Seismicity of Norway and surrounding areas for

Transcription

1 Seismicity of Norway and surrounding areas for 2012 Prepared by Department of Earth Science University of Bergen Allegt. 41 N-5007 Bergen Phone: (+47) Fax: (+47) March 2013

2 CONTENTS 1 Introduction Data availability to the public Velocity models and magnitude relations Events recorded by the NNSN The seismicity of Norway and adjacent areas Seismicity in Nordland Seismicity in the arctic area Seismicity in the Jan Mayen area Seismicity in Storfjorden, Svalbard Haugesund earthquake Fedje earthquake Voss earthquake Earthquakes north of the Faroe isles Felt earthquakes Use of NNSN data during References... 34

3 1 Introduction This annual report on the seismicity of Norway and adjacent areas encompasses the time period January 1 st - December 31 st, The earthquake locations have been compiled from all available seismic stations operating on the Norwegian territory including the Arctic islands of Spitsbergen, Bjørnøya, Hopen and Jan Mayen. In Norway, the University of Bergen (UiB) operates the Norwegian National Seismic Network (NNSN) consisting of 32 seismic stations where 20 have broadband sensors. NORSAR operates three seismic arrays and two seismic stations (Figure 1). One station with real-time data is provided from the Ekofisk field by ConocoPhillips. Data from temporarily installed local networks are also included whenever data are made available. In addition to the NNSN stations, waveform data from other selected stations in Norway (operated by NORSAR), Finland, Denmark, Poland and Great Britain are transferred in real time and included in the NNSN database. In total data from 15 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. 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. Starting from 2008, the NNSN stations were upgraded to provide continuous data in real time. This has resulted in more effective monitoring of earthquake activity in the region. 1

4 2 Data availability to the public All the data stored in the NNSN database are also 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. Figure 1. Stations included in Norwegian National Seismic Network (NNSN). UiB operates the 31 stations (red) and in addition the data are transferred from Ekofisk (EKO1), and NORSAR operates the stations marked in blue including the 3 arrays. Data from stations marked in yellow are received continuously in Bergen, but are operated by institutions in neighbouring countries. 2

5 3 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 1 (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 1. 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) 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: 3

6 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 2) and coda magnitude scale is used (Andersen, 1987). Table 2. 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 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

7 4 Events recorded by the NNSN Based on the criteria mentioned in section 1, a total of 5254 local and regional events, were detected by the NNSN during Of these local and regional events, 64% were large enough to be recorded by several stations and hence could be located reliably. The numbers of local/regional and teleseismic events, recorded per month in 2012 are shown in Figure 2. The average number of located local and regional events recorded per month is 438. The number of recorded local/regional events is higher 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. A total of 957 teleseismic events were recorded in 2012 and the monthly average of teleseismic earthquakes in the NNSN database, is 80. 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 L/R D JAN MAR MAY JUL SEP NOV Figure 2. Monthly distribution of local/regional and distant events, recorded during

8 Figure 3. Epicentre distribution of earthquakes with M 3.0, located by the Norwegian Seismic Network from January to December Teleseismic events recorded only by NORSAR have M 5.0. UiB, as an observatory in the global network of seismological observatories, reports secondary phases from the teleseismic recordings. All events (teleseismic, regional and local) recorded from January to December 2012 with M 3 are plotted on Figure 3. Monthly station recording statistics from January to December 2012 are given in Table 3. 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 3 shows 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 on the stations STOK and KONS and, therefore, the number of recorded earthquakes in this region is higher. The following was observed from Table 3: The seismic activity in the Jan Mayen area was higher between July and September. Some of the activity between June and August was in the same area as the MW=6.7 earthquake on 30 August Aftershocks from this event spread all the way to Jan Mayen and even across south of Jan Mayen. 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. Four stations are located at Jan Mayen. Data from the BB station JMIC are, together with other BB stations, collected for all teleseismic earthquakes 6

