Final answer:
Earthquakes in mid-America can be caused by ancient faults or stress accumulation within the plate, even in regions that do not have obvious fault lines. The US Geological Survey maps show this activity and the history of seismic events like those in the New Madrid Seismic Zone supports this understanding.
Step-by-step explanation:
You might wonder why there are earthquakes in mid-America where there are no obvious fault lines. Even though the majority of earthquakes are associated with tectonic plate boundaries, they can also occur in the interior of plates. The US Geological Survey map shows earthquake activity in relation to tectonic plate boundaries. However, earthquakes in the central United States can be attributed to ancient faults, stress accumulation within the plate, or human activities such as fluid injection from oil and gas operations.
For instance, the motion along fault zones builds up stress in the Earth's crust over time. This stress is released in sudden slippages, leading to earthquakes. If these stresses build up in areas of old faults in the mid-continent, even without recent or visible faulting, they can still generate earthquakes. Additionally, historical data shows that some regions, such as the New Madrid Seismic Zone, have experienced large earthquakes despite a lack of large fault lines on the surface.