Earthquakes, Seismic Waves, and the New Madrid Fault

How far do earthquake waves travel, and what can cause different quakes to be felt at a greater or lesser distance?

This is a fine question, and before we get started, feel free to click these links for some background information from various earlier posts including: tectonics; earthquakes; an introduction to earthquakes; the Richter Scale; and the tectonic setting of the west coast.

Early on 20 December 2022, there was a 6.4 magnitude earthquake just north of the Mendocino Triple Junction near Ferndale, California — this is where the San Andreas fault to the south joins the remains of the East Pacific Rise that runs northward along the floor of the Pacific (starting with the Mendocino Transform Fault in the image, above), and the Cascadia Subduction Zone. There were also a dozen or so lesser aftershocks. Two lives were lost and many were without power for several hours.

At the time, an article came out of southern Oregon that discussed the quake, and how it was pretty much missed by those in the Rogue Valley (Grants Pass to Medford along I-5). It is approximately 120 miles from the epicenter of the main shock to Grants Pass, Oregon as the crow flies (or the seismic waves propagate). I’m somewhat south of Grants Pass, and Susie and I also completely missed any of the ground motion — yes, it was the middle of the night, but we’re old and light sleepers, and if our bed had started shaking, one of us would surely have noticed.

So, why did a respectable quake only a hundred miles from our bedroom fail to cause any noticeable ground motion at all?

As usual, the answer is absurdly simple, and one that we can all understand with an easy, two-part lab experiment. Materials you will need:

            4” X 4” X 48” wooden post

            2 ripe cantaloupes

            Croquet mallet (or sledge hammer)

            Handsaw (or circular saw)

            Cleaning supplies

            Notebook and pencil

Test #1: Start by placing the 4 X 4 post on the ground (I suggest doing this outside), and putting one (1) of the ripe cantaloupes snug against one end. Move to the other end with the mallet, take careful aim, and smack the end of the post as hard as you can. What happened to the cantaloupe? Record your observations in your notebook (and you likely now understand the need for the cleaning supplies).

Test #2: Using the saw, cut the 4 X 4 post into 48 equal pieces, each one inch long (minus the thickness of the saw blade, called the “kerf”). Carefully line up the blocks on the ground so the entire “post” still measures exactly 48” from end to end, leaving a gap between each block the width of the saw kerf. Place the second cantaloupe snug against one end, and then smack the other with the sledge, just like in Test #1. Record what happened to the cantaloupe. (Lab Report guidelines: Leaving the “Observations” section of your lab report blank instead of writing “Nothing happened to the cantaloupe” is not acceptable.)

So, what will you write in the “Conclusions” section of your report? Something along the lines of: “The solid post transferred the energy undiminished from one end to the other, while the cut post lost a bit of energy at each break, and had completely dissipated before it got to the cantaloupe” would go a long way towards getting you full marks for this portion of your write-up.

Seismic energy, as it is transmitted after an earthquake, works exactly the same way, and — like the energy in our lab exercise — is governed by the physical reality of the materials it passes through.

Here on the west coast of the North American plate (an active continental margin), the subsurface is a complex interplay of different rock types, all separated by broken contacts and faults (like Test #2 in our lab). Sure, we get oodles and scads of earthquakes of all sizes, but the energy has trouble traveling for any great distance due to the intensely broken and variable bedrock. This does not mean that the local effects cannot (and will not) be extensive, just that the damage from any potential quake will be somewhat limited. This also explains why Susie and I were blissfully unaware of the 6.4 magnitude temblor a measly hundred miles away.

Not so in the eastern portions of the North American plate. This is a passive continental margin and part of North America’s relatively solid granitic continental crust (and therefore reacts more like Test #1 from our lab). As such, North America east of the Rockies rarely has any earthquakes (at least before the oil companies started hydraulic fracturing (a.k.a. fracking), but that’s for a later post).

But this doesn’t mean that there are no earthquakes at all! There are, and the rigid crust also guarantees that any quakes that do happen will be felt over a greater distance. A good example is the series of “New Madrid” quakes of December 1811 through early February of 1812. The quakes ranged in magnitude (estimated) from 7.0 to 8.2 on the Richter Scale (estimates vary greatly, but since these predated Richter by over a hundred years, that is not surprising).

Intense effects were widely felt in the central and eastern United States over an area of 50,000 square miles, with lesser effects extending the festivities to over one million square miles—it was reported that the shaking rang church bells in Boston, over a thousand miles away, and was felt by President James Madison and his wife Dolly in the White House.

(For comparison, the 9.2 Alaska quake of 1964 “wobbled” the Space Needle about twelve hundred miles away. While this may sound comparable to the bells in Boston, it may be instructive to note that the Alaska quake was a hundred to a thousand times stronger than the New Madrid event that shook up Dolly and Jim… and it barely made it to Seattle.)

Anyway, it’s probably a good thing that the eastern United States was so sparsely populated at the time. Some of the more obvious surface effects from the New Madrid earthquakes included:

• Radical changes in the land surface, resulting in the creation of new swamps and lakes.

• The Mississippi River actually flowing backwards for several hours.

• Several waterfalls in the Mississippi that disrupted the passage of boats and rafts until they were finally washed away and grade was restored.

• North-south trending ground fissures up to five miles in length that required that settlers drop trees to span them so they could get across.

• Missing residents, apparently swallowed by cracks that then closed over them.

• Sand boils and tar balls.

• Lights flashing from the ground (caused by quartz crystals being squeezed) resulting in a phenomena called seismoluminescence.

• Disturbed domestic animals and wildlife, before (and surely after) the event. This may in part have led to the California Earthquake Commission’s reported study where dogs and cats are under permanent observation, monitoring them for erratic behavior as a potential prediction tool.

Click here for a website from New Madrid, Missouri, that gives greater detail on these and myriad other effects.

So yeah… the surface effects were widespread, although the sparse population at the time limited the economic impact. Unfortunately, times have changed, and a similar series of earthquakes along the New Madrid system today would likely result in a significantly greater impact.

In a report filed in November 2008, the U.S. Federal Emergency Management Agency (FEMA) warned that a large quake along the New Madrid Seismic Zone could result in “the highest economic losses due to a natural disaster in the United States.” The report further predicted “widespread and catastrophic” shaking from a 7.7 magnitude or greater temblor that could cause damage to “tens of thousands of structures affecting water distribution, transportation systems, and other vital infrastructure.”

I think it’s possible that many of us may have trouble awarding too many grains of salt to any prediction from FEMA, but… I think that all of us would (or at least should) accept that “The Big One” in California along the San Andreas system (or north along the Cascadia) would possibly pale against the much wider-spread national tragedy that would result from any significant rupture along the New Madrid Fault Zone.