GEOSUFFOLK RIGS
25
AN IMPACT SCENARIO FOR THE COLCHESTER EARTHQUAKE OF 1884 A-J. S. WESTON Introduction This paper briefly outlines research undertaken to develop an impact scenario for the area surrounding the Colchester earthquake of 1884. A detailed discussion of this research can be found in Weston (2004). The Colchester earthquake was one of the UK’s most damaging earthquakes to date, reaching intensity VIII on a twelve point scale of intensity (the Medvedev-SponheuerKarnik (MSK) intensity scale, Burton et al., 1984). It occurred in an area of southeast England which has been otherwise more or less devoid of earthquakes. Its Richter local magnitude has been estimated from historical macroseismic studies (studies of the felt effects of earthquakes) to have been 4·7ML. This magnitude is by no means extreme by UK standards and the average expected recurrence of a similar magnitude earthquake is 10 years in the UK. Extensive damage was caused to over one thousand buildings within the epicentral area south of Colchester town. This high damage level relates to the very shallow source (2–3 km) on land which was indicated by the rapid decay of intensity with distance near the epicentre. It was felt as far north as Hull, as far west as Exeter and even in Europe (Ostend and Boulogne). An Earthquake Impact Scenario (EIS) is a method of estimating the seismological risk to an area i.e. the expected amount of damage due to particular historical earthquakes or specific hypothetical earthquakes. An EIS first forecasts the level of ground shaking (a measure of expected intensity at locations across an area due to seismic activity) and then assesses the vulnerability of buildings (seismic vulnerability) and damage resulting from the expected levels of intensity at each location. The former predicts the seismic hazard for an area, while the latter estimates the seismic risk, where: Seismic Risk = Seismic Hazard × Seismic Vulnerability (Ove Arup, 1993). Musson (2000) summarises these terms as: “seismic hazard is purely a product of natural processes, seismic risk is dependent on societal exposure in terms of the built environment or human population. The fragility of structures is expressed in terms of vulnerability” (Musson, 2000: 353). Methodology Figure 1 summarises the layered methodology of the underlying EIS conducted within a Geographical Information System (GIS). EIS Layer 1 – Observed Intensity – Colchester 1884 macroseismic database – point data and isoseismals. The observed macroseismic intensity data from the British Geological Survey’s (BGS) reassessment of the Colchester earthquake (Musson et al., 1990) forms EIS Layer 1. The locations for which an observed intensity was assessed as greater than or equal to intensity VI MSK (practical threshold of building damage) were entered into the GIS. In this way the observed macroseismic field for the Colchester 1884 earthquake was modelled.