How an Oklahoma earthquake showed danger remains after years of quakes becoming less frequent

A couple of major earthquakes in Oklahoma in recent weeks have raised concerns after years of a declining number of temblors. A 5.1-magnitude quake last week was one of the strongest in years. Most scientists agree the quakes in Oklahoma are related to the high-pressure injection deep underground of wastewater left over from oil and natural gas production. After an uptick in quakes, regulators years ago placed new restrictions on disposal wells. The Oklahoma Geological Survey says its data shows the number of earthquakes in the last decade greater than 2.7 magnitude dropped from nearly 2,000 in 2015 to less than 40 last year.