ORI GIN AL PA PER

Earthquake vulnerability and seismic risk assessmentof urban areas in high seismic regions: applicationto Chania City, Crete Island, Greece

A. Sarris • C. Loupasakis • P. Soupios • V. Trigkas • F. Vallianatos

Received: 2 February 2009 / Accepted: 10 October 2009� Springer Science+Business Media B.V. 2009

Abstract The earthquake vulnerability and the seismic risk assessment for the urban

center of Chania in the island of Crete is approached through the development of a GIS-

based application that takes into consideration the structural and geological domain of the

region. Considering a localized model, the various structural and geomorphologic attri-

butes of the region were assigned specific weights of significance that allowed the creation

of a modular application that was tested for the city of Chania, and it was verified based on

the recent seismic activity of the area. The proposed risk map and model can become a

significant tool for confronting crises resulting from future earthquake incidences.

Keywords Earthquake vulnerability � Seismic risk � GIS � Chania �Crete

1 Introduction

Seismic risk is the probability or likelihood of damage and consequent loss to a given

construction or group of constructions, over a specified period of time. It is important to

note the distinction between risk and vulnerability. Risk combines the expected losses from

all levels of hazard severity, also taking their occurrence probability into account, while

vulnerability of an element is usually expressed for a given hazard severity level.

A. Sarris � V. TrigkasLaboratory of Geophysical-Satellite Remote Sensing and Archaeo-Environment, Institute forMediterranean Studies, Foundation for Research and Technology, Hellas (F.O.R.T.H.), Nik. Foka 130,Rethymno, Crete, Greece

C. Loupasakis (&)Engineering Geology Department, Institute of Geology and Mineral Exploration, Olympic Village,Thrakomacedones, 13677 Athens, Greecee-mail: cloupasakis@yahoo.gr

P. Soupios � F. VallianatosLaboratory of Geophysics and Seismology, Technological Educational Institute of Crete, 3 Romanou,73133 Chalepa, Chania, Crete, Greece

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Nat HazardsDOI 10.1007/s11069-009-9475-z

A number of GIS-oriented applications related to the seismic risk and seismic hazard

have been developed in the past, some of which are dealing with the application of

emergency support systems (Voulgaris et al. 2003; Youhai et al. 2006) or simulation and

modeling of earthquake disaster episodes (Xu et al. 2008; Ren and Xie 2004; Zaincenco

and Alkaz 2005) and the development of earthquake information systems or hazard mit-

igation databases (Yamazaki 1996; Umemura et al. 2000; Giammarinaro et al. 2003;

Martelli et al. 2007; Navarro et al. 2008; Bartolomei et al. 2008; Inel et al. 2008).

In the context of urban centers, seismic vulnerability analysis is mainly focused in the

type of structural, geological and spatial information of buildings which are directly related

to potential human and economic damages in case of a seismic episode. Although there are

a few GIS-based tools, such as HAZUS (1999) and RADIUS (2000), that approach the

seismic risk assessment through either generalized expert information or localized obser-

vations and measurements, they are often difficult to be applied in regions that lack the

necessary information background. This is especially true in the region of the Mediter-

ranean, where due to the fact that most of the cities contain a number of historical buildings

and archeological monuments there are no detailed inventories and studies of the structural

integrity of them. Similarly, there have been no detailed geological and engineering studies

that can provide a sufficient input for the particular applications.

There are a few examples of the application of GIS in studying the earthquake vulner-

ability and seismic risk in urban centers. Lantada et al. (2003) applied a GIS methodology

for studying the vulnerability and seismic damage scenarios for Barcelona using an index of

average vulnerability associated to the residential building typologies of the city. Petermans

et al. (2006) used a combination of geological data, field measurements and numerical

modeling in a 2D and 3D GIS environment for assessing the local seismic hazard in the

urban center of Brussels. A similar method (VULNERALP) that was based on a simplified

approach (‘‘seismic inventory’’ of buildings) for vulnerability assessment in moderate-

to-low seismic hazard regions has been applied to Grenoble, France (Gueguen et al. 2007).

In Greece, Seismocare, a GIS scenario-based system was recently developed for studying

the regional damage and loss estimation due to earthquakes (Anagnostopoulos et al. 2008).