9 recorded at any of the NNSN stations. Therefore, the number of recorded teleseismic earthquakes is higher than for the other stations at Jan Mayen. DOMB is detecting teleseismic earthquakes quite well. However, no detections were made in November and December due to communication problems. The stations KONS, STOK and MOR8 continue to record a relatively large number of small earthquakes in the area. 7

10 Table 3a. 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 SNART STAV STEI STOK SUE TBLU TRO NORSAR ARCES SPITS

11 Table 3b. 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 SNART STAV STEI STOK SUE TBLU TRO NORSAR ARCES SPITS

12 5 The seismicity of Norway and adjacent areas A total of 3366 of the recorded events are located inside the NNSN prime area, 54 N-82 N and 15 W-35 E. We also show the seismicity for the Arctic region including the Barents Sea defined by the coordinates N and 20 W-50 E. During analysis and using the explosion filter (Ottemöller, 1995), 31% of these events were identified as probable explosions. Figure 4 shows all local/regional events in the prime area, analyzed and located during Figure 4. Epicentre distribution of events analyzed and located in Earthquakes are plotted in red and probable and known explosions in blue. For station locations, see Figure 1. Figure 5 and Table 4 show the 180 local and regional earthquakes (known and probably explosions are removed) located in the prime area, with one of the calculated magnitudes greater than or equal to 3.0. Among these, 95 are located in the vicinity of Jan Mayen. 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 10

13 earthquakes are underestimated. Most of the recorded earthquakes in this area have magnitudes above 3.0 if they are recorded on Norwegian mainland stations. Figure 5. Epicentre distribution of located events with one of the calculated magnitudes above or equal to 3.0. All earthquakes are listed in Table 4. for station location, see Figure 1. 11

14 Table 4. Local and regional events in prime area with any magnitude above or equal to 3.0 for the time period January through December Only magnitudes reported by the University of Bergen are included. In cases where all BER magnitudes are below 3 but the event still is included in the list, NORSAR of 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 Mb Mw L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L * NAO NAO L BER BER L BER NAO L BER NAO L BER BER L BER NAO L F BER NAO L BER BER L BER BER L BER BER L F BER BER L BER BER L F BER BER L BER BER L F BER BER L BER BER L F BER BER L BER NAO L BER BER L BER BER L BER NAO L F BER BER 4.9 PDE L BER BER 4.1 PDE L BER BER L BER BER 4.0 PDE L F BER NAO L FF NAO BER L BER NAO L BER BER L F BER BER L BER BER L BER NAO L BER BER L BER NAO L F BER BER L BER BER L BER BER 12

15 L BER BER L F BER NAO L F BER BER 5.7 PDE 6.1WCM L BER BER L F BER BER L BER BER L F BER BER L F BER BER L F BER BER L F BER BER L F BER NAO L F BER BER L F BER NAO L F BER NAO L F BER BER L BER BER L BER BER L F BER BER L F BER BER L F BER BER L F BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L F BER BER L BER BER L BER BER L BER NAO L F BER NAO L BER BER L BER BER L F BER BER L BER BER L BER BER L F BER BER L F BER BER 4.5 PDE L BER BER L BER BER L BER BER L BER BER L BER BER L F BER BER L BER NAO L BER BER L BER BER 4.7 PDE L F BER BER D F BER NAO 5.1 BER L BER BER L F BER BER L BER BER L BER BER L BER BER L BER BER D BER NAO 5.9 PDE 6.8WCM D F BER NAO D BER BER D BER NAO L F BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER L BER BER D F BER BER L BER BER L BER BER L BER BER 13

