AN IMPACT SCENARIO FOR THE COLCHESTER EARTHQUAKE OF 1884

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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.

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Figure 1 – Schematic illustrating the GIS-based EIS layers (numbered 1–8).

EIS Layer 2 – Forecasted Intensity – intensity due seismic attenuation.

A surface of intensity forecasting the decrease in seismic energy (i.e. the intensity of shaking) with distance from the earthquake’s source was generated using a theoretical attenuation relationship fitted to the damaging near-field observed intensity data (i.e. ≥VI MSK).

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EIS Layer 3 – Surficial Geology Boundaries.

A map of surficial geological boundaries was developed using the BGS digital data in order to consider potential amplification effects i.e. the increase in the intensity of the earthquake when the seismic waves pass from rock into less rigid material such as soil.

EIS Layer 4 – Soil Amplification Intensity Increments.

A scheme whereby each of the geological units within the study area (i.e. EIS Layer 3) is linked to its potential amplification increment relative to zero amplification for bedrock, was developed based on estimated shear wave velocities.

EIS Layer 5 – Expected Intensity (EIS Layer 2 + EIS Layer 4) – expected intensity due to seismic attenuation and soil amplification A surface of intensities that would be expected by the Colchester 1884 earthquake considering both the attenuation of seismic energy away from the earthquake source and amplification effects, was generated by combining EIS Layers 2 and 4. This surface is a model of the macroseismic field for the Colchester earthquake and represents the seismic hazard in terms of the levels of ground shaking that would be expected for such an event.

EIS Layer 6 – Building Types The built environment was analysed in terms of building type (e.g. detached), age (e.g. 1930s) and building material (e.g. timber frame) to generate EIS Layer 6. A building classification scheme was developed and in order to consider the level of detail necessary for a seismic risk assessment, three resolutions of analysis were conducted – high (HR), medium (MR) and low (LR). HR and MR analysis used all the available data, results were spatially referenced to individual buildings, and building type, age and materials were classified. More detailed fieldwork was carried out for HR analysis than for MR analysis. LR analysis did not involve fieldwork, rather it relied on data from local authority House Condition Surveys. LR results were not spatially referenced to individual buildings but to local authority administrative boundaries, all buildings were assumed to be masonry and only building age was classified.

EIS Layer 7 – Vulnerability Types In order to be able to consider the potential damage of earthquake ground shaking to the built environment, a classification relating the building type, age and material data (i.e. EIS Layer 6) to seismic vulnerability was established and the building polygons reclassified in terms of six vulnerability types. Vulnerability functions developed by The Martin Centre (Cambridge University), from a worldwide dataset of damage surveys were used for this study. Damage Probability Matrices (DPMs) and vulnerability curves, which consider buildings of different vulnerability types in terms of probabilities of different levels of damage resulting from particular levels of intensity, were developed for 6 common UK building types (Weston & Burton, in prep).

28 EIS Layer 8 – Probability of Damage DPMs and vulnerability curves developed for this study were combined with the surface of expected intensities (i.e. EIS Layer 5) to generate surfaces of probabilities of damage for each of the six vulnerability types (i.e. EIS Layer 7) and to each of the five grades of damage within the MSK intensity scale i.e.

D1 to D5 where: D1 – negligible to slight damage; D2 – moderate damage; D3 – substantial to heavy damage; D4 – very heavy damage; and D5 – near total collapse.

These damage probability maps were combined with the building vulnerability types (i.e. EIS Layer 7) in order to assess the seismic risk in terms of the number of buildings of each vulnerability type damaged to each grade of damage and the costs of this damage. This seismic risk assessment was considered for a “what if” scenario representing a repeat of the Colchester 1884 earthquake in the same location of Essex.

In summary, EIS Layers 1 to 5 (Figure 1) represent the seismic hazard analysis i.e. the modelling of the observed macroseismic data for Colchester 1884 and EIS Layers 6 to 8 the seismic risk assessment for a scenario earthquake representing the potential damage and costs involved for a modern day repeat of the Colchester earthquake.

Results Seismic risk results for the repeat 1884 scenario show that approximately 40 % of the building stock within a study area would be expected to suffer some damage. More than half of this damage is expected to be grade one (D1 – negligible to slight damage) and the combined percentage of damage at grades four ( D4 – very heavy damage) and five (D5 – near total collapse) is expected to be minimal. Damage costs for this repeat 1884 scenario are estimated to be in the region of £1,000 million (at modern values). These results lead to an important finding: that, although the media might emphasise the striking devastation at D5, even D1 losses are financially significant. This is of concern to the insurance and reinsurance industry in the wider context of the total costs arising from damaging earthquakes.

Conclusion The comprehensive historical records for the Colchester 1884 earthquake provide an ideal platform on which to build this EIS study and in this way the 1884 event acted as a calibration for estimating future damage for a “what if” scenario.

GIS provides a powerful environment for an EIS layered approach to seismic hazard, vulnerability and risk assessment. Layers are easily updated to allow for the dynamic nature of the built environment.

The poorer level of spatial disaggregation of the LR database compared to both the HR and MR databases (i.e. to local authority administrative boundaries rather than individual buildings) is less suitable for a detailed seismic risk assessment.

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References Burton, P. W., Musson, R. M. W. & Neilson, G. (1984). Studies of historical British earthquakes, Global Seismology Unit, British Geol. Survey, Report No 237.

Musson, R. M. W., Neilson, G. & Burton, P.W. (1990). Macroseismic reports on historical British earthquakes XIV, 22 April 1884 Colchester, British Geol. Survey, Report No WL/90/33.

Musson, R. M. W. (2000). Intensity-based seismic risk assessment, Soil Dyn. & Earthq. Eng., Vol. 20, 353–360.

Ove Arup (1993). Earthquake hazard and risk in the UK, Technical Report, Ove Arup & Partners, London.

Weston, A-J. S. (2004), Earthquake Impact Scenarios: A GIS-based case study for Colchester, UK, PhD thesis, University of East Anglia.

Weston, A-J. S. & Burton, P.W., Vulnerability functions and damage probability matrices for UK building stock (in prep).

A-J.S. Weston School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ a-j.weston@uea.ac.uk