A Fault Line

Seismologists are in the process of establishing a dashed line.   The line is being drawn through the mountains less than a dozen miles from my cabin.  The line will define the location of a previously unmapped strike-slip fault—a fault that sees two sections of the Earth’s crust grating against one another like a pair of cars side-swiping in a tight parking lot.  The result is something of a rumbling and rolling earthquake.

This all started back on July 6 when a magnitude 5.8 rattled the hell out of everyone within several hundred miles of the Lincoln epicenter.  That was the largest quake Montana has experienced in 40 years. Since then, according to earthquaketrack.com, activity near Lincoln has been fairly constant.  As of this morning, the Lincoln area has seen 20 earthquakes in the last 7 days, 61 in the past 30 days.  The other day, a magnitude 3.5 rumbled through.

There are three types of faults.  One type is the strike-slip fault such as the one near Lincoln.  A second type occurs where two sections of the Earth’s crust abruptly pull apart, forming a valley.  The third type of fault thrusts one section of curst overtop another. 

Determining where the strike-slip fault lies is detailed work.  In addition to studying seismic graphs, scientists study creeks and rivers to see if they can identify points where the flow suddenly jogs sideways: an indication that the landscape has shifted laterally underneath them.  

At the same time, this is nothing new in these parts.  Many of our iconic mountain ranges were thrust skyward by thrusting earthquake faults. The Big Belt Mountains, rising immediately behind my house, are an example.  In those mountains you will find, at 8,000 feet, tickled by passing clouds, the limestone sea beds (complete with sea shell fossils) of an ancient ocean upheaved by eons of earthquake activity.

We are still in the process of making mountains and shifting rivers out here.  Unfortunately, it’s something akin to making sausage.  The process is not all that pretty.  Following the magnitude 5.8, I found cracks in the concrete slabs of my driveway some 40 miles from the epicenter, cracks in the concrete siding at the base of my cabin walls, and waking to the shaking and rumbling was a bit unnerving.

--Mitchell Hegman.