Annual report for the Norwegian National Seismic Network

Transcription

1 Annual report for the Norwegian National Seismic Network 2013 Supported by University of Bergen and Norwegian Oil and Gas Association Prepared by Department of Earth Science University of Bergen Allegaten 41, N-5007 Bergen March 2014

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3 CONTENTS 1 Introduction Data availability to the public Operation NNSN Field stations and technical service NNSN achievements and plans The NORSAR Stations and Arrays New developments at NORSAR Systems Recording Performance Detections Combined NORSAR-UiB data analysis Seismicity of Norway and surrounding areas for Velocity models and magnitude relations Events recorded by the NNSN The seismicity of Norway and adjacent areas Seismicity in the arctic area Earthquakes in the southern North Sea Felt earthquakes Explosions Scientific studies Source study of the Jan Mayen fracture Zone strike-slip earthquakes Research plan for PhD project An Application of Coda Envelopes for Estimation of Stable Event Magnitudes in Southern Norway Research plan for the NNSN grants at NORSAR Publications and presentations of NNSN data during Publications Reports Oral presentations Poster presentations References... 47

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5 1 Introduction This annual report for the Norwegian National Seismic Network (NNSN) covers operational aspects for the seismic stations contributing data, presents the seismic activity in the target areas and the associated scientific work carried out under the project. The report is prepared by the University of Bergen with contributions from NORSAR. The NNSN network is supported by the oil industry through the Norwegian Oil and Gas Association and the University of Bergen (UiB). 1.1 Data availability to the public All the data stored in the NNSN database are available to the public via Internet, or on manual request. The main web-portal for earthquake information is It is possible to search interactively for specific data, display the data locally (waveforms and hypocenters) and then download the data. Data are processed as soon as possible and updated lists of events recorded by Norwegian stations are available soon after recording. These pages are automatically updated with regular intervals. 2 Operation NNSN The University of Bergen (UiB) has the main responsibility to run the NNSN and operates 32 of the seismic stations that form the NNSN. NORSAR operates 3 seismic arrays, which also include broadband instruments, and two single seismometer stations (JMIC and AKN) (Figure 1). In total, NORSAR provides data from 12 broadband stations to the NNSN. One station with real-time data is provided from the Ekofisk field by ConocoPhillips. The station HSPB is operated jointly between NORSAR and the Geophysical Institute, Polish Academy of Sciences, Warsaw, Poland and the station BRBA is a collaboration between NORSAR and the Kola Science Centre, Russian Academy of Sciences, Apatity, Russia. In addition to the NNSN stations, waveform data from other selected stations in Finland, Denmark, Sweden and Great Britain are transferred in real time and included in the NNSN database. In total data from 16 stations located in or operated by neighbouring countries are recorded continuously in Bergen and can be used for locating earthquakes. Phase data from arrays in Russia (Apatity), Finland (Finess), Sweden (Hagfors) and stations operated by the British Geological Survey (BGS) are also included when available. The seismicity detected by the network is processed at UiB, however NORSAR also integrates their results in the joint database at UiB. 1

6 Figure 1. Stations contributing to the Norwegian National Seismic Network (NNSN). UiB operates 32 stations (red) and NORSAR operates the stations marked in blue including the three arrays, and stations AKN and JMIC. Data from stations in yellow are received continuously in Bergen, but are operated by other agencies. Data from station EKO1 is made accessible by ConocoPhillips. UiB is in the process of upgrading the NNSN by changing short period (SP) to broadband (BB) seismometers. A further effort is made to install additional high quality digitizers. The current status of this upgrade is shown in Figure 2. As of today the numbers of SP, BB stations and stations with real time transmission are listed in Table 1. Table 1. Overview of UiB seismic stations Short Period Broadband Real time Number of stations (18 with natural period greater than 100 sec) 29 (not real time are 2 short period and 1 broadband stations on Jan Mayen) 2

7 Figure 2. Status of the NNSN stations operated by UiB as of 31 December Left: Overview of digitizers, still to be upgraded are of type Sara. Right: Overview of seismometers. The operational stability for each station is shown in Table 2. The downtime is computed from the amount of data that are missing from the continuous recordings at UiB. The statistics will, therefore, also show when a single component is not working. This is done as the goal is to obtain as complete continuous data from all stations as possible. Also, communication or computing problems at the centre will contribute to the overall downtime. In the case of communication problems, a station may not participate in the earthquake detection process, but the data can be used when it has been transferred. Thus, the statistics given allow us to evaluate the data availability when rerunning the earthquake detection not in real-time. The data completeness for the majority of the stations is above 95%, except for the following stations FOO, SNART, SUE and TBLU (see technical service overview for details). The completeness for KBS and KONO is low due to data exchange problems, and more complete data are available from the IRIS data center. 3

8 Table 2. Data completeness in % for 2013 for all stations of the NNSN operated by UiB. Station Data completeness % Askøy (ASK) 95 Bergen (BER) 100 Bjørnøya (BJO) 99 Blåsjø (BLS) 99 Dombås (DOMB) 100 Florø (FOO) 68 Hammerfest (HAMF) 100 Homborsund (HOMB) 98 Hopen (HOPEN) 98 Høyanger (HYA) 99 Jan Mayen (JMI) 99 Jan Mayen (JNE) 99 Jan Mayen (JNW) 99 Karmøy (KMY) 98 Kautokeino (KTK) 99 Station Data completeness % Kings Bay (KBS) 94 Kongsberg (KONO) 95 Konsvik (KONS) 100 Lofoten (LOF) 97 Mo i Rana (MOR8) 97 Molde (MOL) 98 Namsos (NSS) 98 Odda (OOD1) 99 Oslo (OSL) 98 Skarslia (SKAR) 95 Snartemo (SNART) 90 Stavanger (STAV) 100 Steigen (STEI) 98 Stokkvågen (STOK) 98 Sulen (SUE) 87 Blussuvoll (TBLU) 93 Tromsø (TRO) Field stations and technical service The technical changes for each seismic station are listed below. It is noted if these changes are carried out by the respective local contact and not by the technical staff of UiB. When a station stops working, tests are made to locate the problem. Sometimes the reason cannot be found and the cause of the problem will be marked as unknown. Major changes during this reporting period of 2013 were: Ask (ASK) Bergen (BER) Bjørnøya (BJO1) : Data lost from due to faulty power supply and monitor : Station down from due to PC problems. New PC was installed and the station was in operation from midnight. No visit or technical changes. No visit or technical changes. 4

9 Blåsjø (BLS) Blussvoll (TBLU) Dombås (DOMB) Florø (FOO) Hammerfest (HAMF) Homborsund (HOMB) Hopen (HOPEN) Høyanger (HYA) Jan Mayen (JMI) JNE JNW Karmøy (KMY) No visit or technical changes : Station down from due to PC problems. Local contact was able to fix the problem : Visit. The aluminum box was too small to use the standard insultaion shield for the sensor. The box was insulated on the outside : Visit. The Trillium 120PA sensor was replaced by a Trillium 120P due to signal problems : Station down since due to PC problem. PC replaced by local operator : Visit. Since the power supplied to the sensor has been unstable due to bad connection in the junction box. Data lost. No visit or technical changes : Visit. Sensor centred as it had not been stable : Visit. A new concrete base was made in the aluminium box as the previous one had deteriorated. Trillium 120P changed and thermo shield was installed. New PC installed : The router for communication was replaced. Due to the problem with the router the communication to the station has been unstable since the beginning of November : Problems with GPS antenna. New GPS antenna was sent from Bergen and installed by local staff : GPS signal unstable : New 3G router for GSM communication is installed : New Patton modem installed by base personnel, to avoid telemetry gaps : New Nanometrics accelerometer model "Titan" installed : The PC was replaced with a PC with larger harddrive : UPS installed. No visit or technical changes. No visit or technical changes : Visit. The vertical sensor (SS-1) was relocated from the temporarily location in the house to a permanent site outside and a new cable installed. New GPS antenna and Guralp digitizer installed : Station down between due to bad connection. Problem solved by local operator. 5

10 Kautokeino (KTK) Kings Bay (KBS) Kongsberg (KONO) Konsvik (KONS) Lofoten (LOF) Mo i Rana (MOR8) Molde (MOL) Namsos (NSS) Odda (ODD1) Oslo (OSL) : Digitizer gain remotely changed from 1x to 8x. The station had several problems with digitizers which have been replaced two times by the local operator. No visit or technical changes. No visit or technical changes. No visit or technical changes : Communication/PC problem. Local contact restarted the station Data lost between at 11:00 to at 00: : PC problem. New PC sent from Bergen and installed by local contact. Data lost between at 02:00 to at 16: : Communication down. No data lost. During May 2013 it was observed water in the aluminium-box. The local contact removed the water and tried to seal the box. The data was noisy and all connections and the sensor must be checked. At the time with water in the vault, the data had spikes : Visit. A new concrete base in the box was made and waterproof primer was applied. The sensor was insulated inside the box and insulation was also used outside the aluminium-box. Work was done to drain water from the site : Visit. The sensor (Guralp CMG-6TD) was replaced by one of the same type. No visit or technical changes : Visit: A new GPS antenna, power suply and a digitizer (Guralp CGM-EAM) were installed. No visit or technical changes. Skarslia (SKAR) : Station installation completed and in operation for the first time : Visit by local electrician : Visit by local contractor : Visit for inspection : Station down due to thunderstorm which resulted in a power failure. Power reconnected by local inspector of the renseanlegg : A new digitizer for the GPS was installed. GPS now ok. 6

11 : Visit for inspection and documentation : Hole in concrete tank for grounding electrode wires sealed by local company. Snartemo (SNART) Stavanger (STAV) Steigen (STEI) Stokkvågen (STOK) Sulen (SUE) : Router for communication installed by local operator : Visit. Digitizer (GuralpEAM DM24) installed. GPS antenna replaced :Visit. New PC installed. Data lost between The station was visited several times during the summer. Several attempts were made to improve the signal : The GPS was changed by the local operator : Visit.The sensor was moved from the reception area and reinstalled in the basement of the building : Station down due to power failure. Data lost for this period of time. No visit or technical changes : Visit. Station location and the sensor were found broken due to tree cutting. The station had to be reinstalled. No data until April : Visit: A new site was made and the station was reinstalled. Sensor: Trillium 120PA : Heavy lightning during morning hours caused power failure and communication failure. Station down between th October. The communication to the station was down between 17 th October and 7 th November : Visit. The trillium PA seismometer was replaced with a Trillium P. Tromsø (TRO) No visit or technical changes. 2.2 NNSN achievements and plans The overall purpose of the NNSN is to provide data both for scientific studies, but equally important for the routine observation of earthquakes. This in principle means that broadband seismometers are desired at all sites. However, in areas where additional stations are deployed for local monitoring, short-period seismometers are sufficient. The number of broadband seismometers in the network will be increased to replace existing short period instruments. A general goal for the future development has to be to achieve better standardization in particular with the seismometers and digitizers. The total number of stations for now should remain stable, but it is important to improve the overall network performance. 7

12 2.2.1 Achievements in 2013 Upgrade: Stations KMY, ODD1 and SNART were upgraded with Guralp digitizers. The new station at Skarslia, Ål municipality, was installed 21 March The station has been performing very well and has very low noise levels. Preparations for a new station near Fauske, northeast of Bodø, have started and the installation is scheduled for spring Data from the new NEONOR2 project with temporary deployments in Nordland will be available to the NSNN. At the end of November, 20 of 26 planned stations are operational with communication to 12 of them. All stations are equipped with high quality broadband stations. There is no cost to the NNSN project from this deployment. Research and development: o UiB: A PhD candidate (Andrea Demuth) has started at UiB in September. The PhD project will have a duration of three years. The proposed topics are evaluation of the network, magnitude scales, attenuation tomography and earthquake source parameters. Additional research work has been carried out on the large Jan Mayen fracture zone earthquakes and the Storfjorden earthquakes. Both of these studies are nearly completed. o NORSAR: Work has been carried out on the use of the mb(lg) scale which is based on coda waves. o NORSAR have recruited a researcher to work on the NNSN project. Ilma Janutyte has her education from Vilnius University, Faculty of Natural Sciences and is completing her PhD this year. An accelerometer has been installed in parallel to the seismometer at the JMI site. NNSN website: Progress has been made on the selection of helicorder plots, although this is not yet implemented on the external web pages. The Nordic Seminar on Detection Seismology was arranged in Bergen September 16-18, with 40 participants from all Nordic countries and the Baltic states Plans for 2014 Upgrade: Station OSL will be upgraded with a broadband seismometer. Install the new seismic station at Fauske. This is planned for the early spring The vault was constructed in February Search for a site for a new permanent seismic station in north-eastern Norway that would be installed in Planning for upgrade of the two Jan Mayen stations JNE and JNW will begin depending on agreement with Fiskeri og Kyst Departementet (FKD) and continuation of operations on Jan Mayen. Hopen: The station is to be improved by re-siting and new construction of the vault. The existing STS2 seismometer has to be checked and will temporarily be replaced with a Nanometrics sensor. Timing of this will also depend on the costs involved. Research and development: This activity will continue in close collaboration between UiB and NORSAR. 8

13 Strengthen the collaboration with NORSAR on data processing through technical visits. Collaborate with other Scandinavian countries to improve identification of explosions, particularly in Northern Scandinavia. Revising the questionnaires and the processing of the questionaries for felt earthquakes. 3 The NORSAR Stations and Arrays NORSAR contributes data from a number of stations in Norway to the NNSN, this section gives an overview of the operation of these stations. NORSAR is operating the following installations: NOA (southern Norway, array, 42 sites, 7 3C broadband sensors and 35 vertical broadband sensors) ARCES (Finmark, array, 25 sites, 1 3C broadband sensor, 4 3C short-period sensor and 21 vertical short-period sensors) SPITS (Spitsbergen, array, 9 sites, 6 3C broadband sensors and 3 vertical broadband sensors) NORES (Hedmark, array, 25 sites, currently 9 3C short-period sensors) JMIC (Jan Mayen, 3C broadband sensor ) AKN (Møre og Romsdal, 3C broadband sensor) IS37 (Bardufoss, infrasound array, 10 sites) In addition, NORSAR receive and process data in near realtime from: FINES (southern Finland, array, 16 sites, 2 3C broadband sensor, 1 3C short-period sensor and 15 short-period vertical sensors, operated by Institute of Seismology, Helsinki, Finland) HFS (Hagfors, Sweden, 10 sites, 1 3C broadband sensor and 9 short-period vertical sensors, operated by the Swedish Defence Reseearch Agency, Stockholm, Sweden) EKA (Eskdalemuir, United Kingdom, 20 sites, 1 3C broadband sensor and 20 shortperiod vertical sensors, operated by the United Kingdom National Data Centre, AWE Blacknest, UK) Map of all stations providing realtime data to NORSAR is given as Figure 3. All NORSAR waveform data and parametric data are openly available and can be accessed through webinterfaces or direct means. The NORSAR webpage provides access to general station information, to automatic and reviewed seismic bulletins, to real-time plots of short and long-period data, and to an AutoDRM request form for waveform data retrieval.the seismic array data are automatically processed and analysed. 3.1 New developments at NORSAR a) In 2010 NORSAR and the Kola Science Centre, Russian Academy of Sciences, Apatity installed the seismic station BRBA in Barentsburg. Data from this station are transferred in near-realtime from Barentsburg to Apatity to NORSAR. In 2013 BRBA data have been included into the data stream forwarded to NNSN. The data continuity 9

14 is still somewhat flawed due to communication problems in Barentsburg (poor mobile network coverage), but it is hoped that this will improve in future. b) An infrasound array (IS37) was built close to Bardufoss. This array is part of the International Monitoring System of the Comprehensive Test Ban Treaty Organization and can detect local and regional (up to several 1000 km distance) infrasound sources (surface and atmospheric blasts/explosions, volcanic eruptions, bolides, gas flares from production platforms, etc.). The array consists of 10 sites with microbarometric sensors distributed over about 1.5 square kilometer. c) In 2013 NORSAR included a large number of seismic stations in Scandinavia and in the northern Arctic (all UiB stations, Finnish and Danish stations, including 2 on Greenland). The data of these single stations are NOT processed automatically (NORSAR processes only array data automatically), but are used for manual event localization and characterization for the reviewed event bulletin. 10

15 Figure 3. Map of all stations providing realtime data to NORSAR (red/pink symbols: stations operated by NORSAR, green symbols: stations operated by of University of Bergen, yellow symbols: other cooperating/contributing stations). 3.2 Systems Recording Performance All data recorded at NORSAR are continuous. The following table provides a monthly overview on the data availability of 12 main data streams provided by NORSAR to NNSN. Table 3. Systems recording performance (in % of data completeness) for 12 main data streams provided from NORSAR to NNSN. ARE0 JMIC NAO01 NB201 NBO00 NC204 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec NC303 NC405 NC602 SPA0 AKN HFC2 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Detections The NORSAR analysis results are based on automatic phase detection and automatic phase associations which produce the automatic bulletin. Based on the automatic bulletin a manual analysis of the data is done, resulting in the reviewed bulletin. The automatic bulletin for northern Europe is created using the Generalized Beam Forming (GBF) method. This bulletin ( is subsequently screened for local and regional events of interest in Fennoscadia and in Norway, which in turn are reviewed by an analyst. Regional reviewed bulletins from NORSAR are available from 1989 and from

