In the past 24 hours, moderate movement at three areas across the United States which share something in common.
A 4.1M (and swarm over the past few weeks) occurred at a dormant volcano located near the Arizona / New Mexico border.
A 3.2M (and swarm over the past 4 years) occurred at a fracking operation in Central Oklahoma.
All three locations share one thing in common: Weak points in the shallow crust of the Earth due to underground cavities.
Whether they be man made drilling shafts for extracting natural gas (fracking), mining operations hollowing out areas underground in search for Gold, or dormant volcanoes with semi-hollow underground tubes / shafts created by long hardened magma.
Diagrams for each show how similar these underground cavities are: