Nat Hazards (2014) 74:1209–1222 DOI 10.1007/s11069-014-1240-2 ORIGINAL PAPER A multi-criteria evaluation model of earthquake vulnerability in Victoria, British Columbia Blake Byron Walker • Cameron Taylor-Noonan • Alan Tabbernor • T’Brenn McKinnon • Harsimran Bal • Dan Bradley • Nadine Schuurman John J. Clague • Received: 30 October 2013 / Accepted: 13 May 2014 / Published online: 27 May 2014 Ó Springer Science+Business Media Dordrecht 2014 Abstract Researchers have recently examined the geographic variability of the vulnerability of populations to earthquakes. These studies focus mainly on the complex modelling of geophysical processes or identification of socio-economically disadvantaged populations. However, no studies to date have integrated different components of vulnerability with metrics of travel distance to hospitals and trauma centres (systemic vulnerability). We argue that this previously unaccounted component is an important conceptual and practical aspect of earthquake vulnerability. Accordingly, this paper presents a multi-criteria model for combining physical, social, and systemic components, moving towards a more comprehensive assessment of vulnerability. An analytic hierarchy process is used to produce a place-specific index of social vulnerability, which we combine with soil liquefaction and amplification index and a road network model for access to hospitals and trauma services. Using a geographic information system, we implemented this model for the Greater Victoria region (483 km2, 2011 population: 345,000) in British Columbia, Canada. Clustering of total vulnerability was found in outlying areas, highlighting the importance of access to trauma centres. We conclude by identifying challenges in measuring earthquake vulnerability and advocate integration of systemic vulnerability components in natural hazards research. Keywords Earthquakes Risk Vulnerability Disaster response GIS Victoria British Columbia B. B. Walker C. Taylor-Noonan A. Tabbernor T’BrennMcKinnon H. Bal D. Bradley N. Schuurman Department of Geography, Simon Fraser University, Burnaby, Canada J. J. Clague (&) Department of Earth Sciences, Simon Fraser University, Burnaby, Canada e-mail: jclague@sfu.ca 123 1210 Nat Hazards (2014) 74:1209–1222 1 Introduction From 2000 to 2009, earthquakes caused a global annual average of $US 18.65 billion in total economic damage (Guha-Sapir et al. 2011). This estimate and others incorporate the effects of physical damage to the built environment, health care costs, and losses in economic productivity, but they fail to capture neighbourhood-scale variations in damage. Consequently, there recently has been a shift in the literature towards local analyses of vulnerability. Identification of areas that are most vulnerable to earthquakes enables targeted disaster preparedness and response programmes to reduce an earthquake’s total impact. A population’s vulnerability to an earthquake or other disaster depends on a combination of factors that affect its resilience both directly (e.g. structural damage and access to trauma centres) and indirectly (e.g. socio-economic disadvantage) (Rygel et al. 2006; Flanagan et al. 2011; Fekete 2012; Smith 2013). For analytical purposes, we deconstruct vulnerability into three components: physical damage based on geological features and the built environment; socio-economic barriers to resilience and recovery; and access to trauma and other support services. Spatial variations in these components result in different levels of vulnerability to an earthquake (Wisner and Luce 1993). While previous earthquake research has made significant progress in developing vulnerability assessments within the physical and social categories, no researchers have included the effect of access to trauma centres and hospitals and few have sought to integrate them into a comprehensive index. Accordingly, we present a multi-criteria evaluation (MCE) model of earthquake vulnerability that incorporates access to trauma centres and hospitals using a geographic information system to identify and rank residential areas in Victoria, British Columbia. 2 Study area This study focuses on the Greater Victoria region, located on the south-east coast of Vancouver Island along the Cascadia Subduction Zone (Fig. 1). In this area, the oceanic Juan de Fuca plate moves in a north-east direction beneath the continental North American plate. Convergence between the two plates and deformation of different blocks within the North American plate result in frequent earthquakes in south-west British Columbia and the US Pacific Northwest. In 2013, 171 earthquakes were recorded within 100 km of Victoria (National Earthquake Database 2014). Moderate-to-strong earthquakes (moment magnitude 5–7) occur in this region, on average once every 15–20 years, and geological evidence suggests that great and giant earthquakes (moment magnitude 8.0?) have an average recurrence of 500–600 years (Atwater et al. 2003; Frankel and Petersen 2008; Cascadia Region Earthquake Workgroup 2011; Goldfinger et al. 2012; Kulkarni et al. 2013; Natural Resources Canada 2013; Oregon Department of Geology and Mineral Industries 2013). The most recent major earthquake near the study area occurred in 1946 on central Vancouver Island (M7.3), and a smaller M6.9 event occurred in Puget Sound, Washington, in 2001 (Clague 1997; Chakraborty