Dynamic characteristics of an active coastal spreading area using ambient noise measurements—Anchor Bay, Malta

Abstract

S U M M A R YAnchor Bay and surrounding regions are located on the northwest coast of the island of Malta, Central Mediterranean. The area is characterized by a coastal cliff environment having an outcropping layer of hard coralline limestone (UCL) resting on a thick (up to 50 m) layer of clays and marls (Blue Clay, BC). This configuration gives rise to coastal instability effects, in particular lateral spreading phenomena and rock falls. Previous and ongoing studies have identified both lateral spreading rates and vertical motions of several millimetres per year. The area is an interesting natural laboratory as coastal detachment processes in a number of different stages can be identified and are easily accessible. We investigate the site dynamic characteristics of this study area by recording ambient noise time-series at more than 30 points, over an area of 0.07 km 2 , using a portable three-component seismograph. The timeseries are processed to give both horizontal-to-vertical spectral ratio graphs (H/V) as well as frequency-dependent polarisation analysis. The H/V graphs illustrate and quantify aspects of site resonance effects due both to underlying geology as well as to mechanical resonance of partly or wholly detached blocks. The polarization diagrams indicate the degree of linearity and predominant directions of vibrational effects. H/V curves closer to the cliff edge show complex responses at higher frequencies, characteristic of the dynamic behaviour of individual detached blocks. Particle motion associated with the higher frequencies shows strongly directional polarization and a high degree of linearity at well-defined frequencies, indicative of normalmode vibration. The stable plateau areas, on the other hand, show simple, single-peak H/V curves representative of the underlying stratification and no predominant polarization direction. These results, which will be compared with those from other experiments in the area, have important implications for the understanding of ongoing processes in geologically active and unstable coastal environments.

Comprehensive analysis of the local seismic response in the complex Büyükçekmece landslide area (Turkey) by engineering-geological and numerical modelling

“…These findings are in good agreement with previous studies, demonstrating evidences of peculiar site effects on other hillslopes in case of monitored landslide site or unstable slopes (Burjánek et al, 2010;Del Gaudio and Wasowski, 2007;2011;Galea et al, 2014). More in particular, these studies suggest that directional resonance phenomena can be result from a combination of topographic, lithological and structural factors that act together to re-distribute shaking energy, focusing it on site-specific directions.…”

Fracture Network Imaging on Rock Slope Instabilities Using Resonance Mode Analysis

“…Ambient seismic vibration measurements have been increasingly used to characterize unstable slopes (Del Gaudio et al, 2014;Del Gaudio & Wasowski, 2011;Galea et al, 2014;Iannucci et al, 2018;Jongmans et al, 2002;Kleinbrod et al, 2017), prone-to-fall rock columns Colombero et al, 2018;Lévy et al, 2010;Valentin et al, 2017), and rock arches (Moore et al, 2016;Moore et al, 2018;Starr et al, 2015). Of particular interest is such seismic mapping in case of brittle rock failure, where only little prefailure displacements can be registered.…”

Single-block rockfall dynamics inferred from seismic signal analysis

“…The continuous recording ability of seismic networks allows a reconstruction of the gravitational activity on an unprecedented timescale and the monitoring of unstable slopes (e.g. Amitrano et al, 2005;Helmstetter and Garambois, 2010;Levy et al, 2011;Burjánek et al, 2012;Panzera et al, 2012;Galea et al, 2014). More than the detection of these events, recent advances allow determining the dynamics of the largest landslides on Earth from the very low-frequency seismic waves they generate.…”