Some issues of mining-induced seismicity in Khibiny and Lovozero mountain massifs

1. Some issues of mining-induced seismicity in Khibiny and Lovozero mountain massifs Asming V.E., Kola Regional Seismological Centre GS RAS Kozyrev A.A., Lovchikov A.V. , Mining Institute of KB GS RAS Korchak P.A., PHOSAGRO joint-stock company

2. Position of Khibiny and Lovozero Massifs in Kola Peninsula Khibiny Lovozero Khibiny and Lovozero alkaline mountain massifs are situated in the centre of Kola Peninsula, North-West Russia. They are two parts of a single intrusion having similar ages and nearly the same rock types (alkaline rocks formed in several stages of igneous activity).

3. Khibiny Massif Lovozero Massif Karnasurt (underground) Quarries Underground mines Umbozero (underground) The exploitation was started in 1930s. About 27 million ton of apatite ore per year is excavated. Large underground and open-pit blasts (several hundred ton) are carried out almost every week. A lot of small technological explosions Rare metal deposits are mined by underground mines. Umbozero 1984-2004 (watersealed in 2009) Karnasurt 1951-now. But no large explosions are conducted there.

4. Seismic stations used for monitoring the Massifs KRSC (KB GS RAS) stations: AP0Apatity small-aperture array (11 elements, 40/80 Hz sensors) APAApatity 3C station (40 Hz) IRIS/GS RAS station LVZLovozero 3C station FOSAGRO (Apatit) mining seismic networks Kirovsk mine network(29 x 3C stations, 1000 samples per second) Rasvumchorr mine network (11 x 3C stations, 1000 samples per second) (the networks unable to locate events outside the mining areas)

5. Seismicity of Khibiny Massif • Main features: • Large mining explosions here are followed by hundreds of rock bursts but the process relaxes during first several days. Most part of such rock bursts are too weak to be detected in Apatity (usually detected by mining seismic networks only) • The most part of earthquakes or rock bursts in Khibiny is concentrated in mining areas. Background seismicity in the Massif but outside the areas is negligibly small (14 events with maximal magnitude 1.4 during 15 years 1992-2007) • Strong rock bursts or earthquakes sometimes take place with no strict connection with mining explosions. The strongest event occurred here 21/10/2010, M=3.2, No precursors or foreshocks have been observed. • Problems of study: • Very large number of explosions. Small underground explosions are considered as a routine technological process. No information available from mining authorities. • No strict criteria of separation explosions and rock bursts. Some long-delayed explosions can be recognized by spectra but these are minority. • The mining networks were unable to locate events outside their local areas.

6. Uniting the KRSC and “Apatit” seismic networks (2010) Apatit JSC data centre KRSC Disc loop 2 weeks SERVER Client (can request bulletins and wave forms) Daily bulletins UDP protocol CSS 3.0 wave forms Automatic detector/locator Convertors Daily bulletins Location program LORS A client-server application has been developed to transmit KRSC data (bulletins and waveforms) to ‘Apatit’ processing centre and locate seismic events Kirovsk and Rasvumchorr data

7. Rock bursts detected and located by the joint KRSC-Apatit network 1/10/2010-24/03.2012 157 events. M 0.6-3.2 Most part of the events occurred in mining areas. The strongest one (M=3.2) occurred 21/10/2010 in Kirovsk mine

8. An empirical discriminant between rock bursts and explosions P and S amplitudes have been measured in 4 frequency bands : 2-10 Hz, 10-18 Hz, 18-26 Hz, 26-34 Hz. N of earthquakes was increased to be the same as N of explosions (each earthquake used several times) The regression was fit : Where D is 0 for rock bursts and 1 for exposions. A are the regression coefficients. The discriminant correctly recognizes about 70% of events. The future plan is to combine this with another criteria and sources of extra information (spectra, place, time, infrasound signals etc.) in a form of Bayesian network

9. Seismicity of Lovozero Massif • Main features: • No large mining explosions there. All events detected in Apatity appeared to be earthquakes or rock bursts • Seismic events there are observed since 1988 • The strongest was 17.08.1999 earthquake with magnitude 4.0-4.4 (different estimations). It caused large destructions in Umbozero mine. • Problems of study • Geometry of Apatity and Apatity array is bad for event location in the area; • Complex velocity structure of the region leads to event location errors; • Low S/P ratio for events here (looked like typical explosions) led to false identification of Lovozero events as mining explosions; • Before 17.08.1999 in KRSC there was no understanding what we see in Lovozero Massif. From one hand the events were very similar with Khibiny explosions by S/P amplitude ratio. From the other hand we saw no distinct connection between the events and the mining areas.

10. Lovozero rare-metal deposits 1 – worked-out mining areas, 2 – seismic station, 3 – rock-tectonic bursts, 4 –faults, 5 – ellipses of main stresses Mined by : Karnasurt underground mine(1951-present day) Umbozero underground mine (1984-2004, watersealed in 2009)

11. Problems with event location and identification in Lovozero Massif Error ellipse Routine location of Lovozero events. The scattering is caused mostly by onset picking errors and travel time model errors. No distinct connection with the mines Example of Lovozero event location by Apatity (APA) and Apatity array (AP0). Bad configuration led to a big error ellipse S/P ratio Near, but outside Lovozero 1 3 2 1- events outside Lovozero but close to it 2 – events in Lovozero 3 – Khibiny explosions Lovozero events very similar with Khibiny explosions! In Lovozero massif

12. The strongest Lovozero earthquake 17.08.1999 4.44:36, M=4 The event occurred in Umbozero mining area. 600-650,000 m2 of the excavations have been destroyed. The earthquake was felt. Its intensity in the epicenter was 8 points MSK, 2-3 points in Apatity. The stream of Alluaiw that flowed on the surface above the mine went into the ground through the cracks.The Umbozero mine was stopped. The event was followed by several dozens of aftershocks (22 detected in Apatity, the strongest has M=2.9). Wave form of the earthquake was very different from previous events as well as aftershocks Event occurred in 1993 17.08.1999 earthquake Aftershock

13. The strongest Lovozero earthquake 17.08.1999 4.44:36, M=4 Investigation of the earthquake source made by the Mining Institute Kola Sci. Centre RAS. The event was caused by simultaneous excavation of two flat pitch layers (17°) at vertical distance 50-60 m from each other and zone of their joint excavation

14. Comparison of the strongest Lovozero and Khibiny earthquake recordings The recordings at Apatity station are almost identical. This could indicate similarity of the sources.

15. Calibration explosion 10.09.2002 in Lovozero Massif 2 ton of explosive was detonated in a derelict mining gallery. This resulted in 1.7 magnitude seismic event. Local travel-time model was corrected. Several seismic events were re-located. All of them appeared to occur within Umbozero and Karnasurt mining areas.

16. Umbozero mine rock bursts since 2000 Mine conserved Mine watersealed In 2004 the mine was conserved. Since that time number of seismic events there diminished significantly. In 2009 the mine was watersealed. This led to a short increase of seismicity here after that it completely stopped

17. Block structure of Lovozero Massif III rank tectonic block Since 2001 seismic events occurred in Karnasurt mining area. At this time the exploitation of the western wing of Karnasurt has begun. The western wing of Karnasurt and the whole Umbozero mine belong to the same III rank tectonic block. This block is the most stressed in the Massif. Thus, all seismic activity in Lovozero Massif is caused by mining work in the most stressed tectonic block of the Massif.

18. Thank You for attention !