The U.S. Geological Survey put the magnitude at 6.
Iranian media said most of the 42 injuries were minor and happened when people rushed to seek shelter.
The quake also damaged scores of buildings in remote mountainous villages near the epicenter, the town of Hojedk, about 1,100 kilometers (683 miles) southeast of the capital Tehran, the reports said.
The town has a population of 3,000 and is frequently hit by quakes. It is home to farms and coal mines.
Rescue workers were at the site.
In November, a magnitude 7.3 earthquake struck western Iran, killing 530 people and injuring more than 9,000. In 2003, a magnitude 6.6 quake flattened the historic southern city of Bam, killing 26,000.
Iran is located on major seismic faults and averages a quake per day.
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