NORSAR and the seismic monitoring of the European Arctic
Dr. Johannes Schweitzer, Senior Research Geophysicist, NORSAR and Assoc.
Professor, University of Oslo
The viewpoints expressed in this article are solely those of the author and cannot be attributed to the Norwegian Scientific Academy for Polar Research. The Academy hopes that our Newsletter can stimulate debates among our members and partners, which may be edited and posted on our website.
On 29 March 2016, at 12:32 local time the Norwegian settlement Longyearbyen on Spitsbergen, Svalbard Archipelago was shaken by seismic waves caused by a magnitude 5.1 earthquake at a distance of about 130 km, east-southeast in the Storfjorden region close to the coast of Edgeøya. This time of the year is high season for winter tourism on Spitsbergen and beside the local inhabitants many visitors experienced this event in the middle of the day (see also Svalbardposten from 1 April 2016).
Stiftelsen NORSAR (http://www.norsar.no), the Norwegian institute for earthquake research in Kjeller, has a long tradition in monitoring and studying seismic activity in the European Arctic, first only with its installations in southern Norway and then with additional stations in the European Arctic. The 29 March 2016 earthquake was a good reminder that the Earth’s crust in the European Artic is more dynamically active than in Fennoscandia and that our knowledge about the potential seismic hazard in this region is still limited.
Figure 1 shows the network of permanent seismic stations in the European Arctic. The stations in red are NORSAR’s stations, the stations in orange are international stations, which were installed or upgraded in joint projects together with NORSAR, and the stations in white are owned and operated by other institutions. The following text will describe the development of this network, focusing on NORSAR’s involvement.
NORSAR has always had a special interest in monitoring the European Arctic. The large aperture (about 110 km) Norwegian Seismic Array (NORSAR), which gave its name to the whole institute, was built in southern Norway in the late 1960s, after a bilateral agreement between Norway and the USA. A seismic array consists of many seismic sensors of common characteristics installed in a specific geometry to be used as an antenna for seismic signals. Data from all sensors are then jointly analyzed as if they were recorded by one single sensor. The main purpose to build the 132 seismometer-site large NORSAR array in southern Norway as written in the bilateral agreement “is seismological research and experimentation. The system is primary designed to produce data valuable as a means of 2 detecting and distinguishing between signals originating from underground explosions and from other sources, especially earthquakes.”
Any type of explosion or implosion radiates elastic waves, which travel through the solid Earth, the oceans or the atmosphere and which can be recorded by seismometers, hydrophones or microbarographs. But at all times, these sensors are recording signals either from natural or many man-made sources, and the biggest challenge to monitor the CTBT is to separate between all other signals from the possibly quite small signals of a clandestine nuclear test. To fulfill this task, sources of all seismic signals have to be located and screened out as non-problematic. Therefore, seismology has largely profited from this challenge and the need for highly sensitive sensors, better algorithms to analyze and discriminate seismic signals and to locate their sources, better knowledge of seismic activity in the whole Earth, and more accurate Earth-structure models.
Data from the NORSAR array enabled monitoring of nuclear tests worldwide, including the activities on the territory of the former Soviet Union. Many of these activities were carried out in the European Arctic, on Novaya Zemlya, on the Kola Peninsula and in northern Siberia. Although the Russian Federation has ratified the international Comprehensive Nuclear-TestBan Treaty (CTBT), Novaya Zemlya is still, as all the other former nuclear-test sites of the other nuclear powers, of special interest for the international community for monitoring the compliance with the test-ban treaty.
Figure 1: The network of permanent seismic stations in operation in the European Arctic in 2016. The locations of arrays are shown as circles and of single stations as triangles. NORSAR’s stations are with red, stations installed or upgraded in joint projects between NORSAR and international partners are with orange, and all other stations in the European Arctic are with white symbols. BJOA (blue) stands for the planned new array on Bjørnøya. For further details see text.
