Quake!

Quite a non-event, really.

Yesterday evening, we felt, and heard a bump – or thump – or muffled boom – that was noticeable, but not lasting.  My initial impression was  that it was a sonic boom.

It turns out that what we actually experienced was a small earthquake, with an epicenter 2.45 miles 3.9 (km) north of us and, according to the US Geological Survey Earthquake Hazards Program, was a magnitude 2.8 at a depth of 4.3 miles (7 km) .

Tiny – no big deal.

Of course, now it begs the question, what about all those others in the past that we attributed to sonic booms?  Scientists speculate that some ‘booms’ are probably small shallow earthquakes that are too small to be recorded, but large enough to be felt by people nearby¹.

The following trace was recorded at Cathedral Cave in the Onondaga Cave State Park in Missouri.  Time 0:00:00 is the time of the event.  The colored marks are the times that various seismic waves reached the seismic monitoring equipment there. The location of an earthquake – and its depth are determined by comparing the times with other seismic stations.

The following map shows 3526 magnitude 1.0 or higher earthquakes in a square centered on Arkansas  recorded since we moved here in August, 1980. There were only 632 at or above magnitude 2.5, 57 at or above 3.5, and 1 at or above magnitude 4.5.   In the same period, USGS only has 53 quakes recorded of a magnitude less than 1.0 in the same region.

During the same period, the following map shows 6338 earthquakes in the region of the New Madrid fault.

In this part of the country, earthquakes are more of a curiosity than a real danger.

1. Earthquake Booms, Seneca Guns, and Other Sounds (USGS)