16 D BER BER L BER NAO D F BER BER L F BER BER D F BER BER L BER BER L BER BER L BER NAO L F BER BER L F BER BER L BER BER L BER BER L BER NAO L BER NAO L BER BER D F BER BER D BER BER D BER BER L BER BER L BER BER L BER BER L F BER NAO L BER BER L BER BER 2.9BER L BER BER L F BER NAO L F BER NOA L BER BER L BER BER L BER NAO L BER BER L F BER BER L F BER BER L F BER BER L F BER BER L BER BER L FF NAO NAO L F BER BER L F BER BER L BER BER L BER BER L F BER BER L BER BER The largest local or regional earthquake in 2012, recorded on Norwegian stations and within the prime area, occurred on August 30 at 13:43(UTC) northwest of Jan Mayen. The earthquake had a magnitude of MW=6.7 (GCMT) and was strongly felt at the island 74 km away. Another large earthquake occurred on the Mohns Ridge on May 24 at 22:47 with a magnitude MW=6.3 (M W GCMT). The most significant earthquakes in the vicinity of the Norwegian mainland were: 14 March 2012, 19:22 (UTC): The earthquake was located northeast of Haugesund, Rogaland, with a magnitude of M L =3.2. This earthquake was reported felt in a large area in Hordaland and Rogaland. 24 March 2012, 06:05 (UTC): The earthquake occurred about 18 km southwest of the Fedje island and 15 km west of Øygarden, Hordaland. The earthquake had a magnitude ML=2.9. This earthquake was reported felt in northern parts of Hordaland and in Gulen, Sogn og Fjordane. 24 March 2012, 11:06 (UTC): An earthquake was felt in the Voss area with a magnitude of ML=

17 Offshore, an earthquake occurred north of the Faroe Islands on 5 August. The earthquake was located to N W with magnitudes ML BER =2.7 and ML BGS =3.6. The earthquake was registered both on Norwegian and British seismic stations. 5.1 Seismicity in Nordland Figure 6 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, but specific clusters in time and space have occurred. The temporal distribution shows the cluster in 2005, which is when the three station network was installed. The number of events during and after 2011 was lower than in previous years, probably reflecting that the station FLOS was closed in February This also reduced the number of very small earthquakes that are located in this area, as can be seen for

18 Figure 6. Seismicty in the Nordland area. Bottom: Red circles show seismicity for , and blue circles show seismicity for Explosions are excluded. The green triangles give the station locations. Top: Seismicity in the same area is plotted as latitude versus time of occurrence. 16

19 5.2 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 7. The two largest earthquakes of the year (Mohns Ridge and Jan Mayen) fall into the arctic area. Figure 7. Seismicity in the Norwegian arctic area during A total of 2132 located earthquakes. 5.3 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 2012 a total of 2136 earthquakes were located as seen in Figure 8 and of these, 196 had a magnitude equal or above 3.0. The largest earthquake in the Jan Mayen region in 2012 occurred on 30 August at 13:43 (UTC). This earthquake was located at 71.44N and 10.05W with a magnitude of 6.7. The earthquake was strongly felt by the population on the island only 74 km from the hypocenter. The earthquake had a strike 17

20 slip mechanism as determined through global moment tensor inversion (Figure 9). The NW-SE striking nodal plane is an alignment with the Jan Mayen fracture zone. Figure 8. Earthquakes located in the vicinity of Jan Mayen during The two yellow stars mark the two major earthquakes, the M6.7 northwest of the island and the M5.3 to the southeast A JAN MAYEN ISLAND REGION Date: 2012/ 8/30 Centroid Time: 13:43:35.4 GMT Lat= Lon= Depth= 23.6 Half duration= 5.7 Centroid time minus hypocenter time: 10.2 Moment Tensor: Expo= Mw = 6.7 mb = 5.9 Ms = 6.8 Scalar Moment = 1.63e+26 Fault plane: strike=111 dip=83 slip=-5 Fault plane: strike=202 dip=86 slip=-173 Figure 9. Moment tensor solution provided by the Global CMT project (taken from 18