After some time, it became clear that the large NORSAR array was a perfect but quite expensive instrument for monitoring seismic events at large distances. In 1976, the original array was reduced to an aperture of about 60 km with 42 sensor sites, which still kept the most sensitive parts of the original installation. However, it became also clear that these large aperture arrays were not optimal to monitor regional or local seismicity, which is needed for the European Arctic. Therefore, the concept of regional, small aperture arrays was developed and tested at NORSAR in joint projects with colleagues from the USA. After reaching a final design, the first such array (NORES) was installed in southern Norway in 1984.
Then in 1987, a second small aperture (3km) array (ARCES, Fig. 1) with 25 sensor sites was built close to Karasjok in northern Norway. ARCES was the first seismic array installation in the European Artic and data from this instrument opened new insight into the seismic activity of this region.
During the last years of the Soviet Union, NORSAR researchers came in contact with seismologists working at the Kola Regional Seismological Center (KRSC) of the Kola Branch of the Russian Academy of Sciences in Apatity. In the framework of a joint project, the old analog seismic station in the town of Apatity was upgraded with modern digital equipment in summer 1991 (APZ9, Fig. 1), and a second seismic array in the European Arctic was installed on the Kola Peninsula close to Apatity in autumn 1992 (APA0, Fig. 1). With an aperture of about 1 km and 9 seismic sensor sites this array is smaller than ARCES.
Also in autumn 1992, NORSAR built a second 9-element array in the European Arctic about 14 km southeast of Longyearbyen, on Jansonhaugen, in Adventdalen, Spitsbergen (SPITS, Fig. 1). NORSAR’s network of seismic stations was further enlarged with a new single seismic station on Jan Mayen (JMIC, Fig. 1) in autumn 2003.
Then in 2007, NORSAR had the lead in an international project of the International Polar Year 2007-2008. In this context, the already existing seismic station at Hornsund (HSPB, Fig. 1), the Polish Polar Research Base in the south of Spitsbergen, was upgraded to a modern broadband station as a joint activity between the Institute of Geophysics of the Polish Academy of Sciences and NORSAR in September 2007.
Three years later, in September 2010, the seismic station in Barentsburg (BRBA, Fig. 1), the Russian settlement on Svalbard, could be up-graded as part of a project within the POLRES program of the Research Council of Norway in a joint initiative between NORSAR and the colleagues from KRSC, and one year later, a second seismic station (BRBB, Fig. 1) was 4 installed close to BRBA. Today’s configuration of NORSAR’s network of seismic stations in the European Arctic was reached when NORSAR installed two more single stations in Finmark, one in Jettan in November 2014 (JETT, Fig. 1) and one co-located with the infrasound array IS37 in Bardufoss (I37H0, Fig. 1) in July 2015.
Today, NORSAR is not only operating two arrays and three seismic stations in the European Arctic, but has also regular data exchange with colleagues in Russia and Poland from the jointly installed stations. In addition, NORSAR is contributing its data to the Norwegian National Seismic Network (NNSN) operated by the University in Bergen and has access to all NNSN data. NORSAR is also exchanging data with the operators of seismic stations and arrays in Finland and Sweden. Three of NORSAR’s seismic installations (ARCES, SPITS and JMIC), the infrasound array in Bardufoss and a radionuclide station on Spitsbergen (Plateauberget, near Longyearbyen) are part of the International Monitoring System (IMS) in the European Arctic, which has been built up during the last 20 years to monitor compliance with the CTBT.
Figure 2: NORSAR’s analyst reviewed seismic events between October 1991 and June 1998. The locations of 6060 events observed and located with at least two seismic arrays are shown.
In parallel to the extension of the monitoring network of seismic stations, the automatic analysis algorithms for data from seismic arrays and single stations data were developed further. The routine analysis of seismic data at NORSAR is organized in two steps: at first an algorithm is making a fully automatic analysis of the incoming data in near-real time and 5 then an analyst is going through (reviewing) the most interesting automatic results and reanalyzes the data. Fig. 2 shows the recorded seismicity for the time period October 1991 until June 1998. Only events, which had been reviewed by a NORSAR analyst and which had been located with observations from at least two seismic stations, are shown. For this time period, most of the events shown were explosions or mining induced events at the mining and quarry areas in northern Sweden, Finland and the Russian Federation observed by the seismic arrays ARCES and Apatity. In addition, the seismicity along the mid-ocean ridges between Jan Mayen and Svalbard and a lot of diffuse activity in the Svalbard Archipelago could be observed.