16 onwards they are directly accessible via internet ( Table 4 gives a summary of the phase detections and events declared by GBF and the analyst. Table 4. Phase detections and event summary. Jan. Feb. March April May June Phase detections Associated phases Un-associated phases Screened GBF events for Fennoscandia/Norway No. of events defined by the analyst July Aug. Sep. October Nov. Dec. Phase detections Associated phases Un-associated phases Screened GBF events for Fennoscandia/Norway No. of events defined by the analyst Combined NORSAR-UiB data analysis Array processing is fundamentally different to single-station processing and there is no straightforward way to merge and commonly process array and single-station waveform data. However, on a higher level parameters like phase arrival readings from array beams and single stations can be combined and be used for event location. At NORSAR the parameters of analyst-reviewed events are converted into parameter files in Nordic format and forwarded via ftp to UiB on a daily basis. The magnitude threshold has been lowered to about M 1.5 for regional events of potential interest for the NNSN. After transferring the parameter files, the NORSAR analyst logs into the the UiB data base using SEISAN and integrates the events. Integration means to merge NORSAR and UiB events, which may require to repick seismic phases, to include new phase readings, to edit double phase readings and to relocate the seismic event with the new parameters. 12

17 4 Seismicity of Norway and surrounding areas for 2013 The earthquake locations presented have been compiled from all available seismic stations as described above. All phase data are collected by UiB, and a monthly bulletin is prepared and distributed. All local and regional earthquakes recorded on NNSN stations are presented on the web pages and the largest are also ed to the European-Mediterranean Seismological Centre (EMSC) to be published on the EMSC web pages. A brief overview of the events published in the monthly bulletins is given in this annual report. Macroseismic data for the largest felt earthquakes in Norway are collected, and macroseismic maps are presented. Local, regional and teleseismic events that are detected by the UiB network are included. The merging of data between NORSAR and UiB is based on the following principles: i) All local and regional events recorded by NORSAR that are also detected by the NNSN network are included. ii) All local and regional events with local magnitude larger than 2.0 detected by NORSAR and not recorded by the NNSN are included. iii) All teleseismic events recorded by NORSAR and also detected by the NNSN are included. iv) All teleseismic events with NORSAR magnitude M b 5.0 are included even not detected by the NNSN. Data from the British Geological Survey (BGS) are included in the database in Bergen following similar criteria as mentioned above, however only events located in the prime area of interest, shown in Figure 1, are included. 4.1 Velocity models and magnitude relations The velocity model used for locating all local and regional events, except for the local Jan Mayen events, is shown in Table 5 (Havskov and Bungum, 1987). Event locations are performed using the HYPOCENTER program (Lienert and Havskov, 1995) and all processing is performed using the SEISAN data analysis software (Havskov and Ottemöller, 1999). Table 5 Velocity model used for locating all local and regional events, except for the local Jan Mayen events (Havskov and Bungum, 1987). P-wave velocity Depth to layer (km/sec) interface (km)

18 Magnitudes are calculated from coda duration, amplitudes or displacement source spectra. The coda magnitude relation was revised in 2006 (Havskov & Sørensen 2006). The coda wave magnitude scale (M C ) is estimated through the relation M C = log10(t) D where T is the coda length in seconds and D is the epicentral distance in km. The new scale made M C more consistent with M L since M C in general is reduced. For this report all data are updated using the new magnitude scale. When instrument corrected ground amplitudes A (nm) are available, local magnitude M L is calculated using the equation given by Alsaker et al. (1991): M L = 1.0 log(a) log(d) D where D is the hypocentral distance in km. The moment magnitude M w is calculated from the seismic moment M 0 using the relation (Kanamori, 1977) M w = 0.67 log(m 0 ) 6.06 The unit of M 0 is Nm. The seismic moment is calculated from standard spectral analysis assuming the Brune model (Brune, 1970) and using the following parameters: Density: 3.0 g/cm 2 Q = 440 f 0.7 P-velocity = 6.2 km/s S velocity = 3.6 km/s For more computational details, see Havskov and Ottemöller, (2003). For the Jan Mayen area, a local velocity model (see Table 6) and coda magnitude scale is used (Andersen, 1987). Table 6. Velocity model used for locating local Jan Mayen events. P-wave velocity Depth to layer (km/sec) interface (km) The coda magnitude scale for Jan Mayen which is used in this report is given by Havskov & Sørensen (2006). This scale was implemented in 2006 but all events used in this report are updated during April/May M C = 3.27 log(t) D 14

19 where T is the coda duration and D is the epicentral distance in km. The regional and teleseismic events recorded by the network are located using the global velocity model IASPEI91 (Kennett and Engdahl, 1991). Body wave magnitude is calculated using the equation by Veith and Clawson (1972): Mb = log(a/t) + Q(D,h) Here h is the hypocentre depth (km), A is the amplitude (microns), T is period in seconds and Q(D,h) is a correction for distance and depth. Surface wave magnitude Ms is calculated using the equation (Karnik et al., 1962): Ms = log(a/t) log(d) where A is the amplitude (microns), T is period in seconds and D is the hypocentral distance in degrees. Starting from January 2001, the European Macroseismic Scale, EMS98, (Grünthal, 1998) has been used. All macroseismic intensities mentioned in the text will refer to the EMS98 instead of the previously used Modified Mercalli Intensity scale. The two scales are very similar at the lower end of the scale for intensities less than VII. 4.2 Events recorded by the NNSN Based on the criteria mentioned in section 4.1, a total of 6061 local and regional events, were detected by the NNSN during Of these local and regional events, 48% were large enough to be recorded by several stations and hence could be located reliably, and are not classified as probable explosions. The numbers of local/regional and teleseismic events, recorded per month in 2013 are shown in Figure 4. The average number of located local and regional events recorded per month is 421. The number of recorded local/regional events is slightly lower than previous years much due to the large number of fore and aftershocks of the magnitude 6.7 (MW GCMT) earthquake west of Jan Mayen in A total of 1010 teleseismic events were recorded in 2013 and the monthly average of teleseismic earthquakes in the NNSN database, is 84. In addition to the locations determined at UiB and NORSAR, also preliminary locations published by the USGS (United States Geological Survey) based on the worldwide network are included for earthquakes also registered by NNSN stations. 15

