More than 70 small earthquakes have hit Oklahoma in the past week, stoking fears about the potential for a major one shaking the state. Smaller quakes continued Thursday after a magnitude-4.7 earthquake struck late Wednesday night directly beneath a major oil and gas production area. A more powerful, magnitude-4.8 quake hit 30 seconds later, which ranks as the state’s fourth-largest on record. Authorities said there were no reports of damage or injuries. Experts say the frequency in quakes is likely caused by hydraulic fracturing operations’ injection wells, which are used for oil and gas waste disposal. In 2014, Oklahoma recorded at least 5,415 earthquakes—585 of them mangitude-3 or larger— compared with the 109 magnitude-3 quakes in 2013, according to the Oklahoma Geologic Survey.