The Toronto-based miner says it's still assessing the financial impact, but that lower production is expected to "adversely impact" costs at the location.
Production guidance at the Westwood mine is being reduced by about 40 per cent to 60,000 to 75,000 ounces of gold.
The company had forecast 110,000 to 130,000 ounces of gold.
It says part of the drop will be offset by an extra 10,000 ounces of gold production from one of its joint venture mines in Mali.
Iamgold says it remains confident in the project's long-term viability.
"We and the other companies mining underground in the region know that this district is prone to seismic activity and we are taking the necessary measures to keep our excellent safety record intact," said president and CEO Steve Letwin in a statement Monday.
"Westwood is still in its infancy and similar to mines in the area, we expect it to have growing pains as we gain operational experience in this mining environment."