21 Jan Mar May Jul Sep Nov Located earthquakes Figure 10. Monthly distribution of the earthquakes recorded on Jan Mayen during The monthly distribution of located earthquakes in the area (Figure 10) shows the increase during July, August and September related to the activity around the mainshock location before and after the event occurred. Figure 8 shows that aftershocks were located along the mainshock fault plane, but also continued across the island to the south-east. This may be significant as only few earthquakes have been recorded from here in the past. Also the ML=5.3 aftershock occurred away from the mainshock fault south of Jan Mayen. The number of recorded earthquakes in the Jan Mayen area has varied over the last years (Figure 11). 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 Total number of recorded earthquakes Number of events with magnitude equal to or above Figure 11. Yearly distribution of earthquakes located in the Jan Mayen area since

22 The daily numbers of earthquakes recorded in 2012 are shown in Figure 12. The figure shows a relative low activity during the first 5 months of 2012 and then an increase in the activity with three significant peaks. The biggest peak is following the major earthquake on august 30, when 350 earthquakes were recorded on the Jan Mayen stations within a few days. This earthquake was strongly felt on Jan Mayen, where the staff members reported things were falling from shelves and small rock falls occurred, as illustrated by the photos in Figure 13. Figure 12. Daily distribution of the earthquakes recorded on the stations at Jan Mayen during Figure 13. The photos shows damages on Jan Mayen after the august 30 earthquake. (Photos are printed with permission by the staff at Jan Mayen). 20

23 Waveform data of the 30 August, 2012 earthquake are presented in Figure 14. The earthquake was recorded on all NNSN stations. The data recorded on the Jan Mayen stations are saturated. To obtain onscale recordings in the future, an accelerometer has been installed next to the broadband seismometer at station JMI in the beginning of Further analysis of this major earthquake will be done. Figure 14. The Jan Mayen earthquake recorded on some of the stations in the NNSN database. 21

24 5.4 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 15, 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, it appears as if the overall activity is finally decreasing. The total number of events detected in the Storfjorden area in 2012 was 267. Figure 15. 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 16 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 in 2011 and 2012 is mostly concentrated in the southwestern half of the area that has been active since

25 Figure 16. Seismicty in the Svalbard area. Bottom: Red circles show seismicity for yellow circles and blue circles show seismicity for 2011 and 2012, respectively. The blue triangles give the station locations. Top: Seismicity in the same area is plotted as latitude as function of time. 23

26 5.5 Haugesund earthquake The most widely felt earthquake on mainland Norway occurred 14 March 2012 at 19:22 (UTC, 20:22 local time). The earthquake was located to N and 5.58 E with a depth of 13.8km and a magnitude MW=3.5. The seismograms recorded are shown on Figure 17. Figure 17. Seismograms for the earthquake occurring on 14 March, 2012 at 19:22 (UTC) and located northeast of Haugesund, Rogaland. The seismograms are filtered between 5-10 Hz. The horizontal time scale is minutes, first marking at 19:22(UTC). 24

27 The epicenter is about 22 km northeast of Haugesund and the earthquake was felt by a large number of people in Rogaland and Hordaland with a maximum intensity of MMI=V. On the web site skjelv.no questionnaires were published for the public to fill in and send to NNSN. A total of 690 questionnaires were received, being the basis of the macroseismic map presented in Figure 18. Figure 18. Macroseismic map of the March 14, 2012 earthquake northeast of Haugesund. The reported intensities have not been manually evaluated. 25

28 Figure 19. Three-component traces of station DOMB, filtered 3-8 Hz. For several of the stations, it is possible to read direct Pn and Sn waves besides the larger amplitude Pg and Sg direct arrivals. The seismograms for station DOMB are shown in Figure 19. Sn (here marked as ES) is often weak and not recognizable in the signal-generated noise of scattered P waves. Reading different phases with different take-off angle from the source, such as Pg/Pn and Sg/Sn, significantly improves the depth determination (13.8 km). 5.6 Fedje earthquake The 24 March 2012 at 07:05 (local time) an earthquake occurred about 18 km southwest of Fedje island and 15 km west of Øygarden, Hordaland. The earthquake was located to N and 4.40 E with a magnitude ML=2.9 and it was given a fixed depth of 15 km. The earthquake is located at the Øygarden fault where the last larger earthquake was reported 8 December 2000, with magnitude MW=

