Whodunnit? What Caused the M5.2 Arizona Earthquake of 29 June 2014

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Home / Whodunnit? What Caused the M5.2 Arizona Earthquake of 29 June 2014

Location of the 29 June Arizona/New Mexico earthquake, northwest of Lordsburg, New Mexico. Image credit: USGS

Sunday’s magnitude 5.2 (M5.2) earthquake on the border between New Mexico and Arizona is, at present, something of an enigma.

With no information available from the USGS as to its cause, this quake north-west of Lordsburg New Mexico is worth looking at possible culprits for this, one of the largest earthquakes to occur in either of the two states.

The Usual Quake Suspects: Tectonics

Most earthquakes are caused by movement along existing faults. States such as California, Oregon and Nevada are affected by the uplift of the Rocky Mountains and by the relative movements of the Pacific and North American plates.

Even in the eastern and central states, natural or human-induced stresses can reactivate ancient faults, with small to medium earthquakes not uncommon. Clear examples of this are found in New Madrid and in the Appalachians.

There are many mapped faults in both Arizona and New Mexico, including a broad zone stretching along the eastern margin of the Rockies from near the town of Hatch, NM, through the Socorro fault zone right the way north to Denver in Colorado.

This is the location of the state’s largest earthquakes to date (the earliest and probably the largest, in 1906, has no assigned magnitude). Arizona’s largest historic earthquake, an M5.6 in 1959, is also probably associated with faulting along the margin of the Rockies.

The 29 June New Mexico/Arizona earthquake, however, seems not to fit this bill. The area is distant from plate boundaries and although there are mapped fault zones, the epicentre of the most recent tremor was not located directly on any of these. The closest faults mapped by the USGS on the real time earthquake map are the two short lengths of the Pearson Mesa faults, around 20km to the west. Nor are there any clues from the maps of past seismicity or seismic hazard.

Volcanoes can cause earthquakes – but not in this case. Image credit: National Parks Service

Whodunnit: Nature or Humans?

Volcanoes also cause earthquakes – and there’s plenty of evidence of volcanicity in both New Mexico and Arizona. But again there’s a problem in pointing the finger. Arizona’s active volcanic fields are far distant to the west and the rest of the state’s volcanoes, along with those in New Mexico, are either long extinct or haven’t erupted for thousands of years.

This doesn’t mean that they can’t – or won’t – erupt, of course. But there’s no evidence to link the most recent earthquake with volcanic activity. In addition, it would certainly be worthy of note if even a currently-active volcano had triggered an earthquake of so large a magnitude.

So was it humans? Human-induced seismicity has been linked to earthquakes in Oklahoma and elsewhere. But the area surrounding the epicentre is not actively or extensively exploited for fracking. Or mining? Again, there’s no obvious candidate – and in both cases, the size of the earthquake is again problematic, with the largest mining-related earthquakes registering around M5 and the largest fracking-related ones possibly slightly smaller (although this area is subject to ongoing research).

New Mexico/Arizona Earthquake Verdict: Not Proven

At the time of writing there’s not enough available information to point the finger conclusively at one cause or another, although the emergence of further data may well shed more light upon the event.

Reviewing the location (at the edge of the Peloncillo Mountains) and depth (5km) of the tremor in the light of geology and economic activity leads to the conclusion that the probable cause of the tremor was slippage along a previously unmapped fault.

As in all the best whodunits – it looks as if it was the obvious suspect all along.

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