Microseismic monitoring of the controls on coastal rock cliff erosion

Abstract

The aim of this thesis has been to improve understanding of the controls on coastal rock cliff erosion, utilising microseismic ground motion. Coastal cliff erosion has remained poorly understood, in part confounded by the challenges associated with monitoring changes to and controls upon steep slopes in the coastal zone. As a result the relative contribution of marine to subaerial and episodic to iterative forcing is based upon models with only limited field validation. For two years, from July 2008 to July 2010, cliff top microseismic ground motions were monitored using a broadband seismometer, installed on top of a 70 m high hard rock cliff of Jurassic mudstone, shale and sandstone, on the North York Moors National Park coast, UK. Concurrently cliff face erosion was monitored using high-resolution 3D terrestrial laser scanning. Regional-scale marine and weather data for the monitoring period and modelled nearshore wave conditions were used to establish the conditions under which cliff microseismic ground motions were generated. Distinct ground motion frequency bands were found to correlate with a range of marine and subaerial processes that transfer energy to the coastline and cliff. Fundamentally, microseismic sources were identified both at the cliff face from, for example, direct wave impact during cliff toe inundation, but also at more distal locations resulting from the transfer of energy from gravity and infragravity waves. Further analysis demonstrates statistically significant correlations between rockfall and cliff ground motion generated by wave impacts and wind at the cliff face, but also surprisingly waves across the nearshore and offshore, implying direct environmental controls on cliff erosion rate. The significance of longer period ground motion, representative of ocean gravity and infragravity waves also identifies an almost constant dynamic loading of the cliff rock mass, highlighting a potential for progressive deterioration of the cliff rock. The analysis demonstrates that cliff top microseismic ground motion provides a valuable proxy for marine and atmospheric forcing at coastal cliffs, overcoming the limitations in quantifying and testing controls on cliff erosion. The findings of this study are used to develop a new conceptual model of the environmental processes and failure mechanisms that control rock cliff erosion.