et al. 2005). Greater Victoria has a population of 345,000 and numerous old masonry buildings that make it particularly vulnerable to damage from earthquakes. Although much of the metropolitan area is located on firm rock, there are areas of thick Holocene and late Pleistocene sediments that are prone to ground motion amplification, as well as beaches and areas of artificial fill along the shoreline that are vulnerable to liquefaction. Potential seismic sources include the megathrust fault separating the Juan de Fuca and North 123 Nat Hazards (2014) 74:1209–1222 1211 Fig. 1 The study area, highlighted in green, is located on the south-east coast of Vancouver Island. Victoria is situated near the west edge of the North American plate, above the subducting Juan de Fuca plate. Traces of known crustal faults are shown in red American plate, crustal faults in the area, and the subducting Juan de Fuca plate, which flexes beneath the Salish Sea as it moves down into the asthenosphere. Great earthquakes on the megathrust trigger large tsunamis that are likely to reach elevations of somewhere between 2 and 5 m at Victoria (Clague et al. 2003). 3 Methods 3.1 Social vulnerability The concept of social vulnerability draws on empirical and theoretical evidence of less favourable disaster outcomes among socio-economically disadvantaged populations. The ability of a disadvantaged population to recover from an earthquake is impaired by limited economic and political capital (Schmidtlein et al. 2011). Numerous studies examining social vulnerability use quantitative indices, often modelled using census data (Wisner and Luce 1993; Morrow 1999; Delor and Hubert 2000; Lindsay 2003; Few 2007; Hewitt 2013). Most of this literature originates in the USA and thus tends to emphasise the proportion of minority populations as an explanatory variable (Schmidtlein et al. 2008). Other widely 123 1212 Nat Hazards (2014) 74:1209–1222 used metrics of social vulnerability are average income, age, family composition, language, and time spent living at one address (Chou et al. 2004; Chateau et al. 2012; Noriega and Ludwig 2012). Of these metrics, average income is commonly ranked as the leading predictor, followed by average or median age (Cutter et al. 2003; Rashed and Weeks 2003). However, geographic variations in population composition and social structures require that such indices be place-specific (Bell et al. 2007). This article details the development of a social vulnerability index specific to the study area and based on census data. The study area contains 452 census dissemination areas; data from seven of the 452 areas are unavailable due to publication restrictions. Where possible, we inferred missing values from these seven areas using lower-resolution census data. We selected nine census variables on the basis of previous findings in the literature (Table 1) and tested them for multi-colinearity (Table 2). Strong correlations between variables, for example, dependent population and seniors living alone, suggest that these are robust indicators of social vulnerability in the study area. We recalculated all values as standard scores and, where necessary, reversed their signs so that higher scores correspond to higher social vulnerability. We derived variable weights using an analytic hierarchy process, a method used in previous natural hazards research (Saatay 1994; Youssef et al. 2011). This method quantitatively ranks variables based on pair-wise comparisons and assigns weights as model coefficients. Reference to the literature is crucial when using this method, as it enables the implementation of expert opinion and prior knowledge. The input priority and pair-wise combinations were thus established based on the literature and observed multi-colinearity (Table 2), and mediated by team discussion. We then multiplied resulting weights (Table 3) by the relevant census variables and entered them into an additive model for each census dissemination area to produce the social vulnerability index scores. These scores were regressed against a similar algorithm for nearby Vancouver and found to correspond (r = 0.25, p \ 0.01), suggesting a suitable model fit (Bell et al. 2007). 3.2 Physical vulnerability Physical vulnerability to earthquakes can be conceptualised as a population’s risk of adverse impacts from ground shaking and secondary phenomena, such as liquefaction, landslides, surface rupture, and tsunamis (Douglas 2007; Duzgun et al. 2011; Fekete 2012). Distance from the hypocentre influences the intensity of ground shaking as well as the dominant components of the seismic wave spectrum, but near-surface and deeper geological conditions are also important (Sica et al. 2014). Amplification of ground motion, for example, can occur as waves travel through loose sediment, and steep slopes can reflect or refract seismic waves (Clague 2002). In some cases, liquefaction, landslides, or tsunamis can be more damaging than the primary ground motion itself (Clague 2002; Pathak and Dalvi 2013). Liquefaction is a process whereby saturated, non-cohesive sediment is transformed from a solid to a liquid state through intense shaking (Xue and Yang 2014). River deltas, shorelines, and reclaimed land are particularly susceptible to this phenomenon. Shaking during earthquakes can also trigger landslides that damage and bury buildings