Similarly, RISK-UE project dealt with the seismic risk of the city of Thessaloniki (and six more

large European cities) by taking into consideration deterministic and probabilistic seismic

hazard scenarios together with the vulnerability of the elements at risk (Pitilakis et al. 2006).

In all the above mentioned studies, the main elements that are used include the spatial

distribution and engineering characteristics of the buildings and the geological and geo-

morphologic conditions of the ground. In some studies, vulnerability curves and seismic

zonation are established through the analysis of postseismic data. In our case, the vul-

nerability indices are established by taking into consideration the structural parameters of

the buildings, their usage and the ground conditions of the area where they are located.

The specific paper presents part of the results of the Interreg III SeRisk project

(Advanced Techniques for Seismic Risk Reduction in Mediterranean Archipelago

Regions) concerning the city of Chania. The SeRisk project dealt with the development of

a general and modular methodology for creating earthquake risk scenarios with emphasis

on the distinctive features of Mediterranean Archipelago towns and sub-regions, including

both current and historical buildings. The employment of the different approaches applied

within the framework of the project allowed the development of a GIS-based prototype

application that provides information to decision makers related to the earthquake

vulnerability and seismic risk at local and sub-regional geographic scales. Seismic risk

modeling was carried out by considering multiple weighted evaluation criteria. The

weighting scheme of analysis for combining various variables provides a greater degree of

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freedom, and it is based on the experts’ knowledge of the region under study. Furthermore,

it is flexible and the user can change the coefficients of the model interactively and apply it

with certain modifications in other regions. The particular methodology has been applied in

the past for seismic risk and hazard assessment (Lin 2008; Petersen et al. 2007; Onur et al.

2005; Nath 2004; Rashed and Weeks 2003; Matsuoka and Midorikawa 1995; King et al.

1994). By taking into account the geographic dimension, the GIS prototype can address

issues related to the vulnerability of population and infrastructure, the evaluation and needs

of the standards of building control and the management and control strategies that are

needed from the corresponding policy-maker agencies.

2 The city of Chania

The city of Chania is located to the west Crete and is the capital of the Prefecture of Crete

(Fig. 1). It is a modern city hosting a population of about 55,000 people, and it is the

Fig. 1 a Satellite image from Google Earth pointing the several sections of Chania City. b Panoramic viewof the historical center of Chania around the old port. c Four-storey historical building. The top floors wereconstructed by wood. d Old houses constructed by masonry embracing the Venetian period wall. e Top floorextension in a building constructed during the 1960s or the 1970s. The top floor was constructed withoutextending all the columns. The response of this construction under seismic loading can be really unfavorable

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second city in population in Crete, after the capital city of Heraklion. The city consists of a

historical center that is located around the old port. The historical center contains narrow

streets and well-conserved traditional houses that attract the tourist activity during the

summer months (Fig. 1). When it became the capital of Crete, in the early phases of the

re-union of Crete with Greece (1913), the city expanded around the old town with a

number of neoclassical buildings. Overcoming the difficulties that the World War II

(WWII) had left, the city of Chania slowly regained its normal pace of development during

the 1950s. The extensive construction activity characterized the second half of the

nineteenth century up to now.

The buildings of the city can be distinguished in four categories based on their con-

struction period and as a result based on their response on seismic loading. The first

category includes the buildings constructed until 1961. These are the historical and the

neoclassical buildings of Chania as well as the common residential buildings constructed

before and after the WWII. All those constructions are one or two and rarely three or four

floors buildings constructed by stone masonry, reinforced or not. Occasionally, the top

floors were made by wood (Fig. 1). The second category includes the simple concrete

frame buildings constructed after the enactment of the First National Earthquake Design

Code, between 1960 and 1984–1985. The buildings constructed after the revision of the

First National Earthquake Design Code, between 1985 and 1992, can be distinguished in an

independent category as they were constructed with heavier concrete frames incorporating

some anti-seismic concrete walls. In 1992, the New National Earthquake Design Code

(NEAK) was enacted. This code was revised twice, in 2000 and in 2003, providing one of

the strictest Earthquake Design Code of the World, incorporating Eurocode 8. The concrete

frame and the metallic buildings constructed following the rules of this code definitely

constitute an extra category. The concrete frame buildings designed following the rules of

NEAK are constructed with numerous oversized anti-seismic concert walls replacing the

majority of the typical concert columns.

Unlikely, many old buildings, constructed between 1970 and 1992, were subjected to

variant alterations and extensions without following the rules of any code (Fig. 1). Those

alterations are not recorded and as a result the response of those building in case of

earthquake cannot be incorporated in the applied risk assessment procedure, and they have

to be analyzed as separate units.

The aforementioned categorization of the constructions is vital for the estimation of the

seismic vulnerability of the city.

3 Geological—geomorphologic settings

The urban center of Chania is founded on two main types of geological formations,

Neogene sediments and Quaternary deposits. The main part of the city (namely the one

expanding toward the north and northeast coast) is located at Neogene formations, while

the quaternary deposits are met toward the west and south (mainland of the island)

directions (Fig. 2).

According to the geological map of the city (IGME 1971), the upper horizons of the

Neogene formations consist of loose marly sandstones, composed by intercalations of sandy

silts and silty/clayey fine calcareous sands. Based on numerous micro-fauna fossils, the age

of those formations was determined in Pliocene. The lower Neogene horizons are occupied

by Miocene marls alternating with beds of marly sandstones and platy marly limestones.

Those horizons are compact, and they occur in depths varying from 11 m, to the North, up to

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the surface, to the South, determined by the paleo morphological relief. The quaternary

deposits consist of loose loamed clays, sands and gravels of small thickness (up to a few

meters). Occasionally cohesive sandstones, remains of old sea terraces, appear.

Regarding the tectonics of Chania, no main active faults are found in the vicinity of the

city. A few possible faults are mentioned in the area, most of which running in a direction

E–W, N–S and NW–SE. Some of them are mentioned toward the west side of Akrotiri hill,

while a few more are mentioned to pass through the city. On the other hand, most of the

particular faults are characterized as geological faults, and they are not active.

The geotechnical characteristics of the foundation formations are critical for the eval-

uation of the seismic vulnerability of the constructions. As known, the looser the foun-

dation formations the higher the ground acceleration values in case of earthquake. Thus,

evaluating the above-described formations occurring in the study area, it can be understood

that the quaternary depositions provide less favorable foundation conditions compared with

the Neogene formations. This fact was taken under consideration during the design of the

vulnerability maps (see rating values in Table 1).

The city of Chania expands from the coast up to an elevation of 217 m above sea level.

The most abrupt slopes (\30�) are found toward the hilly elevations of the Akrotiri

promontory, located at the east side of the city (Fig. 3). This morphological uplift was

caused by a geological fault of NW–SE direction, and it was finally formed by the erosion

mechanisms.

This kind of topography may lead to intense amplification of seismic ground motion

along the slope and at neighboring (within a few tens of meters) points behind the crest,

especially for high frequency excitations (Bouckovalas and Papadimitriou 2005; Yu et al.

2008). Taking into consideration the effect of slope geometry on the dynamic performance

of a site, the seismic risk hazard weighting–rating system adopted in this study (Table 1)

was adjusted accordingly.

Fig. 2 Generalized Geological map of the area of Chania (CRINNO Project 2006)

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4 Historical and recent seismicity, damages and casualties

The Hellenic Arc is the seismically most active area in Mediterranean region due to the

subduction of the oceanic African lithosphere beneath the Eurasian plate. Specifically,

Table 1 Various data layers and seismic risk weighting–rating system adopted in this study

(i) Data layers Classes WeightingWi

RatingRi

A1 Construction year [1960 8

1961–1985 6

1986–1995 4

\1996 2

Under construction 2

A2 Construction materials Concrete Construction year Ri 3

Metal 2

Reinforced masonry 8

Masonry 9

Wood 8

A3 Number of the floors Ground floor Construction year Ri 1

1 Floor 2

2 Floors 4

3–5 Floors 6

[6 Floors 8

A4 Roof cover Inclined roof Construction year Ri 5

Terrace 0

A5 Density of adjacent buildings 0–10 Construction year Ri 1

11–20 2

21–30 3

31–40 4

41–50 5

51–60 6

61–70 7

71–80 8

81–90 9

91–100 10

B1 Geology Loose quaternary deposits 8 10

Neogene 7

B2 Faults Inactive (distance \ 100 m) 10 7

Inactive (distance [ 100 m) 0

B3 Morphology (inclination of surface) 0–5 10 1

6–15 2

16–30 5

31–45 8

46–60 9

[61 10

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Crete Island lies in the fore-arc of the Hellenic subduction zone being bored by numerous

earthquakes during the centuries. Additionally, along the interface between the subducting

African lithosphere and the Aegean lithosphere, south of western Crete, over Chania

Prefecture, high interplate seismicity was found (Engdahl et al. 1998). The lateral width of

the seismogenic zone due to interplate seismicity is about 100 km in NE–SW direction

south of western Crete decreasing to a width of about 30 km south of central Crete.

Interplate seismicity is found along the interface between the plates from about 20 to

40 km depth.

According to earthquake data taken from the database of the Geophysical Laboratory of

the University of Thessaloniki (Papazachos et al. 2000), from 550 BC until September

2007, 525 earthquakes were historically or instrumentally recorded for M C 4.5 in a

distance of 100 km from Chania. This number refers to the main shocks as foreshocks,

aftershocks and earthquake swarms were removed from the initial data (Papazachos and

Papazachou 1997). Many of these earthquakes cause damages or even worse casualties.

Based on historical sources, the 1846 March 28, earthquake with an epicenter close to

Heraklion damaged severely 20 houses in Chania. The 1856 October 12, earthquake with

an epicenter, also close to Heraklion caused several injuries in Chania and even more some

casualties in the wider region (Bardiani and Bardiani 1864; Schmidt 1879). Similarly, the

February 18, 1910 earthquake with an epicenter close to Chania ruined several houses and

caused the death of six people (Sieberg 1932; Platakis 1950). These cases are a small

sample of the historically recorded incidents presenting the vulnerability of the city during

the last two centuries.

One of the most recent strong earthquakes that caused several damages in Chania was

on January 8, 2006 at 11:34 GMT (13:34 local time), with a moment magnitude of 6.7

which occurred in southern Greece, off the eastern coast of the island of Kythira. The

earthquake’s epicenter as estimated by the European Mediterranean Seismological Center

(EMSC-CSEM, http://emsc-csem.org) was 36.31�N, 23.24�E, and the focal depth was

60 km. The shock was felt in a spatially extended area that covered Greece, Italy, Turkey,

Egypt, Cyprus, Israel, Syria, Jordan and Lebanon (Konstantinou et al. 2006, 2009; Boore

et al. 2009; Nikolintaga et al. 2008). The recorded PGA (peak ground acceleration) in

Fig. 3 A 3D projection of the Quickbird satellite image of the city of Chania. The city expands to thenorthern coastal side in the west part of the island of Crete, along a relatively flat area, with the exception ofthe east section of the city, where the hilly slopes of Akrotiri promontory are lying

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Chania (93 km, epicentric distance) was 41 mg (Konstantinou et al. 2006). This earth-

quake affected a total number of seventy buildings, twenty correspond to public ones, six

are schools and the rest are residences (Kouli and Vallianatos 2006). The level of the

damages as well as the spatial distribution and the constructional characteristics of the

damaged buildings provided substantial data for the verification of the earthquake

vulnerability maps provided by the current study.

5 Primary data and data layer preparation

The key issue for studying the earthquake vulnerability and seismic risk of urban areas is

the availability of maps and statistical information that concern the environmental settings

of the region and the infrastructure of the urban centers. For the best possible assessment of

the vulnerability and risk, it is necessary to have the maximum possible information such

as the one proposed by HAZUS risk assessment earthquake model. As it has been men-

tioned in the introduction paragraph, not all of these parameters are usually available and

vulnerability and seismic risk assessment is then limited to the availability of the infor-

mation for a specific region. The validity of the modeling is then based on the consider-

ation of the most vital parameters that reflect the actual factors that are involved in the

seismic risk of the region.

For the case study of Chania, data were collected from different agencies, and thus they

varied in quality and resolution. The particular datasets consisted of:

1. Topographic maps (scale of 1:5,000) of the Geographic Service of the Hellenic Army.

The particular maps contain information regarding the elevation (elevation contours of

4 m), the rural and national road network, the location of villages and settlements, etc.

Instead of using the elevation lines of the topographic maps (produced before 1960), a

SPOT stereoscopic image was employed to produce an accurate digital elevation

model (DEM) of the region with resolution (pixel size) of 20 m (Fig. 4).

2. A generalized geological map (Fig. 2) was created from the more detailed 1:50,000

scale maps of the Institute of Geological and Mineral Exploration of Greece The

geological attributes of the maps include geological formations and faults.

3. Statistical data (2001 registry) of the National Statistical Service of Greece that refer to

the block units and contain information related to the buildings and the corresponding

Fig. 4 Digital elevation map (left) and the corresponding slope map (right) of the urban center of Chania

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population. The cartographic information included linear data such as roads and

topographic characteristics as well as polygons such as buildings and building blocks.

The building inventory that covers a large part of the modern city contained

information (averaged to the block level) concerning geographic code, census sector,

census department, building block, the total number of houses, buildings adjacent to

neighboring constructions, form of building’s facing, the construction material (stone

masonry, reinforced masonry, wood construction, metal construction, concrete frame,

other material), the age of the constructions (before 1919, 1919–1945, 1946–1960,

1961–1970, 1971–1980, 1981–1985, 1986–1990, 1991–1995, after 1996, under

construction, not declared), the number of stories, roof type (terrace loft, tiles, slides,

other materials) and usage type (Exclusive usage, Mixed usage, Main mixed usage,

Secondary mixed usage). The later includes categories such as residences, hospitals,

churches, schools, offices, hotels, laboratories, parking lots. The total number of

houses contained within the different blocks is presented in Fig. 5, and it was also used

as an index of the density of the buildings per block. It has to be mentioned that in

order to obtain meaningful distribution maps, all the corresponding attributes of the

buildings were reduced to percentages over the total number of buildings per block

(Figs. 6, 7).

The population of the city was recorded to be around 4,000 people in 1700, and it was

raised to about 25,000 people in the beginning of the twentieth century. According to

the 2001 census, the population of the urban center of Chania was 53,373 people with

a continuing growing tendency as the people from nearby villages are gathered in the

capital of the prefecture. On the other hand, as it becomes also obvious from the

satellite images, the urban plan of the city did not expand severely since 1985, but

rather a higher density of buildings was constructed within the existing plan (Fig. 8).

Currently, the urban center covers an area of about 7.5 square kilometers and has a

population density of about 7,100 people/sq. km.

4. Geophysical data from previous studies (Papadopoulos et al. 2005; Sarris et al. 2005,

2006a, b; Soupios et al. 2005, 2008) were collected, evaluated, analyzed and used in

order to verify and refine the geological and tectonic information provided by the

Fig. 5 Total number of structures within the cadastral blocks of the urban center of Chania. More than halfof the area of the city is relatively sparsely inhabited although there are also areas that contain more than 20structures per block

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generalized geological maps. Specifically, electrical resistivity tomographies (ERT),

seismic refraction measurements, spectral analysis of surface waves (SASW) applying

linear configuration and horizontal to vertical spectral ratio (HVSR) measurements

were acquired in the past in the urban area of Chania city to determine the shallow

tectonic regime of the area under investigation and the general characteristics

(geometrical, lithological, etc.) of the subsurface. Specifically, HVSR measurements

helped the classification of geological structures in terms of the estimated amplifi-

cation factor and the different lithological units. The extracted information was

confirmed by the resulted versus models as estimated from the SASW method. The

particular measurements indicated that the upper layers consist of anthropogenic

materials, quaternary sediments and sands. Right below of the cover layer, cohesive

marls and marly limestones were suggested by the ERT and the refraction seismic

measurements. The basement seems to expand below the 50 m depth.

The refined geological and tectonic information, by means of the geophysical data,

were taken under consideration during the adjustment of the seismic risk hazard

weighting–rating system adopted in this study (Table 1).

