ALLENDALE -- The U.S. Geological Survey says a 3.6-magnitude earthquake rumbled across southern Illinois.

The quake at about 2:15 a.m. was centered in Allendale, which is in southeast Illinois near the Indiana border.

Geophysicist Julie Martinez says it was an aftershock to the 5.2-magnitude temblor on April 18th near West Salem.

The geological survey says more than 25 aftershocks have been linked to the West Salem quake, ranging in magnitude from about 2.0 to 4.0.

Martinez says the last recorded aftershock was May 1st.

Seismologists say aftershocks can continue for days or months depending on the region or the geology. Many of the aftershocks go unnoticed by people without specialized measuring equipment.

On the Net: U.S. Geological Survey.