The study has found that the provocation window, which refers to the number of days or weeks between a North Korea provocation and a South Korean election, has become narrow over time, Cha said.

"Under (North Korean leader) Kim Jong-un, the average window for a North Korean provocation bracketed around all ROK elections is 6.5 days (about 1 week). The average for presidential elections is 15 days or about two weeks," he said.

That represents a significant change because under the leadership of previous leaders Kim Il-sung and Kim Jong-il, grandfather and father of the current leader, the window was an average of about 10 and 11 weeks, Cha said.

"This pattern suggests a provocation as early as two weeks before the South Korean elections on May 9th. The start of the provocation window falls on Military Foundation Day (April 25), a holiday in North Korea, and 10 days after the 105th anniversary of Kim Il-sung's birthday on April 15th," he said. (Yonhap)