Watching the Wasatch: Earthquakes in Utah

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Populated areas of Utah are vulnerable to a major earthquake. Image by NASA.

I have some (possibly) surprising news for you. Utah, sitting plumb in the middle of the continental US and a long, long way from the nearest tectonic plate boundary (which, by the way, is the San Andreas Fault Zone) is ripe for a large earthquake.

Don’t believe me? Let’s have a look at the evidence.

Utah’s Tectonic Setting

First up, what’s the tectonic setting of the Beehive State? Tectonically, it’s a part of what geologists term the Basin and Range province, an area which extends for the better part of 1000km, from west Nevada to eastern Utah, and from Idaho to New Mexico.

This area is dominated by extensional tectonics (where blocks of crust move apart) and characterised by typical topography of broadly parallel mountain ridges separated by wide, flat-bottomed valleys — which is where the

Major faults in Utah are at risk of rupture. Image by USGS

name of Basin and Range comes from. This topography is also accompanied by faults which trend in the same direction.

In Utah, the major fault is the Wasatch Fault, which bisects the northern half of the state for around 350km. Though by no means the only fault in the state — there are many other mapped faults and almost certainly many more which haven’t yet been identified — it’s the most significant, not least because, as the USGS points out, the fault zone runs through an area which “is home to nearly 80 percent of Utah’s population of 3 million and more than 75 percent of Utah’s economy.

Significantly, this includes Salt Lake City, which is built on the sediments of a former lake bed — terrain which is particularly vulnerable to earthquakes because the softness of the ground amplifies shaking and potentially increases damage.

Earthquake History

The USGS archive includes the major recorded earthquakes in Utah since 1900. Image by USGS.

A Purdue University Study of 2002 notes that the Wasatch Fault Zone has been relatively quiet in recent years, though it also acknowledges that there’s evidence of prehistoric seismicity. Some of these earthquakes — the more recent ones — are documented. The USGS’s interactive earthquake archive and catalogue includes seven of at least M5.0 in Utah since 1900, the largest being M6.5 (which doesn’t seem very big at all). The most recent of them, in 1989, had a magnitude of M5.4.

But that’s by no means the whole story — not least because the 117 years from 1900 to the present is very little, and we don’t know for certain how many large earthquakes occurred before records began— though we can be sure of their existence. As the USGS notes: “Earth scientists have shown that the Wasatch Fault has repeatedly experienced strong earthquakes of magnitude 7 or larger and will continue to do so in the future.”

In an area with a limited history of settlement with written records, it’s inevitable that the earthquake history will be both limited and inaccurately reported. Studies of physical deformation and other topographical features have established that the absence of recent major earthquakes isn’t an indicator that they don’t occur.

The Future For Utah’s Quakes: What Next?

Forecasting earthquakes, at least in specific terms, is a mug’s game. You might try to pick a time and a place and a magnitude for an earthquake in Utah, and you might get lucky — but most of the time, you won’t.

That doesn’t mean we can’t be sure that earthquakes occur,

These are the forecast probabilities for a large earthquake along the Wasatch Fault Zone. Image by USGS.

and measurement of strain and deformation indicates that, at some point in the future, the one of the active segments of the Wasatch Fault Zone will give way, and an earthquake is inevitable.

Recent work from the snappily-titled Working Group on Utah Earthquake Probabilities has come up with some forecasts — not of when or where an earthquake will occur (which would be a prediction) but of the likelihood of an earthquake occurring in a particular area within a particular timescale (which is a probability forecast).

It makes alarming reading (see the full probability table, in the attached image). The headline is that, somewhere on the Wasatch Fault Zone, “there is a 43 percent probability [of] at least one M6.75 or greater earthquake in the next 50 years… a 57 percent probability of one or more M6.0 or greater earthquakes in the region in the next 50 years.” The greatest chance is not on the Wasatch fault itself.

This risk is big. In its reporting on the study, the Salt Lake Tribune reports lead author Ivan Wong as saying: “The hazard and thus the risk on the Wasatch Front is higher than anyone in the working group had anticipated… The attitude that the ‘big one’ will not occur in one’s lifetime and that the threat can be ignored must change.”

Man, Nature and Probability

It’s tempting to become obsessed by the possibility of Utah’s own “Big One”, but in a review of the state’s seismic activity, it’s sometimes interesting to look at the small stuff, as well. An interesting study on Utah’s seismicity, published in December 2016, distinguishes between the many natural (small) earthquakes recorded and those which result from human activity, most significantly mining.

This is of particular interest in the light of the forecasts above, which were published in April of 2016. The later study concludes (according to a report on the American Geophysical Union’s blog) that “mining-induced events, which do not signify a larger earthquake is on the way, can confuse researchers generating hazard maps. If they count mining-induced events as tectonic earthquakes, they could predict larger earthquakes more frequently than they should.

I don’t know whether these mining-induced earthquakes have been considered in the Working Group report, but as probabilities are constantly updated as new evidence emerges, they may very well be.

One thing’s for certain. We won’t know how accurate these probabilities are until a large earthquake occurs.

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