Microseismic investigation of an unstable mountain slope in the Swiss Alps

Risks associated with unstable rocky slopes are growing as a result of climate change and rapid expansions of human habitats and critical infrastructure in mountainous regions. To improve our understanding of mountain slope instability, we developed a microseismic monitoring system that operates autonomously in remote areas afflicted by harsh weather. Our microseismic system comprising 12 three-component geophones was deployed across ∼60,000 m² of rugged crystalline terrain above a huge (30 million m³) recent rockfall in the Swiss Alps. During its 31-month lifetime, signals from 223 microearthquakes with approximate moment magnitudes ranging from -2 to 0 were recorded. Determining the hypocenters was challenging for several reasons: (1) P wave velocities were highly heterogeneous, varying abruptly from <1.5 km/s to >3.8 km/s. (2) First-break picks were either inaccurate or lacking for some microearthquakes. (3) There were no reliable S wave picks. (4) Numerous microearthquakes occurred just outside the network boundaries. These issues were addressed by using a three-dimensional (3-D) P wave velocity model of the mountain slope determined from refraction tomography in a nonlinear inversion for hypocenter parameters and their probability density functions. Recordings from geophones at different altitudes and in boreholes constrained microearthquake depth estimates. Most microearthquakes were concentrated within 50-100 m of the surface in two zones, one that followed the recent rockslide scarp and one that spanned the volume of highest fracture zone/fault density. These two active zones delineated a mass of rock that according to geodetic measurements has moved toward the scarp at 1-2 cm/yr.