Microseismic events on the Åknes rockslide in Norway located by a back-projection approach

Abstract

The Aknes rockslide in Western Norway is characterised by a steady movement of a rock mass with an extent of about 1 km2 and a yearly deformation rate in the range of 2 to 4 cm. A seismic network consisting of 8 three-component geophones records tens to hundreds of local and distant seismic events daily. Depending on their character, local seismic events show a variety of waveforms, both with abrupt and emergent onsets. Along with the relatively low frequencies of the waveforms, standard location procedure using arrival time measurements is difficult to apply. Based on the event envelopes, we classify local events and obtain their approximate location by stacking STA/LTA ratios of back-projected waveforms. To suppress the influence of complex surface morphology and related complicated wave propagation, we constrain hypocentres to the surface of the slope and determine only horizontal coordinates. The method was successfully tested by locating two types of ground truth data: calibration shots and a block collapse. The test proved the ability of the method to determine the position of sliding events with an uncertainty of less than 36 m, which allows to distinguish amongst several foci of rockslide activity. Application of the method to 8 years of monitoring data shows continuous seismic activity, which is concentrated in the centre and at the western edge of the monitored area. Most likely, microseismic events recorded by the seismic network originate within the body of the rock slope and are related to its disintegration or potentially to sliding on the detachment fault.