By RUSTY DENNEN

A month after Virginia's second-largest earthquake, small aftershocks continue as geologists try to learn more about the tremor and whether more could be on the way.

To date there have been hundreds of aftershocks--most of them imperceptible, detected only by seismic monitors. But others are still being felt near the epicenter, a few miles from Mineral in Louisa County.

Within hours after the magnitude-5.8 quake that struck at 1:51 p.m., on Aug. 23, geologists and seismologists rushed to Mineral to inspect the damage and to install additional monitors to record aftershocks.

"This is the first large quake in the eastern part of the U.S. in modern times," said Martin Chapman, a seismologist and director of the Virginia Tech Seismological Observatory.

The only stronger one was a magnitude-5.9 quake in the Giles area of Southwest Virginia, in 1897.

Chapman drove to Louisa two days after the quake, with students and research associates, to put out monitoring instruments.

"Now we have seven portable instruments, and a permanent network of stations" to monitor the aftershocks, Chapman said. The observatory, several other universities and the U.S. Geological Survey now have 40 or more new monitors in place.

Researchers want to know the area of the fault that ruptured to generate an earthquake of that size. Chapman said one thing is already known: "This was a complex rupture, not a single slip. There were two main slips separated by six tenths of a second."

There are two active earthquake areas in Virginia--one around Giles, and the other known as the Central Virginia Seismic Zone. Six ill-defined faults crisscross the zone between Charlottesville and Richmond.

"By locating the aftershocks and doing studies of the main shock, we'll understand a lot more about how they rupture," he said.

Earthquakes in the West generally occur along where the North American and Pacific plates meet. Stresses build up as they crunch past each other.

It's a different scenario in the East, which sits far from a plate boundary.

"These are not young faults. It's a reaction of old faults in a very different tectonic setting," Chapman said.