5. Satellite data of medium (Landsat) and high (Quickbird) resolution were also

implemented in the project. The Landsat images (Fig. 9) consisted of a time series of

1988, 1994, 1999 and 2003 captures that indicate the structural expansion of the city of

Chania. All the images were rectified based on relatively uniform distributed GCPs

(Ground Control Points) using the nearest neighbor re-sampling interpolator,

achieving a root mean squares (RMS) error of less than ±1 pixel for each spectral

Fig. 6 Distribution of the percentage of structures (within the blocks) depending on their material type.Most of the stone buildings are located near the old neighborhood of the city close to the port. In contrast,the majority of the most recent buildings are made of reinforced concrete. Small percentages are noticed formetal and reinforced masonry buildings

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band. The images were processed with different spatial and spectral filters and

different color composites were formed for each dataset.

The Quickbird image (Fig. 9) was employed in order to delineate, vectorize and overlay

the polygons that corresponded to the main infrastructure buildings of the city, such as

public buildings, archeological monuments and historical buildings, hospitals, parking

lots, museums, galleries, stadiums, bus stations, schools, educational and research

facilities, coastal facilities, streets and any other kind of architecture of importance. The

particular types of structures were digitized corresponding to different thematic layers,

some of which were grouped according to their usage (public concentration areas, main

public buildings and areas, streets, other buildings) (Fig. 10).

6 Earthquake vulnerability maps—design methodology

The statistical data were preprocessed in order to create a series of thematic maps (layers)

that could be used directly in the modeling procedure in the GIS platform. All data were

rectified to be in the same projection system of the Greek Cadastral (HGRS’87—Hellenic

Geodetic Reference System) so that they could all be tied to the same reference system. GIS

spatial tools were used for creating surfaces from independent measurements (e.g. slope and

aspect maps from the DEM), buffers around linear features (e.g. faults) and carrying out

specific map calculations (algebraic calculations between maps, Boolean operations, etc.).

The raster and vector layers (weight factors) were then classified in specific categories that

Fig. 7 Distribution of the percentage of structures (within the blocks) depending on their number of floors.Most of the buildings consist of 1–2 floors. Very small percentages of buildings with 3–5 floors (and evenless for more than 5 floors) are noticed. The largest percentages of these buildings are noticed at theboundaries between the old neighborhood and the new city and they concern offices and hotels

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corresponded to specific weights of significance (rating). The specific weight factors and

ratings are given in Table 1. The contributing layers were classified to those dealing with the

construction risk (construction year, construction material, number of floors, roof type and

density of adjacent buildings) and those concerning the geological risk (geological forma-

tions, faults and morphology of the terrain—slope). The intermediate steps involved the

Fig. 8 Distribution of structures depending on their construction age in the urban center of Chania.Generalized categories were formed from the data provided from the National Statistical Service of Greece,in order to classify buildings according to their age-vulnerability and the compliance to the anti-seismicconstruction law that was established after 1985, and it was updated in 1995. The 1995 anti-seismic code(NEAK), incorporating Eurocode 8, imposed certain directives in the construction of buildings. It is obviousthat most of the historical buildings of the city are located close to the port of Chania. Most of theconstruction activity was carried out in the period of 1961–1985, slowing down the following year, without asignificance expansion of the city, but rather an intensification of it within the existing area of the urbannucleus of it

Fig. 9 Landsat satellite images of 1988 (left) and 2003 (middle) indicate the slow expansion of the urbansettings of the city of Chania. The 2002 Quickbird image (right) indicates the details needed to map the mainpublic buildings and infrastructure of the city

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construction of maps presenting the above-mentioned risk categories (Fig. 11), and in the

end, the following formula was used for computing the total seismic vulnerability index

(TSVI, Risk Coefficient) (Fig. 12):

TSVI ¼Xi¼A2

A5

RiWi þXi¼B1

B3

RiWi

where R and W are the rating and weighting, respectively, presented in Table 1. A2–A5 and

B1–B3 are the data layers used for the estimation of the construction and geological risk,

respectively.

Fig. 10 The Cadastral map of the city of Chania. The main public buildings and infrastructure are outlined.A number of areas have been also assigned as population concentration areas by the Civil ProtectionDivision of the Municipality of Chania in case of emergency

Fig. 11 Maps presenting the construction risk (left) and the geological risk (rights) for the urban center ofChania. The former map was based on the construction attributes of the buildings and the latter on thegeological and geomorphologic characteristics of the region

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7 Verification and evaluation of the earthquake risk map

Evaluating the construction risk map (Fig. 11), the city can be distinguished in three zones.

