The quake hit offshore at 4:18 am local time at a depth of around 36 km, the US Geological Survey (USGS) said. Its epicenter was in the Pacific Ocean 40 km from the town of Acari.
The tremor hit the Iranian province of Kermanshah where an earthquake killed 620 people in November, Iranian state TV said.
The landmark polls cap the country's transformation from a Hindu monarchy to a secular republic, following a 10-year Maoist insurgency that left more than 16,000 people dead.
The western Iranian province of Kermanshah was hardest hit by the earthquake, which struck in a remote mountainous region of Iraq.
Iranian authorities scrambled on Tuesday to bring help to thousands of people left homeless by a major quake that killed more than 400 people.
More than 300 people were killed in Iran when a magnitude 7.3 earthquake jolted the country on Sunday, state media said, and rescuers were searching for dozens trapped under rubble in the mountainous area. At least six have died in Iraq as well.
Most schools in Mexico City remained closed on Monday after last week's deadly earthquake, but children outside the capital were set to return to their classrooms even though aftershocks are still jolting the country.