Moderate Earthquake Rattles North-West Nevada

Moderate Earthquake Rattles North-West Nevada

Mw 4.6, 63km ESE of Lakeview, Oregon, 07:23 UTC, 05/11/14.

A moment magnitude 4.6 (body-wave magnitude 4.6) earthquake has struck the Sheldon Contiguous Study Area in north-west Nevada. This is the just the latest in a large swarm of minor earthquakes that has been rumbling on since mid-July. The earthquake had a focal depth of 8 kilometres and was centred about 63 kilometres east-south-east of Lakeview, Oregon. The quake was the result of normal faulting, with a minor strike-slip element; the strike of the fault is roughly north-north-east to south-south-west.

In the past few days the earthquake swarm has surged in intensity after a short period of inactivity, with two earthquakes of Mw 3.9 (30^th October) & Mw 3.8 (4^th November) the main quakes in this new period of activity. The largest earthquake previous to this appears to have been an Mw 4.0 event on the 1^st October. All three of these events have been the result of oblique normal faulting, indicating that the quakes are the result of tensional forces within the tectonic setting of this region.

The greater part of Nevada is a small section of what is generally known as the Basin & Range Province, stretching from north-western Mexico all the way up to south-east Oregon. Boundaries of the region in the western United States are the Sierra Nevada Mountains to the west and the Wasatch Fault to the east. A magnitude 6.0 earthquake which struck the town of Wells, Nevada in 2008 was the largest earthquake to occur in this region since 2000.

A swarm similar to the Sheldon Area Sequence occurred in 2011. The Hawthorne Swarm of March to May 2011 included several events over magnitude 4, the largest a magnitude 4.6 on the 16^th April 2011. Most of these events occurred at depth of 10-15 kilometres, although later on in the swarm events occurred at shallower depth of between 2-5 kilometres.

The Sheldon Area Sequence has now been going on for nearly four months and has included many moderate earthquakes of the high-magnitude 3 to low-magnitude 4 range. Today’s earthquake may well indicate the height of seismic activity in this sequence, but there have been quite a few aftershocks so far to today’s earthquake (including a moment magnitude 3.5 quake three hours after the Mw 4.6*). This swarm is certainly something of interest to keep an eye on for the weeks to come.

*This aftershock fits the Gutenberg-Richter Law of aftershock magnitudes relating to their mainshock almost perfectly, being just over 1 magnitude below the mainshock’s.

This article was written on the 5^th November 2014 by J H Gurney.