HELENA, Mont. (AP) -- Eight aftershocks rumbled through southwestern Montana early Tuesday in the wake of an earthquake late Monday measuring a magnitude 5.6 that rattled windows, buildings and nerves, but caused no serious damage.

The U.S. Geological Survey said the earthquake and its aftershocks were centered about 13 miles northwest of Dillon, although residents across a wide swath of Montana, Idaho and Washington reported feeling the main temblor at about 10:10 p.m.

Reports came in from as far away as Seattle, 490 miles to the west; and Hardin, 246 miles to the east; Troy, more than 248 miles to the north; and Bonners Ferry, Idaho, 303 miles to the south.

Some residents reported two distinct tremors, several seconds apart, lasting about eight to 10 seconds in all.

The aftershocks measured between 2.8 and 3.8, and occurred between 10:30 p.m. Monday and 6:45 a.m. Tuesday, the USGS said.

In Dillon, Beaverhead County Commissioner Mike McGinley said there were scattered reports of broken glass and other minor damage around town. Bricks had fallen off several buildings and chimneys, but the quake's epicenter was on an old geologic fault line with "no population in the area," Chief of Police John Gutcheck said Tuesday.

One man suffered a heart attack at a Dillon motel following the quake, but survived. No other injuries were reported.

The USGS categorized the earthquake as "moderate." Initial reports said it occurred about 25 miles underground, but the USGS later said the quake was at a much more shallow depth -- only about 3 miles below the surface.

Bud Revious, the deputy response coordinator for the state Disaster and Emergency Services office in Helena, said officials there were busy monitoring the state's scattered wildfires when the office shook.

"We thought maybe we were having one of those microbursts like we had a couple weeks ago, but we looked around and there was no wind," he said.

Revious called the quake "heavy" but confirmed state officials had not received any immediate reports of significant damage.

Charity Watt-Levis, state Transportation Department spokeswoman, said Tuesday the epicenter was just 6 miles east of Interstate 15. Maintenance crews from Butte initially checked the nearest interchange, located midway between Dillon and Glen, Monday night and found no damage, she said. Another inspection was planned Tuesday.

The largest earthquake in Montana's history struck Hebgen Lake near Yellowstone National Park on Aug. 18, 1959. It registered 7.3, killed 28 people and caused $11 million in damage. The most disastrous effect of the quake was a massive landslide on the wall of the Madison River Canyon, blocking the river's flow and forming a new lake.