and infrastructure. Most tsunamis are triggered by the sudden displacement of the sea floor during earthquakes and can be extremely destructive in coastal areas, particularly if there is little time to evacuate before they make landfall (Xie et al. 2012). Further, building design and construction have been shown to significantly influence physical vulnerability (Hengjian et al. 2003). Significant 123 Nat Hazards (2014) 74:1209–1222 1213 Table 1 Census variables used to construct the social vulnerability index based on previous findings in the literature Census variable References Average income Chou et al. (2004) Cutter et al. (2003) Kuhlicke et al. (2011) Housing ownership Martins et al. (2012) Morrow (1999) Noriega and Ludwig (2012) Percentage of single-parent families King and MacGregor (2000) Kuhlicke et al. (2011) Martins et al. (2012) Percentage of no high school completion Armas (2008) Cutter et al. (2003) Martins et al. (2012) Unemployment rate Armas (2008) Cutter et al. (2003) Kuhlicke et al. (2011) Percentage of dependent population Cutter et al. (2003) Morrow (1999) Schmidtlein et al. (2011) Percentage of seniors living alone Chakraborty et al. (2005) King and MacGregor (2000) Schmidtlein et al. (2008) Percentage of foreign language only Noriega and Ludwig (2012) Morrow (1999) Percentage of recent movers (within the past year) Cutter et al. (2003) King and MacGregor (2000) Morrow (1999) Morrow (1999) Schmidtlein et al. (2011) Morrow (1999) Morrow (1999) progress has been made in recent years in modelling structural risk at the scale of individual buildings (Ventura et al. 2005; Onur et al. 2006; Mück et al. 2013; Xu et al. 2014), but structural data availability poses a significant limitation in this field (Dell’Acqua et al. 2013; Ehrlich et al. 2013). For our analysis, we define physical vulnerability as the potential risk of structural damage from amplification and liquefaction. Thicker clay, peat, or organic soil layers are substantially more prone to amplification than other materials, and anthropogenic fill and shoreline sediments are most susceptible to liquefaction. We used the combined liquefaction and ground motion amplification ratings created by the British Columbia Geological Survey, with six soil categories ranked on a scale of increasing amplification/ liquefaction risk (Monahan et al. 2000). We calculated an average soil amplification and liquefaction rating for each census dissemination area. These ratings were used as physical vulnerability index values (top-left map in Fig. 3). Although this procedure does not account for building conditions and some secondary damage phenomena, it provides a baseline to implement a MCE. Future research will examine the potential for incorporating terrain modelling and structural risk indices in a MCE model. 3.3 Systemic vulnerability Medical geography research has demonstrated that travel distance to trauma centres is significantly negatively correlated with the probability of patient survival (Amram et al. 2011). Immediately after a major earthquake, populations farther from a trauma centre are at a greater risk of adverse health outcomes (Gitis et al. 2012). In this context, access to 123 123 -0.13* Unemployment -0.02 Recent movers 0.73* -0.01 0.06 0.16* -0.15* 0.28* 0.16* Home ownership 0.04 0.08 -0.19* -0.35* 0.13* 0.20* Single parent 0.52* 0.24* 0.09 -0.04 0.12* No high school 0.00 0.12* 0.05 -0.09* Unemployment Values are Pearson’s R (linear); asterisks indicate significance of correlation for the study area (p \ 0.05) -0.01 -0.12* Seniors living alone Foreign language only 0.24* -0.25 No high school Dependent population -0.27* 0.18* Average income Single parent Home ownership Average income Census variable Table 2 Colinearity matrix of selected census variables for the social vulnerability index -0.03 -0.08 0.65* Dependent population -0.01 -0.03 Seniors living alone 0.15* Foreign language only 1214 Nat Hazards (2014) 74:1209–1222 Nat Hazards (2014) 74:1209–1222 1215 Table 3 Variable weights for the social vulnerability index, derived using an analytic hierarchy process Factor Average income Weights Rank 0.2855 1 % of seniors living alone -0.2371 2 Dependent population -0.1528 3 % of single-parent families -0.1239 4 0.0771 5 Unemployment rate -0.0412 6 % of population without high school completion -0.0349 7 Number of movers within the past year -0.0239 8 % of population without knowledge of an official language -0.0237 9 % of home ownership medical care is commonly defined as a function of distance, time, and travel barriers to care facilities (McLafferty 2003). The two primary ways of measuring access are distance-based and area-based. Distance-based measures focus on the time required for people at risk to reach a care facility, whereas area-based measures describe the ratio of population to services in an area (McLafferty 2003). Measuring access based on area suffers from a dependence on the spatial containers used (e.g. municipalities vs. provinces), a phenomenon known as the modifiable areal unit problem. Distance-based measures circumvent this problem and thus are commonly more accurate than area-based measures. Previous studies have used geographic information systems to integrate census and road network data to measure population access to hospitals and physicians (Brabyn and Skelly 2002; Luo and Wang 2003; Schuurman