Seismic characterization of a clay-block rupture in Harmalière landslide, French Western Alps

Abstract

SUMMARY In late June 2016, the Harmalière clayey landslide (located 30 km south of the city of Grenoble, French Alps) was dramatically reactivated at the headscarp after a 35-yr-long period of continuous but limited activity. The total involved volume, which moved as sliding blocks of various sizes, was estimated to be about 2 × 10 6 m3. Two seismometers were installed at the rear of the main headscarp in August 2016, on both sides of a developing fracture delineating a block with a volume of a few hundred cubic metres. For 4 months, they continuously recorded seismic ambient vibrations and microearthquakes until the block broke. Five seismic parameters were derived from the monitoring: the cumulative number of microearthquakes (CNe), the seismic energy (SE), the block resonance frequency (fB), the relative variation in Rayleigh wave velocity (dV/V) deduced from noise cross-correlations between the two sensors and the associated correlation coefficient (CC). All parameters showed a significant precursory signal before the rupture, but at very different times, which indicates the complexity of the rupture mechanism in this clay material.

Ambient Seismic Noise and Microseismicity Monitoring of a Prone-To-Fall Quartzite Tower (Ormea, NW Italy)

“…Spectral ratio computations were performed to enhance the presence of these frequency peaks in the spectral content [5,7,12], both on single stations (e.g., H2/V2: horizontalto-vertical spectral ratio on S2) and site-reference configurations (e.g., H2/H1: horizontalto-horizontal spectral ratio between S2 and S1). For each station, the horizontal spectrum H was computed as total horizontal energy, by the composition of E and N hourly spectra.…”

“…Standard methods for ambient seismic noise cross-correlation were applied [5,12,23,24]. Cross-correlation was performed in the 2-20 Hz frequency range of vertical components.…”

“…Spectral analysis and cross-correlation of the recorded noise are applied to extract resonance frequency variations and seismic velocity changes within the investigated volumes. Both seismic parameters can show reversible fluctuations driven by external modification in air temperature and precipitation [1] and irreversible drops in their values when failure is approached [2][3][4][5].…”

“…The spatial orientation of the resonance frequencies can also identify the potential direction of collapse and give further information about the constraints of the potentially unstable volume. For vertically elongated rock columns and prisms, separated from the stable rock mass by one or more near-vertical fractures, the first vibration mode is generally related to the bending of the unstable volume, perpendicular to the direction of the main fractures separating stable and unstable compartments [5][6][7][8][9][10]. The second vibration mode usually bends perpendicular to the first mode, i.e., parallel to the fracture orientation.…”

“…These seismic events are extracted from ambient seismic noise recordings by means of detection algorithms. Their classification, source location and temporal trends can help to identify the most unstable compartments and track the evolution to failure [5,[13][14][15].…”