By David Jacobson, Temblor
At 3:59 p.m. local time, a M=5.0 earthquake shook the island of Spitsbergen, the largest island in the Svalbard archipelago in the Arctic Circle. According to the USGS, the quake occurred at a depth of 7.7 km, and was centered 133 km from Longyearbyen, the archipelago’s largest settlement, which is home to a little over 2,000 people. This M=5.0 quake was preceded several hours earlier by a M=3.0. While very few people likely felt this earthquake, it highlights a lesser-known area, and the idea that seemingly inactive faults can rupture in earthquakes.
The Svalbard archipelago represents an uplifted portion of the Barents Sea, which separates Svalbard from Northern Europe. To the west of the archipelago is the Mid-Atlantic Ridge, and the Spitsbergen Fracture Zone, a divergent plate boundary between the North American and Eurasian plates. Much of the seismicity in this part of the Arctic Ocean occurs on or near the Mid-Atlantic Ridge, which has a spreading rate of approximately 2.5 cm/yr (1 inch). However, by examining the historical seismicity of the island of Spitsbergen and the rest of Svalbard, it is clear that this remote archipelago is no stranger to earthquakes.
Along the western margin of the island of Spitsbergen is the West Spitsbergen Orogenic Front, a series of both compressional and extensional faults. If the location of today’s earthquake is accurate, it likely occurred on a compressional fault, based on data from the Norwegian Polar Institute. However, from the historical seismicity shown above, western Spitsbergen is nearly seismically inactive. What this potentially shows is that this ancient thrust belt is still capable of generating moderate earthquakes.
Another aspect of the historical seismicity that is worth addressing is that the earthquakes are extremely scattered around the southern part of the archipelago. Why this is the case, we must concede we don’t really know. This remnant of oceanic crust represents a relatively stable continental platform close to active spreading centers. Furthermore, journal articles on the seismicity of Svalbard are sparse, and most suggest that the earthquakes are associated with stress related to the spreading ridge to the west. However, scientists still appear to be searching for answers to explain the spatial orientation of historical seismicity. It should also be stated that there does not appear to be a recent increase in the number of earthquakes in the region due to global warming. While glacial unloading has been shown to cause earthquakes worldwide, it does not appear to be taking place here.
Even though there may be no concrete explanation for earthquakes in Svalbard, it should be pointed out that the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor, picks up this area as a zone capable of experiencing earthquakes with moderate magnitudes. Therefore, today’s earthquake should not come as a surprise, but more highlights the importance of understanding where earthquakes are possible, and that potentially inactive faults can still rupture in earthquakes.
European-Mediterranean Seismological Centre (EMSC)
Norwegian Polar Institute
B.J. Mitchell, H. Bungum, W.W. Chan and P.B. Mitchell, Seismicity and present-day tectonics of the Svalbard region, Geophys. J. Int. (1990) 102, 139-149