Colorado on a map is a beautiful thing — or rather, I imagine it must be if you’re a mathematician. All those straight lines and right angles showing no regard for the subtleties of the land beneath, but marching instead across hills and valleys like a line of soldier ants.
Earth scientists generally prefer to look at the lines of the land, and Colorado, with the ranges of the Rockies in the west and the Great Plains stretching away in the east, offers us plenty of those.
And as any geographer can tell you, lines on the land rather than the map have a cause; and that cause is often related to geology, to tectonics — and to earthquakes.
Colorado’s Tectonic Setting
Colorado, in the centre of the North American continent, lies far from any of the Earth’s tectonic plate boundaries and thus from the main zones of earthquake activity. But wait…those whipped-cream-peaks that rise up in the west? What are they?
Well, they’re the Rockies. Obviously.
Without going into too much detail (there are various theories) it seems probable that the subduction of an ancient plate at a very low angle beneath the North American continent may have uplifted the vast area of the Rocky Mountains. But, as they say, what goes up must come down and whatever is going on deep beneath the western USA has resulted in the formation of the long, parallel trends of mountains and valleys that we know as the Basin and Range.
These ranges and valleys are part of an extensional setting, whereby blocks subside along normal faults — something which you might realistically expect to produce occasional, if not particularly large, earthquakes. Does this happen? And has it ever?
Colorado’s Seismic History: Rockbursts and the Trinidad Earthquake Swarm
Given its setting, Colorado is suspiciously quiet, seismically speaking. A search of the United States Geological Survey’s earthquake database (which may be incomplete, given that monitoring has increased over time and some earlier earthquakes may not have been recorded) lists just 394 tremors of at least magnitude 2.5 since April 1915 — that’s less than four each year.
To place that in context, California had 11 in the week prior to the time of writing and no-one in the Golden State will have turned a hair; while Oklahoma’s frenzy of wastewater injection (probably) produced 27.
The USGS’s earthquake history page for Colorado describes (or so it seems) pretty much every one of them and is well worth a read if you’re struggling to sleep one night. It’s easier to look at the database, form which we can comment on the earthquake pattern shown on the map.
The first observation shouldn’t really surprise anyone, given the steep nature of the terrain and the human history of the area: it’s that very many of the recorded events are rockbursts or explosions associated with mining. But that also means that if you strip them out the levels of natural seismicity are even lower than they first appear.
It’s interesting that the largest tremor on the map is also human-induced (an explosion of M5.4 in 1973). The largest in historic times (in 1882) predates the development of modern seismology and is estimated to have had a magnitude of M6.6 — significantly larger than anything on the map. Remember this: it’s important (even though the earthquake may actually have had its epicentre in neighbouring Wyoming).
A second area with much going on is the area west of Trinidad, where hundreds of small earthquakes have been recorded, many of them recently. A paper by Meremonte et al in 2002 linked a sequence of these, observed in August-October 2001, to the disposal of wastewater. Sound familiar? It should. Such deep injection of wastewater has been shown to reactivate faults in Oklahoma and elsewhere. It’s worth noting that a significant number of these have occurred in the last five years — including one of M5.3 in August 2011
Suspiciously Quiet? The Rio Grande Rift
Meanwhile, in the Rockies, there has been what almost amounts to seismic silence. The Rio Grande Rift, an extensional feature running from Colorado down into Mexico, might be expected to produce significant earthquakes as the land subsides — but there’s nothing significant on record.
This might seem to suggest that the rift, for some reason, does not produce earthquakes; but that is not the case. Utah State University, in its FAQs about the Rio Grande Rift, observes that: “There is geologic evidence that multiple major earthquakes (7.0 to 7.5 magnitude) have occurred in Colorado and New Mexico within the past 15,000 years. Although it is less likely to have large-scale seismic activity than regions such as the San Andreas Fault in California, a large earthquake (7.0 magnitude or larger) will occur in the Rio Grande Rift area at some point in the future”.
Colorado Ain’t Finished Yet
So what should we make of Colorado? Human activity has been a significant contributor to the observed levels of seismic activity which have occurred there within the state’s recorded history. But longer-term studies and the overall tectonic setting suggest that these are minor in the context of the power of nature.
Remember that M6.6 of 1882? Depending on its exact location (which is uncertain), it may have something to tell us — and even if it isn’t associated with the Rio Grande Rift it’s a reminder that large earthquakes aren’t just a thing of the distant past.
“The next large earthquake to strike the RGR will be a complete surprise,” warns seismologist Robert Yeats, “and will cause more casualties if building codes of major cities have not been upgraded.” Such an earthquake may not strike in Colorado — the Rio Grande Rift is, after all, a continental-scale feature — but the state should be prepared in case it does.