides” on the bottom and banks of artificial lake Douglas in the northeast of the state Tennessee.
This reservoir, created by the state authorities during the Second World War to build hydroelectric power plants, becomes shallow and almost completely dry out during the winter, which allows scientists to study the geological history of the Appalachians and the entire east of the USA, not arranging for this large-scale excavations.
While carrying out similar expeditions on the “bottom” and on the shores of this lake, the Cox team came across traces of two unusual “landslides”, during which the seams shifted by about six meters, and the resulting cracks were filled with sand, clay and other sedimentary rocks .
Studying these cracks, the scientists came to the conclusion that they were caused not by a landslide, but by powerful earthquakes, which, judging by the age of the surrounding rocks, occurred approximately 15 and 25 thousand years ago. The power of these shocks, as the calculations of geologists show, was about 6 or 7 points on the MMS scale. This compares with the strength of the earthquake in Los Angeles in 1994 or the spurts in Spitak in 1988, which caused massive destruction and deaths of thousands of people.
How could these earthquakes occur? Scientists believe that they could be connected with those processes that occurred inside the earth’s crust after the glaciers retreated to the north and the gradual “ascent” of North America, squeezed by the ice shield during the glacial epoch. This process has not been completed so far, and therefore scientists do not rule out that in the future such tremors can happen again.
Why is it important? As Cox notes, in the vicinity of the fault several nuclear power plants and dams of hydroelectric power stations have been built, which can fail if they are not prepared for powerful tremors. Therefore, scientists will continue to study the bottom of the lake in search of other traces of ancient earthquakes, and plan to begin searching for their tracks in other regions of the fault.