M3.6 Earthquake in Dallas, Texas: What Was the Cause?


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A series of small earthquakes has shaken the outskirts of Dallas, TX. What’s causing these tremors? Image by USGS.

So now it’s Texas.

With an ongoing earthquake swarm in Oklahoma spreading into Kansas, it’s tempting to look at the magnitude 3.6 (M3.6) earthquake which struck just to the northwest of Dallas on 6 January as an extension of this activity — but is it?

The M3.6 Dallas Earthquake

The 6 January Texas earthquake was the largest in a series of small tremors (between M1.6 and M3.6) which began with a single event on 2 January and escalated on 6 January; the series of aftershocks may well continue beyond the time of writing.

The United States Geological Survey’s event page shows that light shaking was recorded in and around the city of Dallas, which has a population of almost 2.5 million, although local media reported no damage or injuries.

Texas Earthquakes: Unusual?

It’s fair to say that Texas isn’t earthquake country. Located well away from the planet’s major seismic zones around the boundaries of the earth’s tectonic plates, it has no major fault zones and is largely tectonically stable.

That doesn’t mean that there are no earthquakes in Texas (very few places on earth are completely immune) because there is regular, low-level movement on existing faults in the rock. But as the map of recent seismicity shows, the Lone Star State is reasonably quiet, with occasional small, shallow earthquakes.

Historically, larger events do occur. The largest recorded tremor in Texas is an M5.8 in 1931 which, (like most of the significant events listed by USGS in the state) occurred in west Texas, an area which is closer to the western Cordillera, an area of seismic uplift.

More recently, however, there’s been an increase in smaller earthquakes elsewhere in the state.

What Caused the Dallas Earthquake?

Texas seismicity 1973-2012

Texas has low levels of seismicity (tremors shown from 1973-2012). Image by USGS.

At present the jury is out and seismologists will want to look closely at the most recent tremors, which, as noted above, are just the latest in a series of small earthquakes to strike across Texas.

Although minor earthquakes are by no means unusual, the USGS summary observes that “Few earthquakes east of the Rockies, however, have been definitely linked to mapped geologic faults… Scientists who study eastern and central North America earthquakes often work from the hypothesis that modern earthquakes occur as the result of slip on preexisting faults that were formed in earlier geologic eras and that have been reactivated under the current stress conditions.”

The last few words are key. Texas, like many areas of the US, is an area where oil extraction is widespread; and elsewhere, seismic activity has been linked both to shale gas extraction (most notably in a study published just this week on fracking and seismicity in Ohio) and to wastewater disposal (currently thought to be the cause of the Oklahoma earthquake swarm). These processes are capable of doing exactly what the USGs describes – reactivating existing faults.

Texas Quake and Human Activity: the Most Likely Cause

On the basis of the available evidence — what we know of this series of tremors and of those elsewhere — it would be surprising if the Dallas earthquake of January 6 and similar recent events in other parts of the state are not associated in some way with human activity. That said, the USGS observes that: “regions with frequent induced earthquakes may also be subject to damaging earthquakes that would have occurred independently of human activity.”

In the light of this, perhaps Dallas — and other previously stable parts of Texas — can expect more low level earthquakes tin the future.

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