29 Figure 20. Macroseismic map of the March 24, 2012 earthquake southwest of Fedje, Hordaland. The reported intensities have not been manually evaluated. 5.7 Voss earthquake On 24 March 2012 at 11:06, a second earthquake was reported felt in Hordaland. The earthquake was located to Voss at N and 6.40 E with 15.3 km depth and magnitude ML=3.0. People at Voss reported a strong sound and shaking, and many feared an avalanche. Having a reasonably good azimuthal coverage for this earthquake, it was possible to determine a fault plane solution (Figure 22). The solution shows strike-slip faulting on fault planes oriented either NW-SE or SE-NW. The maximum compression (P axis) is nearly horizontal and oriented EW in agreement with the regional stress pattern. 27

30 Figure 21. Macroseismic map of the March 24, 2012 earthquake at Voss. The reported intensities have not been manually evaluated. Figure 22. Fault plane solutions determined thtough program HASH and FPFIR for the Voss earthquake on 24 March

31 5.8 Earthquakes north of the Faroe isles During last half of 2012 and beginning of 2013 earthquakes have been located north of the Faroe islands. An earthquake occurred on 5 August 2012 at 21:19. The earthquake was located to N W with magnitudes ML BER =2.7 and ML BGS =3.6. The earthquake was registered both on Norwegian and British seismic stations. On 5 December 2012, two slightly smaller earthquakes occurred in the same area with magnitude ML BER =2.3 and ML BER =2.4 and 4 March 2013 an ML BER =2.8 and ML BGS =3.6 earthquake were located to the same area. In 2007, similar magnitudes earthquakes were observed in this area. Figure 23 shows the location of earthquakes from 2000 to March Figure 23. All earthquakes located north of the Faroe Islands after January Earthquakes recorded in 2012/2013 in blue and earthquakes recorded in 2007 in yellow. 29

32 6 Felt earthquakes In total, 22 earthquakes were reported felt and located within the target area during 2012 (see Table 5 and Figure 24). Not all earthquakes were felt in Norway. For the Jan Mayen island the number of felt earthquakes is expected to be larger then reported. Figure 24. Location of the 22 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 30

33 Table 5. 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 :04 III M L =2.2(BGS),M L = N / 07.61W :35 III M L =2.6(BGS),M L = N / 06.35W :31 II M L =2.9(BGS),M L = N / 00.84W :14 III M L =2.8(BGS), M L = N / 06.34W :25 III M L =2.1(BGS),M L = N / 06.35W :55 III M L =2.4(BGS),M L = N / 04.84W :59 III M L =3.2(BGS),M L = N / 01.67W :23 III M L =2.8(BGS),M L = N / 04.76W :22 V M L =3.2,M L =3.6(BGS) 59.52N / 05.58E Y :05 V M L =2.9,M L =2.6(NAO), M L =3.3(BGS) 60.82N / 04.88E Y :06 V M L =3.0,M L =3.2(NAO), 60.64N / 06.40E M L =3.3(BGS) Y :53 III M L =1.9, 54.46N / 02.97W M L =2.0 (NAO) :07 III M L =2.1,M L =2.8 (NAO) 66.53N / 13.35E N :03 V M L =2.9,M L =2.7 (NAO) 60.46N / 06.22E Y :36 II M L =2.4,M L =3.1 (NAO) 66.06N / 13.35E N :59 III M L =2.0,M L =2.3 (BGS) 56.56N / 05.69W :43 VI Mw=6.7,MB=6.9, M L =6.2 (NAO) 71.44N / 10.05W N :24 V M W =2.9,M L =3.0, 62.19N / 05.17E M L =3.2 (NAO) Y :59 II M L =2.7,M L =2.9 (BGS) 55.97N / 05.94W :46 III M L =2.0 (BGS) 55.77N / 06.26W :37 III M L =2.2, M L =2.1 (BGS) 54.51N / 02.98W :23 V M L =4.6,M L =4.7 (NAO) 70.97N / 07.09W N W 31

