Second Moderate Earthquake In 24 Hours Rattles North-West Nevada

North-West Nevada Swarm Still Energetic

Mw 4.3, 68km ESE of Lakeview, Oregon, 08:34 UTC, 06/11/2014

A moment magnitude 4.3 (body-wave magnitude 4.5) earthquake has struck the Sheldon Contiguous Study Area in north-west Nevada. This is the second-largest earthquake to strike in a nearly four-month long swarm that has rattled this rural part of the American West. The earthquake had a focal depth of 8 kilometres and was centred 68 kilometres east-south-east of Lakeview, Oregon. The quake was the result of a normal faulting mechanism, with a major strike-slip element; the strike of the fault plane was roughly north-east to south-west.

After yesterday’s Mw 4.6 earthquake a large number of moderate high-magnitude 3 earthquakes have afflicted the region, including an Mw 3.9 event at 15:18 UTC on the 5^th November, with four more earthquakes of similar size striking today (including a preliminary local magnitude 4.2 at 15:58 UTC). This is a much more energetic phase within the sequence than at any point in the past and may well indicate that we are in the peak of activity within the swarm. A similar situation was seen during the Hawthorne Sequence of 2011 where eight earthquake over magnitude 3.5 struck within a 36-hour period (15^th to 16^th April 2011), including a magnitude 4.6 event.

Only the largest events have been felt people, mainly due to the extremely rural nature of this region and the timing of the largest quakes (from the late hours of night until the early hours of the morning). If this swarm had been occurring in a less rural region then it is likely that many people would have felt even some of the smaller shocks (M2.0-3.0), mainly because of the extremely shallow depths of these earthquakes. Examples of this are present throughout the world, but were highlighted recently by the Christchurch earthquakes, where small quakes of around magnitude 2.7 have been felt very well in the southern and eastern suburbs.

My personal belief is that this swarm will perhaps continue at this sort of intensity for another 12-36 hours before beginning to fade away. This does not mean there will not be further magnitude 3.5+ events weeks or even months from now, but the frequency with which these earthquakes strike should decrease over time. Unlike the earthquakes in central and northern Oklahoma, these earthquakes are tectonic in nature; therefore there is a limit as to how much energy can be released as the stress being released cannot be quickly accumulated again.

It is plausible, though less likely than the scenario described above, that the swarm will continue at similar intensities for longer than the 36-hour window I predict. This would be of great interest as seismic observations in this part of Nevada are scarce and may help answer some of the questions regarding the causes of the seismicity in this part of the United States. In this scenario the swarm would continue at similar intensities as at present, punctuated by several magnitude 4 events, before losing energy very quickly with the frequency of earthquakes fading fast.

A far less likely scenario is that this swarm is a precursor for a much larger event, perhaps in the order of magnitude 5-6. This is considered very unlikely as there is no evidence currently that the fault or faults rupturing in this swarm are large enough to generate such an earthquake. Nevertheless similar occurrences have happened in the recent past, with a large swarm preceding the Mw 8.1 Iquique Earthquake in northern Chile. This is extremely unlikely to occur as the Nevada quakes are not located near a subduction zone and therefore the chance of this swarm preceding a larger earthquake is extremely remote.

This article was written on the 6^th November 2014 by J H Gurney.