Alexandra Witze, at Nature,
writes:
Fifteen quakes of magnitude 4 or greater struck in 2014 — packing more
than a century’s worth of normal seismic activity for the state into a
single year. Oklahoma had twice as many earthquakes last year as
California — a seismic hotspot — and researchers are racing to
understand why before the next major one strikes.
Whatever they learn will apply to seismic hazards worldwide. Oklahoma’s
quakes have been linked to underground wells where oil and gas
operations dispose of waste water, but mining, geothermal energy and
other underground explorations have triggered earthquakes from South
Africa to Switzerland.
This map, from the article sums it all up:
