Seismology on Kola: Monitoring earthquakes and explosions in the Barents region

Elena Kremenetskaya, Vladimir E. Asming Kola Regional Seismological Centre, Apatity, Russia

The Kola Regional Seismological Centre of the Russian Academy of Sciences (KRSC) is a small organization in the town of Apatity on the Kola Peninsula, North-West Russia. Since 1982 we have been continuously monitoring seismic events in North-West Russia and the adjacent seas.

In the past, we have primarily used our own set of seismological stations (see Table 1), but in recent years we have also been using data from IRIS stations (KBS, LVZ, KEV, ARU, ALE, NRI, etc.) and the Scandinavian seismic arrays (ARCESS, SPITS, FINESA, HFS, NORESS) for analysis of complicated events.

TABLE 1. Kola Seismic Network

As a result, a large amount of information has been collected, including seismic bulletins and catalogues, digital wave forms data, digitized seismograms for selected events, results of spectral processing, etc.

Because of the mining activity in our region, a lot of artificial seismic signals have been registered here including open-pit, underground and underwater explosions. These events give us a good basis for developing some criteria to discriminate explosions and earthquakes.

Kola regional seismic network

Before 1992, KRSC used only analog seismic stations. All of our stations were equipped with SKM-3 short-period seismometers with the same amplitude-frequency response (amplification 50000 in frequency range 1.25-2 Hz). In addition, the Apatity station included long-period sensors SKD (Ts=25 sec). Seismograms from all the stations (excluding KHE) are stored in Apatity. Data from KHE were transferred by daily telex into KRSC.

In 1991, a cooperative program between KRSC and NORSAR (Norway) began. As the result, three digital seismological stations have been installed.

1. A seismic array (aperture 1 km) comprising 11 short-period sensors (Geotech S-500) is located about 17 km to the west of Apatity.
2. In the town of Apatity there is a 3-component broad band digital station (Guralp GMG-3T).
3. A micro-array, with aperture about 150m, was installed in Amderma in 1993. The array is situated in an underground fluorite mine and comprises 3 vertical sensors and a 3-component station in the center. The sampling rate is 40 samples per second.

Events Location

Because the IASPEI-91 model is not suitable for the Barents region [1], we use local travel-time curves developed from a set of strong explosions with known locations. In addition, a 350 ton underground calibration explosion was carried out in the Khibiny Massif on September 29, 1996 [2]. Our velocity model is a combination of the NORSAR model for shallow depths (above 200 km) and the IASPEI-91 model for below 200 km.

TABLE 2.

 

Date	Our location	JHD	Remark
18.08.83	73.289N, 54.893E	73.358N, 54.943E
01.08.86	72.945N, 56.549E	73N, 56.7E	Located by Marshall
02.08.87	73.298N, 54.398E	73.324N, 54.597E
07.05.88	73.275N, 54.436E	73.314N, 54.557E
24.10.90	73.304N, 54.634E	73.317N, 54.803E

To confirm it we relocated several previous seismic events. As seen in Table 2, a comparison of our locations with the known locations indicates the model's capability. In addition, we have sufficient data to locate the small nuclear explosion [3] on the Novaya Zemlya test site on August 26, 1984. The result is shown in Figure 1.

Figure 1. Location of the smallest Soviet nuclear explosion using data by stations PYR, BRB, APA and AMD.

Data analysis

The KRSC detection and location software is based on our algorithm which is close to the generalized beamforming approach [4]. It operates well when data from several seismic stations are available. The Amderma station, however, is far from other seismic stations so we frequently have to locate weak near events using only its data. The small aperture makes it impossible to use beamforming or other procedures to determine array-style back azimuths. Moreover, high noise levels (probably due to construction work) are commonly present. Under such circumstances, the totally automatic processing often results in wrong phase association (true phases may be associated with a noise, etc). To avoid this error, we use a semi-automatic routine. An analyst marks approximately the phases and the automatic procedure is executed for the filtration, STA/LTA detection, and joint polarization analysis both for P and S phases. Although accuracy of this method is not high, it is often suitable for preliminary locations (see examples below).

August 16, 1997 Events

On August 16, 1997, five seismic events occurred near Amderma station. Two of the events were very similar events and occurred in the same location of the Kara sea (distance from Amderma is about 320 km). The wave forms are shown in Figure 2.

Figure 2. Wave forms for two events on August 16, 1997, in the Kara Sea (Amderma stations). Low frequencies filtered below 2 Hz.

The other two events were explosions near Vorkuta (about the same distance but to South-West from Amderma) and one event was too weak to locate.

The result of the site-specific monitoring for this day is shown in Figure 3. The SSM procedure detected and located the two Kara events and the two Vorkuta explosions. False alarms are also shown. The results of semi-automatic location for the Kara event (6.20 GMT) and the Vorkuta explosion (7.02 GMT) are given in the insertions.

Figure 3. Results of site-specific monitoring using Amderma station for August 16,1997. Examples of semi-automatic location (isolines of rating functions and their maxima corresponding to epicenters) are shown in the insertions.

Event Discrimination

As mentioned, we often record seismic events which can not be identified by traditional criteria like P/S ratio. For instance, an event on February 9, 1998 near Murmansk (69.18 N, 32.63 E, 16.51:07) was recorded by seismic arrays ARCESS and SPITS very differently. The S-wave amplitudes for SPITS were much less than the P-wave amplitudes independent of the bandpass filter used. ARCESS recordings, on the other hand, include strong S-wave and even Lg and Rg phases.

A striking event occurred on April 18, 1998 in the Norwegian Sea near Bear Island. Its wave forms (recorded at APA, ARCESS and SPITS), together with our location are shown in Figure 4.

Figure 4. Wave forms together with our location of the strange event on April 18,1998. Signs "? S" mark places where S-onsets could be.

The nearest station is SPITS (about 470 km) and itÕs recording contains no noticeable S wave in any frequency band. ARCESS (670km) recorded strong S, whereas APA (1020 km) registered the P-wave only in the band 8-12 Hz.

Note that the calculated epicenter of this event is the same (within the limits of location errors) as the position where the nuclear submarine "Komsomolets" found its rest in 1989 [6]. Could the nuclear submarine be the source of this mysterious event ?

Acknowledgement

The authors greatly appreciate the support and scientific advice of Dr. Frode Ringdal. We thank Dr. Ralph W. Alewine for his consistent support of KRSC's developments. Finally, we appreciate the encouragement by Dr. Gregory van der Vink to write this article.

References

1. F. Ringdal, E. Kremenetskaya, V. Asming, Y. Filatov. Study of seismic travel-time models for the Barents region. Semiannual Technical Summary 1 October 1996 - 31 March 1997, NORSAR Sci. Rep. 2-96/97, Kjeller, Norway.
2. Ringdal F., Kremenetskaya E., V. Asming, I. Kuzmin, S. Evttuhin, V. Kovalenko (1996) : Study of the calibration explosion on 29 September 1996 in the Khibiny Massif, Kola Peninsula. Semiannual Technical Summary 1 April - 30 September 1996, NORSAR Sci. Rep. 1-96/97, Kjeller, Norway.
3. Mikhailov, V.N. et.al. (1996) : USSR Nuclear Weapons Tests and Peaceful Nuclear Explosions, 1949 through 1990, RFNC - VNIIEF, Sarov, 1996, 63 pp.
4. T. Kvaerna and F. Ringdal. Generalized beamforming, phase association and threshold monitoring using a global seismic network. In : E.S. Husebye and A.M. Dainty (eds), Monitoring a Comprehensive Test Ban Treaty. 1996, Kluwer Academic Publishers. Printed in Netherlands. 447-466.
5. F. Ringdal and T. Kvaerna. Continuous seismic threshold monitoring. Geophys. J. Int. (1992) 111, 505-514 6. The Barents sea, its fisheries and the past and present status of radioactive contamination, and its impact on fisheries. By Lars Føyn and Ingrid Sv¾ren Institute of marine research (IMR) Bergen, Norway. Paper presented at The international conference on Environmental Radioactivity in the Arctic - Oslo, Norway - August 21 - 25, 1995.

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