Under-representation of faults on geological maps of the London region: reasons, consequences and solutions

London lies mainly within an area of long-term tectonic stability known as the London Platform. This is characterised by relatively thin Cretaceous and Palaeogene sequences overlying Palaeozoic basement at shallow depths, less seismic activity than surrounding areas and, according to published geological maps, little faulting. However, observations of temporary exposures and borehole records, and other studies, show that in reality faults are numerous and widespread in the London region. Their relative absence on the geological maps is a consequence of past mapping methods, coupled with the relative uniformity of extensive bedrock units such as the London Clay Formation and the Chalk Group, and the widespread presence of Quaternary and anthropogenic deposits, and of urban development. However, complementary approaches to geological surveying, including the use of geophysical data and satellite-based radar interferometry, together with geological modelling in three dimensions using subsurface information, provide the means to accurately survey fault systems even in the most densely urbanised areas. Such work shows that earth movements in the London area, apparently including near-surface fault displacements, have taken place during the late Quaternary and continue at the present. These findings are important to civil engineering projects and hydrogeological studies in the London area and to understanding local tectonic development.