A 3.5-magnitude earthquake shook central Kansas on Wednesday, jolting an area layered over ancient volcanic rock deep below the surface.
The tremor was detected at 2.14pm local time (3.14pm ET), striking outside Assaria, home to just a few hundred people.
Although Kansas has no active volcanoes, the region marks the southern reach of the Midcontinent Rift System, a massive tectonic event that nearly split North America apart in Earth’s distant past.
When magma forced its way through the crust during that period, it left behind hardened igneous rock and deep fractures that remain buried thousands of feet underground.
The earthquake near Assaria was associated with the Humboldt Fault Zone, a major structural feature stretching across eastern Kansas.
Geologists say the fault is made up of a series of fractures and buried faults formed hundreds of millions of years ago.
While it is relatively quiet compared with faults in more seismically active regions, it remains capable of producing small to moderate earthquakes when stress builds along its hidden fractures.
In areas like Assaria, where the fault intersects layers of ancient volcanic rock, even minor shifts can be detected at the surface as noticeable tremors.
Scientists say the ancient scars from both the Midcontinent Rift and the Humboldt Fault still matter today, as the North American plate is constantly under slow-moving stress.
That pressure tends to concentrate along pre-existing weaknesses in the crust, and if enough strain builds, the rock can suddenly slip, releasing energy in the form of seismic waves.
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