Are they being being triggered or are they natural? That's something we don't know," Arkansas Geological Survey scientist Scott Ausbrook said Sunday.
The chances of so many temblors in the region in such a short time are "Powerball kind of odds," Ausbrook said. "What was unusual was to have four different areas in the state to be active in the same week."
More than two dozen quakes recorded since Wednesday have been centered north of Morrilton, Arkansas. The strongest, on Wednesday and Thursday nights, had reported magnitudes of 3.5 and 3.4.
"Right now all we've gotten reports of is shaking stuff and rattling shelves, but people are definitely noticing them," Ausbrook said.