Regularities of Increase and Decrease of the Triggered Seismicity in the Rock Mass During the Lovozero Rare-Metal Deposit Development

Abstract

The article describes regularities of changes in mining-induced seismicity during development of the Lovozero rare metal deposit. The seismicity at the deposit is shown to arise as a result of rare-metal extraction. The increase in seismicity, including the occurrence of strong events in the rock mass (M > 1), is associated with the joint mining of adjacent gently dipping ore deposits. The location of the Lovozero deposit on the Kola Peninsula map until the 1990s was considered as an aseismic territory, since no earthquakes had been recorded there. The earthquakes with magnitudes M_L > 1 began to occur at the deposit after a new Umbozero mine had been put into exploitation in 1984. Seismic events at this mine occurred after the simultaneous mining of two contiguous gently dipping ore deposits (the distance between the deposits is 40–60 m vertically) at a depth of 200–400 m from the surface. The development of seismicity at the Umbozero mine led to the strongest mining-induced earthquake in the entire history of the Russian mines (magnitude M_L = 5.0, energy class k = 11.8), which occurred in the mine on August 17, 1999. The mine openings were destroyed within an area of 650 thousand m². Shortly after this event, the Umbozero mine was closed (in 2004). The mining-induced seismicity occurred in 2002 at the nearby Karnasurt mine (10 km from the Umbozero mine). The seismological observations have revealed that up to 13 strong seismic events occur in the Karnasurt mine (magnitude M_L > 1) per year. The mine simultaneously produced two gently dipping (incidence angle with horizon of about 10–15°) thin (1.2 m each) ore deposits at a depth of 50–600 m below the surface. The distance between the orebodies is 100 m. It has been established that the cause of seismic events at the Lovozero deposit is simultaneous mining of the adjacent ore bodies in the area of large gravitational-tectonic stresses in the rock massif. The measurements have revealed the horizontal tectonic stresses with a magnitude of 40–60 MPa at the reached depths in the Umbozero and Karnasurt mines. The concentration of gravitational-tectonic stresses in the zone of contiguous mining of ore bodies causes seismic events in the mines’ rock mass.