et al. 2010). Others have used similar approaches to model evacuation and information dissemination in the event of a natural disaster (Cova and Church 1997; Chiba 2011; Wood and Schmidtlein 2012; Zhang et al. 2013). However, no known earthquake vulnerability assessments to date have implemented access to health care. We built upon these approaches by incorporating an index of systemic vulnerability for the study area, which we define as the population’s accessibility to trauma services, contingent on the spatial arrangement and quality of transportation infrastructure and emergency service response time (Rashed and Weeks 2003; Peleg and Pliskin 2004; Nallamothu et al. 2006; Amram et al. 2011; Horner and Widener 2011). To calculate systemic vulnerability, we performed a distance-based network analysis (Brabyn and Skelly 2002). Six hospitals serve the study area, one of which is a trauma centre. Driving distances based on speed limits, travel time, walking time, and the number of road lanes per kilometre were separately calculated from the trauma centre and the nearest hospital to each census dissemination area and entered into a simple additive model. We then standardised the resulting accessibility value for each dissemination area to produce a systemic vulnerability score. Disaster response routes are designated roadways that only authorised vehicles can travel on in the event of an earthquake or tsunami. We removed these routes from the road network when calculating access to hospitals, as we assume personal vehicle use for less severe trauma, but we included them for access to the trauma centre on the assumption that emergency medical services vehicles would use them. Walking was included to account for the potential impassability of roads due to surface rupture. 123 1216 Nat Hazards (2014) 74:1209–1222 3.4 Constructing a multi-criteria evaluation model Multi-criteria evaluation is a quantitative method of geographic problem-solving and decision support that integrates multiple spatial data sets with score areas based on a set of predetermined criteria (Malczewski 1999). With roots in early constraint mapping methods in the 1960s, this process has historically been applied to site selection studies. However, recent studies have applied MCE to natural hazards research, including floods (Scheuer et al. 2011; Youssef et al. 2011; Feizizadeh and Blaschke 2013), landslides (Castellanos Abella and Van Westen 2007; Akgun and Türk 2010; Pourghasemi et al. 2012), and earthquakes (Rashed and Weeks 2003; Anbazhagan et al. 2010; Martins et al. 2012; Armaş and Rădulian 2014). The choice of evaluation criteria has differed since MCE was adopted as a problemsolving and decision support tool, with most studies focussing on geophysical factors (Akgun and Türk 2010; Anbazhagan et al. 2010; Pourghasemi et al. 2012; Feizizadeh and Blaschke 2013); others have integrated social vulnerability (Scheuer et al. 2011; Martins et al. 2012). Armaş and Rădulian (2014) present an algorithmically sophisticated MCE of earthquake vulnerability that includes geological risk, social vulnerability, and building condition. However, previous research has yet to account for the effect of systemic vulnerability, as described in the previous section. To combine the physical, social, and systemic vulnerability components of this MCE, we rescaled each component linearly from 0 to 1 and summed them to produce an equally weighted combined vulnerability score for each census dissemination area. Future research may integrate the analytic hierarchy process with expert input to derive customisable component weights (Youssef et al. 2011). A summary of the components is shown in Fig. 2. 4 Results Each component differs in its spatial distribution throughout the study area, as shown in Fig. 3. Physical vulnerability tends to be higher near shorelines, floodplains, and steep slopes, a finding concordant with previous studies (Clague 2002; Anbazhagan et al. 2010; Sica et al. 2014). Social vulnerability clusters in the urban core of Victoria with smaller pockets dispersed throughout the periphery of the urbanised area (Saanich, Langford, Oak Bay). Systemic vulnerability follows a predictable distance decay pattern along major roads. When all vulnerability elements are combined, the most vulnerable areas are in Langford, Victoria, Saanich, and Oak Bay. The pattern is similar to the physical vulnerability pattern, suggesting the significance of this component in our model. Neighbourhoods in the lowest quintile of overall vulnerability tend to have social vulnerability scores near the average value, but those scores are offset by their proximity to hospitals and their low soil amplification rating. Social vulnerability spatially deviates most from systemic and physical, which we interpret to be a mitigation of socio-economic deprivation by proximity to the expensive city centre. The five most and least vulnerable dissemination areas and their corresponding social, physical, and systemic scores are shown in Table 4. As expected, those with the highest systemic vulnerability are located at the fringes of the study area. These spatial patterns appear to fit with expected outcomes, but future research incorporating heterogeneous 123 Nat Hazards (2014) 74:1209–1222 1217 Fig. 2 Census data, a soil amplification/liquefaction index, and a road network analysis were rescaled to create three components of earthquake vulnerability component weights may yield results that deviate from ours. The analytic hierarchy process is a particularly powerful, yet easy-to-implement tool for this purpose. 