A small earthquake shook Helena and Alabaster Saturday, but few people noticed the mild temblor. The earthquake was the second to hit Shelby County in two weeks.

The 2.8-magnitude quake happened just after midnight Saturday. According to the U.S. Geological Survey, the quake centered about six miles southwest of Helena and six miles south of Alabaster.

Earlier in August, a 3.5-magnitude quake hit near Montevallo and Alabaster. Also, Helena sustained a 3.3-magnitude quake in May and another small temblor in March.

Despite the activity, USGS officials said Shelby County is not at risk of any damaging earthquakes.

According to David Russ, chief geologist for the eastern region of the USGS, earthquakes are noticeable at a magnitude of 3.0 or higher. He said a quake with a magnitude of 4.0 or greater will usually wake people out of their sleep.The largest likely quake for Shelby County is about 4.0, Russ said.

However, a 4.9-magnitude temblor did strike northeast Alabama and Georgia in April 2003.

Damage usually occurs with quakes measuring 5.2 to 5.3, Russ said. Significant damage occurs at about 6 on the Richter scale.

Russ said he believes the recent string of earthquakes in Shelby County is related to the readjustment of geologic structures at the southern end of the Appalachians.

All areas are susceptible to earthquakes due to the dynamic nature of the earth’s crust, Russ said. Earthquakes occur on faults in the earth’s crust when pressure exceeds the strength of the rock and it breaks.

The gradual westward drift of the North American plate shifts rock in the earth’s crust.

According to Russ, the closest area for a large, damaging earthquake is the New Madrid seismic zone in the Mississippi River valley of Missouri, Arkansas and Tennessee.

In 1811 and 1812, three earthquakes measuring greater than magnitude 8.0 occurred within three days along the Mississippi River in New Madrid, Mo