20 L/R D JAN MAR MAY JUL SEP NOV Figure 4. Monthly distribution of local/regional and distant events, recorded during UiB, as an observatory in the global network of seismological observatories, reports phases from the teleseismic recordings. All events (teleseismic, regional and local) recorded from January to December 2013 with M 3 are plotted on Figure 5. Figure 5. Epicentre distribution of earthquakes with M 3.0, located by the NNSN from January to December Teleseismic events recorded only by NORSAR have M 5.0. Monthly station recording statistics from January to December 2013 are given in Table 7 and Table 8. This table shows, for each station, local events recorded on more than one station and recorded teleseismic events. The statistics are based on the analysed data and are taken from the database. Table 7 and 8 show both earthquakes and explosions, and that the large number of detections at KTK mainly is due to explosions at the Kirruna/Malmberget mines in Sweden. The MOR station also records the Kirruna/Malmberget explosions, but in addition the station records a large number of local earthquakes. These earthquakes are also recorded 16

21 on the stations STOK and KONS and, therefore, the number of recorded earthquakes in this region is higher. The following was observed from Table 7 and 8: Skarslia (SKAR) was in operation from March With a high S/N ratio and located in central southern Norway this station records a high number of events. TBLU and OSL are recording mostly teleseismic earthquakes, which is as expected due to their location in noisy environment. Stronger local earthquakes will however be detected. The arctic stations, in particular KBS and SPITS, are recording a much larger number of earthquakes than the mainland stations. The seismic activity around Spitsbergen will be presented in a separate section later in this report. The stations KONS, STOK and MOR8 continue to record a relatively large number of small earthquakes in the area. Temporary stations were deployed during fall 2013 and an increase in smaller events recorded at KONS, MOR8 and STOK is expected during the NEONOR2 project period. 17

22 Table 7. Monthly statistics of events recorded at each station for January-June Abbreviations are: LM = Number of local events recorded at more than one station and D = Number of teleseismic events. JANUARY FEBRUARY MARCH APRIL MAY JUNE STATION LM D LM D LM D LM D LM D LM D ASK BER BJO BLS DOMB FOO HAMF HOMB HOPEN HYA JMI JMIC JNE JNW KBS KMY KONO KONS KTK LOF MOL MOR NSS ODD OSL SKAR SNART STAV STEI STOK SUE TBLU TRO NORSAR ARCES SPITS

23 Table 8. Monthly statistics of events recorded at each station for July-December Abbreviations are: LM = Number of local events recorded at more than one station and D = Number of teleseismic events. JULY AUGUST SEPT OCT NOV DEC STATION LM D LM D LM D LM D L D LM D ASK BER BJO BLS DOMB FOO HAMF HOMB HOPEN HYA JMI JMIC JNE JNW KBS KMY KONO KONS KTK LOF MOL MOR NSS ODD OSL SKAR SNART STAV STEI STOK SUE TBLU TRO NORSAR ARCES SPITS The seismicity of Norway and adjacent areas The main area of interest is defined as 54-82N and 15W-35E. We also show the seismicity for the Arctic region including the Barents Sea defined by the coordinates N and 20 W- 50 E. A total of 3145 of the recorded events are located inside the NNSN prime area, 54 N- 82 N and 15 W-35 E. During analysis and using the explosion filter (Ottemöller, 1995), 44% of these events were identified as probable explosions. After including stations from the NORSAR array the number of located small events has increased in eastern Norway. Figure 6 shows all local/regional events in the prime area, analysed and located during Among these, 22 are located in the vicinity of Jan Mayen. Figure 7 shows the location of every earthquake (known and probably explosions removed) located within the prime area with one of the calculated magnitudes above 3 and Table 9 lists the same earthquakes with all earthquakes located close to the Mid-Atlantic ridge removed. 19

24 Figure 6. Epicentre distribution of events analysed and located in Earthquakes are plotted in red and probable and known explosions in blue. For station locations, see Figure 1. It should be emphasized that the magnitude calculation for the earthquakes located on the oceanic ridge in the Norwegian Sea uses the same formula as for mainland Norway. As the scale is not appropriate for this region, the magnitudes for these earthquakes are underestimated. Most of the recorded earthquakes in this area have magnitudes above 3.0 if they are recorded on Norwegian mainland stations. 20

25 Figure 7. Epicentre distribution of located events with one of the calculated magnitudes above or equal to 3.0. For station location, see Figure 1. 21