34 7 Use of NNSN data during 2012 Data collected on Norwegian seismic stations are made available through the Internet and is provided on request to interested parties. Therefore it is difficult to get a comprehensive overview on the use and all publication based on Norwegian data. Publications and reports Gibbons, S. J., F. Ringdal and T. Kværna, 2012 Ratio-to-moving-average seismograms: a strategy for improving correlation detector performance, Geophysical Journal International, Vol. 190, No. 1. (July 2012), pp , doi: /j x x Gibbons, S. J., 2012.The Applicability of Incoherent Array Processing to IMS Seismic Arrays. Pure and Applied Geophysics In Pure and Applied Geophysics (online only 17 October 2012), doi: /s Havskov, J, P. Bormann and J. Schweitzer, Seismic source location. doi: /gfz.nmsop-2_is_11.1; 36 pp., escidoc:44031 P. Bormann (ed.) (2012), New Manual of Seismological Observatory Practice 2 (NMSOP-2). 2nd (revised) edition, Potsdam: Deutsches GeoForschungsZentrum GFZ, doi: /GFZ.NMSOP-2 Harris, D. B., S. J. Gibbons, A. J. Rodgers and M. E. Pasyanos, Nuclear Test Ban Treaty Verification: Improving Test Ban Monitoring with Empirical and Model-Based Signal Processing. IEEE Signal Processing Magazine, Vol. 29, No. 3. (May 2012), pp , doi: /msp Klinge, K., J. Schweitzer and P. Bormann, Record examples of underground nuclear explosions doi: /gfz.nmsop- 2_DS_11.4; 6 pp., escidoc:44029 P. Bormann (ed.) (2012), New Manual of Seismological Observatory Practice (NMSOP-2). 2nd (revised) edition, Potsdam: Deutsches GeoForschungsZentrum GFZ, doi: /GFZ.NMSOP-2 Schweitzer, J., J. Fyen, S. Mykkeltveit, S. J. Gibbons, M. Pirli, D. Kühn and T. Kværna, Seismic Arrays. doi: /gfz.nmsop-2_ch9; 80 pp., escidoc:44028 P. Bormann (ed.) (2012), New Manual of Seismological Observatory Practice (NMSOP-2). 2nd (revised) edition, Potsdam: Deutsches GeoForschungsZentrum GFZ, doi: /GFZ.NMSOP-2 NNSN related Talks & Posters 2012 Bondevik, S., Sørensen, M.B. and Gjevik, B. (2012). Norwegian fjords sang along observations of seiches following the 2011 Tohoku, Japan, earthquake, oral presentation at ESC General Assembly, Moscow, August

