One of NORSAR’s internal cross-collaborative topics is the seismic monitoring of unstable rock slopes. The main purpose of these long-term installations is to record seismic events directly related with the mass movement of the slide, as well as secondary (triggered) events such as rock falls or small-scale rock avalanches.
In November 2014, upon request from Statens Vegvesen and Nordnorsk Fjellovervåkning (NNFO), NORSAR instrumented the Jettan rockslide with a broadband station. The Jettan rockslide at Nordnes is located in Kåfjord county (Northern Norway). Since Statens Vegvesen started in 2014 to construct a tunnel through Nordnesfjellet in close proximity to the Jettan rockslide, the initial purpose of the seismic station was to monitor the ground motion characteristics produced by the tunneling works, especially the blasting of rock material by using explosives. Since then, the seismic station’s purpose is to further monitor the microseismic activities of the unstable rock slope section.
This field of applied research started in 2005 with the first instrumentation of the Åknes rockslide at Stranda (county of Møre and Romsdal, western Norway) and more recently with the Jettan rockslide at Nordnes (county of Kåfjord, northern Norway) in November 2014.
The main purpose of these long-term installations is to record seismic events directly related with the mass movement of the slide as well as secondary (triggered) events such as rock falls or small-scale rock avalanches. Seismic monitoring systems are considered complementary to direct measuring systems (i.e., extensometers, laser ranging, inclinometers, etc.). In contrast to these direct measurements which provide information on deformation that are constricted to a limited area, the seismic monitoring can cover a much wider range of the rockslide area.
Alerts local authorities
Continuous seismic monitoring of a rockslide can provide data from increased seismic activity, as it is expected that a movement of the slope will be accompanied with an increasing seismic activity.The collected data from the seismic stations that are placed either on top of the unstable slope section or in close vicinity are then processed in real-time mode and made available immediately to local authorities. Thereby, alert messages and warnings can be immediately conveyed to the public that might be endangered by a potential slope failure or its secondary effects, i.e., a triggered flood wave.
Seismic monitoring of the unstable rock slopes Åknes and Jettan
In 2005, NORSAR installed a small-aperture network at the Åknes rockslide at Stranda (county of Møre and Romsdal, western Norway). This network consists of eight high-frequency geophones which were placed along the rockslide’s top rupture line in order to monitor the microseismic activity of the rockslide in a real-time mode. In October 2009, the network was supplemented by a seismic broad-band station which was placed in the midst of the rockslide plane.
Instrumentation works of the unstable rock slope Åknes
Location and extent of the unstable rock slope Åknes with sites of the small-aperture network of seismic sensors.
All seismic stations operate in a continuous recording mode. Real-time data access is provided on NORSAR’s website www.norsardata.no.
Collaboration with NVE
In January 2016, NORSAR and the Norwegian Water Resources and Energy Directorate (NVE) started a collaboration on the geophysical surveillance of rockslides in Norway. The main task of this collaboration is to further develop seismic monitoring procedures of selected rockslides in Norway. The analysis and interpretation of seismic data by also considering meteorological data as well as data from other point-based and area-based displacement measurements is hereby a major task.