26 Table 9. Local and regional events in prime area with any magnitude above or equal to 3.0 for the time period January through December Earthquakes at the mid-atlantic ridge are excluded. Earthquakes at Jan Mayen are discussed separately. In cases where all BER magnitudes are below 3 but the event still is included in the list, NORSAR (NAO) or the British Geological Survey (BGS) has reported a magnitude of 3.0 or larger. Abbreviations are: HR = hour (UTC), MM = minutes, Sec = seconds, L = distance identification (L=local, R=regional, D=teleseismic), Latitud = latitude, Longitud = longitude, Depth = focal depth (km), F = fixed depth, AGA = agency (BER=Bergen), NST = number of stations, RMS = root mean square of the travel-time residuals, Ml = local magnitude and Mw = moment magnitude. Year Date HRMM Sec L Latitud Longitud Depth FF AGA NST RMS Ml Mw Ml L F BER (NAO) L F BER (BGS) L F BER (BGS) L F BER (BGS) L F BER (BGS) L F BER L F BER (NAO) L F BER (NAO) L F BER (NAO) L F BER (NAO) L F BER L F BER (NAO) L F BER (NAO) L F BER (NAO) L F BER (NAO) L F BER (BGS) L F BER (NAO) L F BER (NAO) L F BER (NAO) The largest local or regional earthquake in 2013, recorded on Norwegian stations and within the prime area, occurred on July 27 th at 16:22(UTC) in Storfjorden, Svalbard. The earthquake had a magnitude of MW=4.5. Another two slightly smaller earthquakes occurred in Storfjorden and in the Heer Land region of Spitsbergen on November 21 st at 21:38 and December 27 th at 00:07, respectively. Along the mid-atlantic ridge a large number of earthquakes occur with magnitude above 3. For 2013 more than 20 earthquakes along the ridge had one of its calculated magnitudes above 4 even if the magnitudes calculated for these earthquakes are expected to be underestimated. The earthquake most widely recorded was the one occurring 18 December at 08:22 which was recorded on most mainland stations. The most significant earthquakes in the vicinity of the Norwegian mainland were: March 22 nd 2013, 10:32(UTC): The earthquake was located west of Solund, Sogn og Fjordane, with a magnitude of M L =3.1. Only three hours later another earthquake occurred in the same area. Both earthquakes were reported felt in Sogn og Fjordane. Both earthquakes were recorded on the British seismic stations. April 19 th, 2013 at 09:28(UTC): The earthquake occurred in the Lurøy area in Nordland, about 115km west of Mo i Rana. The earthquake had a magnitude ML=3.3 and was reported felt in Helgeland. November 19 th, 2013 at 23:41(UTC): An earthquake was felt in the Luster area in Sogn og Fjordane. The earthquake was located northwest of Gaupne and had a magnitude of MW=2.9. Offshore, an earthquake occurred December 1 st in the North Sea between Shetland and the Norwegian west coast. The earthquake was located at N E with 22

27 magnitudes ML BER =3.0 and ML BGS =3.4. The earthquake was registered both on Norwegian and British seismic stations Seismicity in Nordland Figure 8 shows the seismicity in Nordland since The area includes the locations of earthquake swarm activity such as Meløy, Steigen and Stokkvågen. The latter two areas are clearly visible on the map. Steigen was active between 2007 and 2008, but has been relatively quiet since then. The activity in the Stokkvågen area has been nearly continuous, and specific clusters in time and space have occurred. Figure 8. Seismicity in the Nordland area. Red circles show seismicity for , and blue circles show seismicity for Explosions are excluded. The yellow triangles give the station locations. 23

28 The Petromaks funded project NEONOR2 started in 2013 and is a collaboration between NGU, Kartverket, the University of Bergen (UiB), NORSAR and NPD. The prime objectives of NEONOR2 are: to promote the understanding of regional-scale stress and strain dynamics in the Nordland area which has proved to be the tectonically most active area in Norway. The secondary objectives are to Obtain a new seismicity map of the Nordland region, relevant for geohazard evaluation Explore combined inversion methods to determine the regional stress field by integrating GPS, DInSAR, seismic data, in situ stress, and mapped faults Quantify the contribution of Pleistocene sediment redistribution on the present-day stress field using numerical modelling Estimate the Pleistocene palaeo-stresses and palaeo-temperatures using numerical modelling Figure 9. Map of stations deployed in Northern Norway in

29 Figure 10. Map of stations deployed in Northern Norway in During the months from August to November 2013, 15 seismic stations were installed by UiB, and 5 stations by NORSAR. Six more stations will be installed by NORSAR in The locations of stations that were operated in 2013 are shown in Figure 9 and Figure 10. Data from 8 of the stations are continuously transferred to Bergen and are used in the daily processing. A total of 32 events are recorded on one or more of these NEONOR2 stations during 2013, mostly teleseismic earthquakes. Among the 15 recorded local events, 6 are assumed to be local explosions. Figure 11 and Figure 12 showexamples of earthquakes recorded on the NEONOR2 stations and also on the permanent local stations LOF, MOR8, KONS and STEI. The earthquakes occurring on 12 February 2014 is located offshore at the continental slope 89 km from the closest station N2SV. The other earthquake is located on Moskenesøya in Lofoten 31 km from the closest station. 25

30 Figure 11. Seismograms for an earthquake registered 12 February 2014 on the NEONOR2 stations. The earthquake is located 68.4N 11.35E, depth 4.4km and M L =1.4 26

31 Figure 12. Seismograms for an earthquake registered 18 February 2014 on the NEONOR2 stations. The earthquake is located N E, depth 4.4km and M L =

32 4.4 Seismicity in the arctic area With the mainland stations on the Lofoten, in Tromsø and Hammerfest, the network on Jan Mayen, and stations on Bjørnøya, Hopen and Svalbard, the network detection capability in the arctic area is relatively good. We define the arctic area as the region N and 20 W-50 E. Most of the activity falls into three areas: Jan Mayen, the Mid-Atlantic ridge and Storfjorden southeast of Svalbard, as can be seen in Figure 13. Figure 13. Seismicity in the Norwegian arctic area during A total of 1425 located earthquakes Seismicity in the Jan Mayen area Jan Mayen is located in an active tectonic area with two major structures, the Mid Atlantic ridge and the Jan Mayen fracture zone, interacting in the vicinity of the island. Due to both tectonic and magmatic activity in the area, the number of recorded earthquakes is higher than in other areas covered by Norwegian seismic stations. During 2013 a total of 200 earthquakes were located as seen in Figure 14 and of these, six had a magnitude equal or above 3.0. The monthly distribution of located earthquakes in the area (Figure 15) shows the increase during July, August and September 2012 related to the activity around the M=6.7 earthquake occurring 30 th August The number of located earthquakes has since then decreased to an expected yearly average after the major number of fore- and after-shocks around the two large earthquakes in

