A study of the seismic behaviour of geomorphological features using ambient noise measurements

Abstract

When an earthquake strikes, one of the main factors affecting ground motion is site response. Although this is mainly due to the geology of the site, other factors such as the geomorphology also contribute to it. The aim of this study is to investigate the ground motion amplification and polarization effects (directional effects) at three particular sites which exhibit different geomorphological characteristics: Il-Prajjet area which exhibits landsliding phenomena due to the soft Blue Clay underlying the rigid Upper Coralline Limestone; Xrobb l-Għaġin and Ras il-Fenek, which are two peninsulas of different dimensions in the south of Malta, and are characterized by the Upper Globigerina Limestone. Ambient noise was measured using Micromed Tromino® portable seismograph. The HVSR method (Horizontal-to-Vertical Spectral Ratio) was used to investigate ground amplification, while the covariance matrix method was used to estimate the polarization characteristics of ground motion and thus investigate any directional effects. At Il-Prajjet, the use of ambient noise has provided, with a few simple and cost-effective measurements, insight into the dynamic behaviour of an area of coastal instability, characterized by lateral spreading, rock sliding and rockfalls. The characteristic lithology of the area, i.e. a layer of compact Coralline Limestone overlying a layer of clays, gives rise to a ubiquitous resonance peak between 1 - 2 Hz, which is observed both on stable areas as well on unstable blocks close to the cliff edge. Closer to the cliff edge, where large scale fracturing and block detachment occurs, the HVSR curves also show higher frequency resonance peaks, characteristic of the individual blocks. Moreover, polarization analysis reveals that these higher frequency peaks represent highly directional particle motion, while ellipticity analysis indicates this motion to be almost linear. This is strong evidence for normal mode vibrational behaviour of the large rock masses. The 1 - 2 Hz resonance peak, on the other hand shows no directivity or linearity of particle motion, confirming that it originates from the interaction of ambient noise surface waves from random directions with the lithology of the subsurface. The curves obtained on the peninsulas, were flat with no amplification. However, Ras il-Fenek, is also characterized by smaller scale fractures and the HVSR curves on this unstable part exhibited sizeable peaks. The ellipticity at these points also dropped to zero, indicating normal mode vibration of the partially detached block. The other points close to the cliff edge of the peninsula were also characterized pronounced directivity, parallel to the peninsula. At Xrobb l-Għaġin, no preferred direction was observed.