Between summer 1998 and spring 1999, the main software package for the reviewing analysis, as well as several parts of the automated processing were upgraded, and NORSAR’s reviewed bulletin started again in October 1998. From then on, only events with a magnitude of 2 or larger were reviewed. Fig. 3 shows the analyst reviewed events for the period October 1998 – December 2007. The upgrade of the automated analysis led to a much better integration of SPITS observations. The new seismic station on Jan Mayen and the (mutual) data exchange with other seismological institutes improved the monitoring capabilities. This all resulted in better monitoring possibilities for the mid-ocean ridges between Jan Mayen and the Svalbard Archipelago, a less diffuse distribution of seismicity on Svalbard and a reduced number and less scatter of seismic events in the known mining and quarry areas.
Figure 3: NORSAR’s analyst reviewed seismic events from October 1998 until December 2007. The locations of 4103 events observed and located with at least two seismic arrays and with a magnitude of 2 or larger are shown.
After the installation and/or upgrading of more stations during and after the IPY, and the dramatic progress in automated exchange of seismic data between station operators in the European Arctic (Fig. 1), the quality of NORSAR’s reviewed bulletin changed again. Fig. 4 shows the reviewed events following the above mentioned criteria (observations from at least two stations, magnitude 2 or larger) for the period 2008 until March 2016. Comparing Figs. 3 and 4 clearly shows that the whole picture becomes even clearer. Seismicity is observed at specific areas, which are much better defined.
Figure 4: NORSAR’s analyst reviewed seismic events from 2008 until March 2016. The locations of 4547 events observed and located with at least two seismic arrays and with a magnitude of 2 or larger are shown.
The steady increase of observed seismic events in the European Arctic during the last 25 years and the improved accuracy of event locations in the region are obvious. The seismicity along the northern end of the mid-Atlantic Ridge system, which is located west and north of the Svalbard Archipelago, has been known since a long time. Little knowledge existed however about the seismicity of Svalbard and the whole Barents Sea. Today’s understanding is mostly achieved by the much denser network of seismic stations and arrays in the European Arctic and the free exchange of seismic data between institutions operating these installations. NORSAR’s activities significantly contributed to this development.
But also temporary installations with NORSAR’s participation contributed significantly. In the late 1970s and early 1980s, NORSAR became involved as project partner in several field campaigns to operate temporary seismic stations in the Svalbard Archipelago; the other project partners were the St. Louis University in Michigan, USA and the Norwegian Polar Research Institute. During the NORSAR led IPY project, temporary stations had also been installed: a small aperture array of 13 sites was temporarily operated on Bjørnøya jointly with colleagues of the University in Potsdam, Germany, and in cooperation with the AlfredWegener Institute in Bremerhaven, Germany, 12 Ocean Bottom Seismometers (OBSs) were deployed in the sea west of Bjørnøya.
Another important component is the increased international cooperation. In particular the joint project with KRSC to build the Apatity array became the base for more than 25 years of successful cooperation in monitoring and classifying seismic events in the European Arctic between NORSAR and KRSC. During the last years, the contact between NORSAR and Russian colleagues was further strengthened by starting a new cooperation with the Federal Center for Integrated Arctic Research of the Ural Branch of the Russian Academy of Sciences in Arkhangelsk. Within the NORRUSS program of the Research Council of Norway, a joint Norwegian-Russian project between the three institutes to compile a joint seismic bulletin for the European Arctic was funded for the years 2014 – 2016.
Due to the increasing interest from the public and policymakers in the Arctic, the need for understanding the natural processes in the Earth’s crust in this region has also increased. NORSAR is very much interested in contributing to solve this riddle. Since many years, NORSAR has been planning to build a third permanent seismic array in the European Arctic on Bjørnøya (BJOA, Fig. 1). The potential of such an array to improve monitoring could be clearly demonstrated with the temporary data recorded during the IPY. Recently, the Research Council of Norway funded this idea within the framework of the Norwegian contribution to the European research infrastructure initiative EPOS (European Plate Observing System) and NORSAR hopes to realize this project in the near future.