33 The largest earthquake in the Jan Mayen region in 2013 occurred on 28 November at 09:24 (UTC) and the magnitude is estimated to 4.1. The earthquake is located slightly north of the island in the Jan Mayen fracture zone and is reported felt by the staff at Jan Mayen. Figure 14. Earthquakes located in the vicinity of Jan Mayen during Jan Mar May Jul Sep Nov Figure 15. Monthly distribution of the earthquakes recorded on Jan Mayen during 2012 and

34 The number of recorded earthquakes in the Jan Mayen area has varied over the last years (Figure 16). The number of relative strong earthquakes (M 3) shows smaller time variation than for the smaller earthquakes. The increases in 2004 and 2005 were due to the M=6.0 earthquake in 2004 and its aftershocks (Sørensen et al., 2007). The same is true for 2011, where the M=6.0 earthquake on 29 January was followed by a sequence of aftershocks. The 30 August 2012 earthquake with its fore and aftershocks clearly increases the number of recorded events in 2012 compared with previous years, making it the largest number of recorded events since There are twice as many large earthquakes but more than four times as many smaller events recorded in the vicinity of the island. For 2013 the number of earthquakes has decreased to an expected yearly average for the smaller earthquakes. With only six earthquakes with magnitude 3 or above, 2013 has fewer larger earthquakes than any of the previous year shown in Figure Total number of recorded earthquakes Number of events with magnitude equal to or above Figure 16. Yearly distribution of earthquakes located in the Jan Mayen area since Seismicity in Storfjorden, Svalbard The Storfjorden area southeast of Svalbard has been seismically active since the M w =6.0 earthquake on 21 February A preliminary study was presented by Piril et al. (2008). One of the main objectives from this study was to resolve the source mechanism, which was found to be oblique-normal. This result was found from the inversion of both regional and teleseismic data. The earthquake had a high number of aftershocks. A total of more than 20 earthquakes with magnitude larger than M=4 have occurred in the area since An interesting question is whether the earthquakes now are aftershocks to the event in From the monthly number of all detected earthquakes, shown in Figure 17, it seems that the activity had decreased until the start of 2009 and then remained relatively low until late In 2010 the number had increased again and remained mostly stable in This is mostly explained by an increase in the number of smaller earthquakes as seen by the almost constant levels of earthquakes with M>2.5 since The better detection was due to usage of the data from the Hornsund (HSPB) station. In 2012 and 2013, it appears as if 30

35 the overall activity is decreasing slightly, although still continuing. The total number of events detected in the Storfjorden area in 2013 was around 173. Figure 17. Monthly number of earthquakes in the Storfjorden area since January The colours show: grey all events, red M>2.5, blue M>4.0. Figure 18 shows both a map with the seismicity since 2000 and the distribution of events over time. The onset of the activity in the Storfjorden area in February 2008 is obvious from the time plot. The seismicity since 2011 is mostly concentrated in the southwestern half of the area that has been active since One of the larger events in 2013 occurred closer to the Heer Land area on Spitsbergen. 31

36 Figure 18. Seismicty in the Svalbard area. Bottom: Earthquakes occurring in 2013 are plotted in yellow circles. Red circles show seismicity for The blue triangles give the station locations. Top: Seismicity in the same area is plotted as latitude as function of time. 32

37 4.5 Earthquakes in the southern North Sea During 2013 two earthquakes were detected and located in the southern North Sea as shown in Figure 19. The southernmost of the earthquakes occurred on March 15, 2013 at 10:43 with a local magnitude 2.6 (BER) and 3.1 (BGS). Figure 19. The two earthquakes located in the southern North Sea during The earthquake occurring on 28 October 2013 at 11:09 was located to N 2.394E with a magnitude M L= 2.2(BER) and M L= 2.8 (BGS). The hypocentre is 110 km north of the Ekofisk filed. Seismograms for this earthquake are shown in Figure 20. The earthquake was recorded on the real-time seismic stations at Ekofisk allowing for quick distance estimation from Ekofisk based on the S-P time. 33

38 Figure 20. Seismograms for the earthquake 28 October Felt earthquakes In total, 18 earthquakes were reported felt and located within the target area during 2013 (see Table 10 and Figure 21). Not all earthquakes were felt in Norway. For the Jan Mayen island the number of felt earthquakes is expected to be larger than reported. 34

39 Figure 21. Location of the 18 earthquakes reported felt during Not all earthquakes are felt in Norway. From 2006 it is possible to report felt earthquakes over the internet. When an earthquake is reported felt by a sufficient number of people, questionnaires are available for the public on the site This result in a higher number of questionnaires returned and therefore a need for new routines when processing the data. The largest felt earthquake during 2013 where questionnaires was published was the earthquake occurring 22 March at 10:32 and located west of Florø. A map with reported intensities is presented in Figure

40 Table 10. Earthquakes reported felt in the BER database in Abbreviations are: M L = local magnitude and M w = moment magnitude, W: questionnaires received on web (Y/N). Earthquakes reported felt in Norway are marked in colour. Nr Date Time (UTC) Max. Intensity (MMI) Magnitude (BER) Instrumental epicentre location :16 III M L =2.4(BGS),M L = N / 04.74W :59 IV M L =2.7(HEL),M L = N / 24.56E W :32 IV M L =3.1,M W =3.3, M L =3.6(BGS) 61.57N / 4.67E W IV M L =2.9,M W =3.3, 61.60N / 4.69E M L =3.5(BGS) W :02 III M L = N / 5.52W :28 M L =3.3, M W = N / 12.23E W :43 III M L = N / 5.58W :18 III M L = N / 5.20W :16 IV M L =3.3, M L =3.8(BGS) 52.85N / 4.76W II M L =2.5, M L =2.5(NAO) 59.43N / 5.72E :04 III M L =2.4, M L =2.8(HEL) 57.73N / 5.71W :33 II M L =1.8,M W =2.1, 61.81N / 6.45E M L =1.6(NAO) :58 III M L =3.1, M L =3.3(BGS) 53.89N / 3.41W :06 II M L =2.4, M L =2.7(BGS) 56.67N / 4.26W III M L = N / 4.39W :41 IV M L =3.0,M W =2.9, 61.50N / 7.16E M L =2.7(NAO) W :36 IV M L =2.3, M L =2.8(NAO) 64.41N / 12.30E W :24 II M L =4.1, M L =4.0(NAO) 71.17N / 8.36W W 36

