Changes in Seismicity, Stress, and Stress Proxies Before and After the Large Seismic Event with Mw 4.2 on May 18, 2020 in Kiirunavaara Mine (Sweden)

Abstract

ABSTRACT: An unusually large event with moment magnitude MW 4.2 occurred on May 18, 2020 in Kiirunavaara Mine (Sweden), followed by very intense aftershock activity for about 2 weeks. A comprehensive interdisciplinary study found that the event was the result of a combination of stress, rock stiffness, strength, structural and mining factors. In this paper, we concentrate on the observed patterns of seismic source parameters, inferred seismic velocity, and inferred stress and strain changes before and immediately after the large event, to appreciate potential warning signs for future similar events. The observations of the relation between investigated source parameters - apparent stress, energy index and seismic velocity changes - and calibrated stress and strain change from a calibrated mine scale numerical model, add to the understanding of system changes during the event and may assist in future to infer the stability state of parts of the mine, and potentially to identify some emerging instability hazards. 1. INTRODUCTION The Kiirunavaara Mine is a Sub-Level Caving (SLC) mine located in the north of Sweden, currently producing 25 Mtones crude iron ore per year. The production areas span ∼4 km along strike with footwall to hangingwall width varying from 30 to 150 m (Figure 1). The mine is divided into 10 production areas (blocks) along the strike, numbered by the central Y-coordinate. The mine was declared significantly seismically active after a 2008 seismic event in Block 19 (22) which caused significant damage across sub-levels from 907-964 m. Today, the mine has one of the world's largest underground seismic systems with approximately 250 geophones (Figure 1) covering the production areas with a typical magnitude of completeness of the seismic system of around ML -1.5 (Dineva and Boskovic, 2017). This study aims to continue the seismological investigation of the M 4.2 event, looking at long- and short-term changes in patterns of seismicity and quantitative seismological measures such as energy index, cumulative seismic moment and apparent stress, and relate them to numerical modeling results. The work also aims to progress the integration, in the mine's workflow for observational management of seismic hazard, of calibrated numerical modelling forecasts of stress with quantitative seismological parameters and seismic wave velocity changes inferred via passive tomography.