Seismic Shift Meaning

Example : 1958, Waldo Frank, "Mexico" in The Romance of North America (Hardwick Moseley, ed.), Houghton Mifflin, p. 127 (Google snippet view):  The seismic shift in Mexico began in 1810 with the movement toward independence from Spain.1999 Dec. 6, Eamon Phoenix, "Peace Is Breaking Out," Time (retrieved 19 May 2015):  For the republican movement, the acceptance of seats in a "partitionist" Assembly signaled a seismic shift in historical attitudes since the division of Ireland in 1921.2007 Dec. 10, "Most Important of 2007," Businessweek (retrieved 19 May 2015):  [T]he U.S. could be on the verge of a seismic shift, where it is possible to envision a time when it will no longer be the dominant economic superpower.2011 Feb. 5, "The Aging of America," New York Times (retrieved 19 May 2015):  Baby boomers began turning 65 in January, heralding a seismic shift in demographics worldwide.