41 Figure 22. Intensities for the earthquake 22 March at 10: Explosions Triggered data files from the NNSN will inevitably detect non-tectonic events such as explosions and identifying these is important. For several years, explosions at mines (e.g. Titania) and explosions made by the navy have been registered into the NNSN database. An explosion filter (Ottemoller, 1995) using location and time-of-day to identify probable explosion has been used since 1995 in addition to a decision based on the waveform signals. To identify and verify more non-seismic events, work has started to contact mines, quarries, road constructions etc. to obtain information, on a more regular basis, about time and size of explosions. Some of these locations are presented below. 37

42 In addition to obtain information about explosions from the mines and quarries, work will be done to establish daily routines that enables the analysist to discriminate between explosions and earthquakes (also see section 5.4) Northern Norway In northern Scandinavia there are several mines and quarries all contributing to an increased number of recorded seismic events. Some of the quarries are shown on Figure 23. Most of the explosions triggering the stations in Northern Norway are located at approximately 67N 20E, at the Kiruna and Malmberget quarries, Northern Sweden. As there are several (+/- 10) events daily, we quickly register these events as likely explosions if they meet the set criteria: Location in close proximity (<1km radius) to a known quarry, the ratio between P and S phase is approximately the same for all stations (indicating that tectonic fault is unlikely the cause of the event) and/or presence of RG waves (indicating shallow source). Figure 23. Map showing location of Swedish, Russian and Norwegian quarries creating explosions that regularly trigger on the NNSN. (Source: Google Maps) From January 2014, the same procedure is applied to events located in or around Russian quarries. For 2013, these events were in general not registered as likely or confirmed explosions. There is only one known Norwegian quarry in the area; Bjørnevatn near Kirkenes. From January 2014 we will receive lists with timing of explosions from this quarry. A selection of frequently registered small events (M>1.5) in and around Varangerfjorden, Finnmark was closer investigated during fall 2013, as a known fault line is present along the fjord and the 38

43 fact that the events did not match the explosions from Bjørnevatn quarry. The events are located at approx. 70 N, 30 E with a certain degree of uncertainty, as they are very low in magnitude and the nearest stations are quite a distance away. The conclusion is that the selected events are likely to be explosions from the Russian quarries close to the Norway- Russian border, based on the presence of surface waves in the larger events (M>1) Mid-Norway There are some known quarries in Mid-Norway. However, few events suspected to be possible explosions are registered in this area Southern Norway Figure 24. Map showing location of quarries in southern Norway with explosions that regularly trigger the NNSN (Source: Google Maps). Lists confirming timing of explosions is received monthly from Titania quarry near Egersund and from Norsk Stein, who operate the quarries at Berakvam and Tau. Events matching the time and location from the lists are registered as confirmed explosions. More rarely, we register events located in close proximity to quarries at Eikefet, Sløvåg and Skiparvik, all western Norway (Figure 24). Lists of confirmed timing of explosions will be received from these quarries from 1 st Jan 2014, allowing these events to be registered as confirmed explosions. Figure 24 show some of the known quarries located in southern Norway. Based on waveform, timing and location, we suspect that quite a lot of the registered events in the eastern part of Norway, around Oslo, are explosions connected to road work/construction work. These are typically detected by the NNSN stations in Oslo and Kongsberg, as well the 39

44 the NOA array. As this type of work is mainly short term projects handled by sub-contractors, it has proved hard to confirm if the registered event is an explosion. Some larger non-seismolgical events have been confirmed during Examples are demolishing of mines from World War 2 in Karmsundet Aug. 23 rd (Figure 25) and in Dybingen Sept. 2 nd, confirmed by the Norwegian navy and DOF Subsea, respectively. Figure 25. NNSN registration of the demolishing of an old mine in Karmsundet, 23 rd August

45 5 Scientific studies This section will present an overview of research work that is carried out under the NNSN project. The main objective of this work is to improve the understanding of earthquakes and the seismological models in the region, mostly by using data recorded by the NNSN. Results will be used to improve the NNSN monitoring service. 5.1 Source study of the Jan Mayen fracture Zone strike-slip earthquakes by Ouetzalcoatl Rodriguez Perez and Lars Ottemöller Seismic source parameters of oceanic transform zone earthquakes have been relatively poorly studied. Previous studies showed that this type of earthquakes have unique characteristics such as the relatively common occurrence of slow events with weak seismic radiation at high frequencies; but also the occurrence of some events that have high apparent stress indicating strong high frequency radiation. We studied 5 strike-slip earthquakes in the Jan Mayen fracture zone with magnitudes in the range of 5.9 < Mw < 6.7 (Figure 26). We determined finite-fault slip distributions with the iterative deconvolution method using teleseismic data (example shown in Figure 27). The slip models were characterized by determing source parameters on the asperities and background areas. We found that the area of the asperities represent about percent of the total rupture area with two different criteria based on average slip and maximum displacement, respectively. Sensitivity tests were carried out to estimate the variability of slip patterns and source parameters. We observed robust solutions and reliable source parameter estimates from the models. We also determined the radiated seismic energy and apparent stress in order to identify possible slow events. Our results suggest that the analyzed events in Jan Mayen have a regular nature in terms of apparent stress, seismic energy and centroid time delay. 41

46 Figure 26. Overview of earthquakes in the Jan Mayen Fracture Zone with M>5.7 since Figure 27. Slip distribution of the 2012 Jan Mayen event (M=6.7). 42