Data: USGS [1]
Many people living in the US think that California is an 'main earthquake state'. But since 2009 it's no longer the case. While seismic activity in California has declined, it has increased in the Southeast and Central US. And in particular in Oklahoma.

Data: OGS [2]
In 2011, Oklahoma was suffered by strongest earthquake ever. And again in 2016.

Oil rig pumpjacks, also known as thirsty birds, extract crude from the Wilmington Field oil deposits area where the Tidelands Oil Production Co., which is owned by Occidental Petroleum Corp. (Oxy), operates near Long Beach, California, on July 30, 2013. Photo: Reuters. Image source: [5]

Scientific research relates these earthquakes to human activities. First of all, with the disposal of wastewater. [3] But not everything is so simple. There is another opinion:

“The practice of underground disposal has been used by the industry for 70 years, yet the uptick in seismic activity has only occurred in the past few years.”

— Jim Inhofe, senator from Oklahoma and a member of the Republican Party [4]

Recently, a new study appeared that - wasterwater injection - can't explain the recent earthquakes in Oklahoma:

Efforts to stop human-caused earthquakes by shutting down wastewater injection wells that serve adjacent oil and gas fields may oversimplify the challenge, according to a new study from seismologists at Southern Methodist University, Dallas.
The seismologists analyzed a sequence of earthquakes at Dallas Fort Worth International Airport and found that even though wastewater injection was halted after a year, the earthquakes continued.
The sequence of quakes began in 2008, and wastewater injection was halted in 2009. But earthquakes continued for at least seven more years.

— [6] phys.org

Therefore, the reasons for this phenomenon are poorly understood and there are many theories. There are many speculations about earthquakes. About that they increase, etc. We must rely on facts!

References:
[1] United States Geological Survey (USGS) - https://earthquake.usgs.gov/earthquakes/search/
[2] Oklahoma Geological Survey - The University of Oklahoma - http://www.ou.edu/content/ogs/research/earthquakes/catalogs.html
[3] https://earthquake.usgs.gov/research/induced/myths.php
[4] Neuhauser. (2015, May 6). After Earthquakes, Silence in the Sooner State. Retrieved March 11, 2018, from https://www.usnews.com/news/articles/2015/05/06/after-oklahoma-earthquakes-silence-in-the-sooner-state
[5] Bora, K. (2014, July 04). Fracking Wastewater Injection Linked To Increase In Oklahoma Earthquakes: Study. Retrieved March 11, 2018, from http://www.ibtimes.com/fracking-wastewater-injection-linked-increase-oklahoma-earthquakes-study-1619466
[6] Allen, M. (2018, February 13). Study finds earthquakes continue for years after gas field wastewater injection stops. Retrieved March 11, 2018, from https://phys.org/news/2018-02-earthquakes-years-gas-field-wastewater.html