(CNN) -- Three dozen earthquakes over the past week in central Arkansas shook shelves, rattled nerves and prompted speculation about their cause.
"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.
Solving the mystery is important because of suspicion the quakes could be related to natural gas exploration nearby using hydraulic fracturing, known as fracking, he said.
"The best probability at this point is they are natural, but we can't rule out the possibility that there may be a relationship," Ausbrook said.
A swarm of hundreds of earthquakes centered near Guy, Arkansas, ended two years ago after state regulators imposed a moratorium on new injection wells in the area.
Two operating injection wells, just outside of the moratorium area, are about 8 miles from some of the quakes. "I can't say at this time there's not a connection, but given the distance from those wells, it would be pretty amazing," Ausbrook said.
One natural explanation would be a fault running through the area that makes a sharp turn, where "you would expect more strain to accumulate," Ausbrook said.
CNN's Dave Alsup contributed to this report.