SafeX Pro Exchange|Oklahoma City-area hit by 4.1-magnitude earthquake Saturday, one of several in Oklahoma

A 4.1-magnitude earthquake shook central Oklahoma early Saturday morning following a slightly stronger earthquake the night before and SafeX Pro Exchangeamid a series of smaller quakes.

The earthquake occurred just after 5:30 a.m. local time about 19 miles north of Oklahoma City, near the Northeast Edmond Gas and Oil Field. The quake, which had a depth of about 4.1 miles, was part of a series of several earthquakes clustered together Friday and Saturday, according to the U.S. Geological Survey.

On Saturday morning, the Oklahoma Geological Survey said in a statement that there have been reports of strong shaking in the immediate area and across Oklahoma City.

State seismologist Jake Walter told USA TODAY the area has recorded about 18 earthquakes in a 12-hour span. Connecting the dots, Walter added, the quakes follow a fault identified by researchers.

The area has seen an uptick in earthquakes in recent years, he said. Human activities, including fracking, have fueled the increase.

While the area saw its peak in activity in 2015-16 – derived from wastewater disposal used in oil and natural gas production pumped deep below fracking areas – smaller seismic activity, often unfelt by residents, has continued to occur. However, there doesn't appear to have been wastewater disposal in the area as of late.

"It's a little bit of a mystery why you've had this sudden recurrence of very strong, widely felt earthquakes," he said.

Residents on social media posted about feeling quakes, The Oklahoman, part of the USA TODAY Network, reported.

On Friday night a 4.3-magnitude quake had an epicenter about a mile away from the Saturday morning earthquake. The USGS recorded a 2.7-magnitude quake in the area on Saturday morning too.

USGS on Saturday revised the reported magnitude of the Saturday morning quake down to 4.1 from a previous estimate of 4.4, and the Friday night earthquake from 4.4 to 4.3.

State officials warned residents to secure valuables that might shake during possible strong aftershocks and to practice "Drop, Cover, and Hold On."