35 Fyen, J., S. J. Gibbons, T. Kværna, S. Mykkeltveit, F. Ringdal, M. Roth and J. Schweitzer, NORSAR s automatic processing. Joint Scientific Commission meeting AFTAC, Florida 6-8- November 2012 Fyen, J. and M. Roth, NORSAR history and field installation in text and pictures. Joint Scientific Commission meeting AFTAC, Florida 6-8- November 2012 Gibbons, S. J., The Applicability of Incoherent Array Processing to IMS Seismic Array Stations. Geophysical Research Abstracts Vol. 14, EGU , 2012 EGU General Assembly EGU 2012 oral presentation, Vienna International Center, 25 April 2012 Knapmeyer-Endrun, B., M. Ohrnberger and J. Schweitzer, Temporal variability of the noise wavefield at different scales: Influence on H/V and FK results. Noise and Diffuse Wavefields, Extended Abstracts of the Neustadt Workshop, November 2012, Deutsche Geophysikalische Gesellschaft Mitteilungen, Sonderband IV/2012, 29-30, 2012 Knapmeyer-Endrun, B., M. Ohrnberger and J. Schweitzer, Temporal variability of the noise wavefield at different scales: Influence on H/V and FK results, Neustadt Workshop Noise and Diffuse Wavefields, November 2012 Kværna, T., Verification-related research at NORSAR. Joint Scientific Commission meeting AFTAC, Florida 6-8- November 2012 Kværna, T., M. Pirli and J. Schweitzer, Overview of the seismological activities in the Arctic region, including plans for a seismic array on Bear Island. Joint Scientific Commission meeting AFTAC, Florida 6-8- November 2012 Kværna, T. and F. Ringdal, Analysis of the IDC reviewed event bulletin for detection capability estimation of the IMS seismic stations. Joint Scientific Commission meeting AFTAC, Florida 6-8- November 2012 Kværna, T, S. J. Gibbons, D. B. Harris and D. A. Dodge, Adapting Pipeline architecture to track developing aftershock sequences and recurrent explosions. Joint Scientific Commission meeting AFTAC, Florida 6-8- November 2012 Ottemöller, Lars, Observations from the Ongoing Storfjorden, Svalbard, Earthquake Sequence. ESC2012. Pirli, M., J. Schweitzer and B. Paulsen, A long earthquake series in the European Arctic: challenges and limitations to seismotectonic interpretation, 33rd General Assembly of the European Seismological Commission, Moscow, August 2012 Pirli, M. and J. Schweitzer, Seismic activity at the region of convergence of the Mohns and Knipovich Ridge, as seen within the International Polar Year 33rd General Assembly of the European Seismological Commission, Moscow, August

36 Roth, M and J. Fyen, Seismic sensors with hybrid response at the IMS array PS27, Norway. Joint Scientific Commission meeting AFTAC, Florida 6-8- November 2012 Schøyen, N. K., 2012 NORSAR infrastrucure and network. Joint Scientific Commission meeting AFTAC, Florida 6-8- November 2012 Sørensen, M.B., Ottemöller, Raeesi, M. and Storheim, B.M. (2012). The 30. August, 2012 M6.6 Jan Mayen earthquake, oral presentation at Nordic Seismology Seminar, Tallinn, October Sørensen, M.B., Bondevik, S. and Gjevik, B. (2012). Norwegian fjords sang along observations of seiches following the 2011 Tohoku, Japan, earthquake, oral presentation at Nordic Seismology Seminar, Tallinn, October References Alsaker A., Kvamme, L.B., Hansen, R.A., Dahle, A. and Bungum, H. (1991): The ML scale in Norway. Bull. Seism. Soc. Am., Vol. 81, No. 2, pp Andersen K. (1987): Local seismicity and volcanism in the Jan Mayen area. McS., Department of geosciences, University of Bergen. Atakan,K., Lindholm,C.D., and Havskov,J Earthquake swarm in Steigen, northern Norway: An unusual example of intraplate seismicity. Terra Nova 6, Brune J.N. (1970): Tectonic stress and spectra of seismic shear waves. Journal of Geophysical Research, 75, Grünthal, G. (1998): "European Macroseismic Scale 1998". Cahiers du Centre Européen de Géodynamique et de Séismologie Volume 15, Luxembourg. Havskov J., and Bungum, H. (1987): Source parameters for earthquakes in the northern North Sea. Norsk Geologisk Tidskrift,Vol.67, pp Havskov, J. and Ottemöller, L. (1999): SEISAN earthquake analysis software. Seism. Res. Letters, Vol. 70, pp Havskov, J. and Ottemöller, L. (2001): SEISAN: The earthquake analysis software. Manual for SEISAN v. 8.0, Department of Earth Science, University of Bergen, Norway. Havskov, J. and Sørensen, M.B. (2006): New coda magnitude scales for mainland Norway and the Jan mayen region. NNSN Technical report no