5 Discussion Spatial interactions between components augment the geographic patterns in earthquake vulnerability, but additional patterns exist within these components that may influence the nature of vulnerability in a specific disaster. For example, vulnerability due to liquefaction is augmented in coastal areas by the threat of tsunami. Similarly, landslide and rockfall danger from primary ground shaking contribute to greater vulnerability due to increased ground motion amplification along hillsides, for example in north-west Saanich. The exclusion of these secondary effects from our model suggests that it may underestimate physical vulnerability in coastal and mountainous regions. Dissemination areas with high systemic vulnerability were identified at the fringes of the Victoria urban area. Beyond the study area, however, few roads penetrate into the interior of Vancouver Island; thus, many small communities are more isolated than the areas examined in this study. Systemic vulnerability thus may be a nonlinear function of distance and time, rather than linear as we have modelled. Little empirical research work has been conducted to examine this effect, presenting an important avenue for future study. Airlift services for medical emergencies are available in British Columbia, but such evacuation resources may be in high demand after a large earthquake, underscoring the importance of systemic vulnerability in future disaster vulnerability research. Furthermore, other resources will be required following an earthquake, such as grocery stores, petrol stations, banks, and other non-medical services. Future research should move towards the development of a weighted ‘local resource’ index for augmenting the applicability of systemic vulnerability. While there may be trade-offs between vulnerability components, the situational implications of the individual components differ. For example, low social vulnerability does not necessarily compensate for structural damage as a result of high physical vulnerability. However, in combination, they certainly amount to an augmented risk. This combinatory conceptualisation of total vulnerability is therefore a crucial concept for risk modelling. 123 1218 Nat Hazards (2014) 74:1209–1222 Fig. 3 Social, physical, and systemic vulnerability scores (top) were equally weighted and combined to produce the resulting earthquake vulnerability map (bottom). Highly vulnerable areas cluster around the edge of the City of Victoria and are recommended as priority neighbourhoods for earthquake preparedness and response programmes 5.1 Study limitations The scale and spatial units used in this study were dictated by census geography, making our findings vulnerable to the modifiable areal unit problem. The index of physical vulnerability is based on interpolated and aggregated data (Monahan et al. 2000); consequently, there is potential error in soil properties between sample sites. In addition, we did not model other secondary geophysical phenomena. The inclusion of building information would improve the accuracy of the model and significantly increase resolution. We determined the weighting of social factors in relation to earthquake vulnerability from the existing literature, which may not be directly applicable to Greater Victoria. The specificity 123 Nat Hazards (2014) 74:1209–1222 1219 Table 4 The five most vulnerable and five least vulnerable census dissemination areas in the Victoria metropolitan area, based on the earthquake vulnerability index Least vulnerable dissemination areas Most vulnerable dissemination areas Community Vulnerability score Community Vulnerability score Oak Bay 0.44 Saanich 2.38 View Royal 0.81 Langford 2.36 Esquimalt 0.83 Langford 2.34 Saanich 0.85 Saanich 2.31 Victoria 0.87 Saanich 2.17 of this model could be improved by adopting an expert survey approach (Bell et al. 2007). Accordingly, other study areas and post hoc evaluations of previous earthquake events might assist in the calibration of component weights. However, data availability and quality may be an inherent barrier in low-resource settings, such as Haiti. Finally, the outputs of any MCE are sensitive to component weights. Accordingly, future research should consider scenario modelling based on individual components, which can then be combined in consultation with disaster preparedness and response professionals (Yu and Lai 2011). The inclusion of expert-determined weights may facilitate the uptake of MCE methodologies in disaster planning, for example in dedicating evacuation resources or disseminating public safety information. 6 Conclusion We present a composite index for scoring a population’s vulnerability to earthquakes, based on the intersection of high social, physical, and systemic vulnerabilities. Our study is among the first to include a systemic component, which is an important consideration in both urban and rural locales. Our methods, including the use of systemic vulnerability indices, may benefit vulnerability assessments of other natural hazards, such as floods and landslides. 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