The first zone includes the old town and the coastal zone, composed by relatively high to

very high risk constructions. The majority of the buildings in this zone are one to two floors

stone (Figs. 6, 7) residential houses constructed before 1961 (Fig. 8). This zone also

includes the small neighborhood in the inner central section of the city consisting of old

houses with the aforementioned characteristics. This neighborhood can be easily located in

the total risk map (Fig. 12) and also in the map referring to the constructions made by stone

(Fig. 7). The second zone includes the bigger section of the city extending from the

northwest coastline to the foot of the Akrotiri promontory, composed by medium to rel-

atively high risk constructions. This is a mixed constructions area composed mainly by two

to five floors concrete frame buildings constructed between 1961 and 1985 and secondarily

by one to two floors stone residential houses constructed before 1961 (Figs. 6, 7, 8). The

third zone includes the inner perimeter of the city as well as the Akrotiri promontory

region. This is a medium-to-relatively low risk constructions zone composed by concrete

frame buildings constructed after 1961 or even better after 1986.

She geological risk map (Fig. 11) separates the city in two sections, the very low risk

section and the medium to very high risk section. The first one includes the main part of the

city occupied by Neogene formations (Fig. 2), and the second one the perimeter of the city

as well as some parts of the Akrotiri promontory region either occupied by quaternary

deposits or located along the most abrupt slopes (\30�).

The inverted distribution of the risk levels between the two maps affects positively the

final risk levels of the total risk map. For example, the fact that the main section of the city

with the medium to relatively high risk constructions coincides with the low geological risk

region reduces the earthquake effect. Moreover, the fact that the perimeter of the city with

the medium to relatively high level of geological risk coincides with the low construction

Fig. 12 Total risk map for the city of Chania which was produced from the synthesis (algebraic sum) of theprevious risk maps (construction and geological risk maps)

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risk section minimize or even more efface the negative influence of the geological structure

to the urban environment. On the contrary, the fact that along the north western coast line

the relatively high level of geological risk coincides with the relatively high to very high

risk constructions influence the total risk map by appointing a very high risk neighborhood.

Thus, the combination of the two risk maps provided an earthquake risk (total risk) map

(Figs. 12, 13) that distinguishes the city of Chania in two zones. The first zone includes all

the sections with the constructions with the seismically unfavorable characteristics (the old

town, the coastal zone and the small neighborhood in the inner central section of the city).

And the second zone includes the rest of the town with the mixed constructions blocks. The

first zone includes the relatively high to very high risk constructions, and the second zone

the relatively low to relatively high risk constructions. Note that the geological risk,

besides the northwestern coastline region, is practically eliminated by the fact that the

relatively high or high geological risk areas coincide with the low construction risk

sections.

In order to verify the accuracy and the efficiency of the risk map, data referring to the

level of the damages as well as the spatial distribution and the construction characteristics of

the damaged buildings of the 2006 January 8 Kythira earthquake were used. According to

these recordings (Kouli and Vallianatos 2006), the suffered buildings are distributed mainly

in the northern sector of the Chania city (Fig. 13). The seriously damaged buildings were all

constructed prior to 1970 while the buildings constructed after 1970 were moderately to no

damage. Thirteen of the suffered buildings were constructed during or after the year 1980,

and five of them were moderately damaged requiring repair. Furthermore, 17 of the seri-

ously or moderately damaged buildings were located in the Pliocene Marly Sandstone

formation, 16 in the Miocene Marls formation and only eight in the Marly Limestone.

Fig. 13 The classified building damages of the 2006 January 8 Kythira earthquake overlaid to total riskmap of the city of Chania. The level of damages was classified into four classes (1, 2, 3, 4) with anincreasing level of damage; 11 buildings were seriously damaged with the need of total reconstruction (class4), 30 were moderately damaged and could be used after repair (class 3), four could be used without repair(class 2) and 23 were very slightly damaged (class 1). The black bullets present the damaged buildingsconstructed after 1980 (Kouli and Vallianatos 2006)

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At first, the fact that all the seriously damaged buildings were constructed before 1970

and that the younger buildings were moderately to no damaged, verifies the grounds of the

rating–waiting procedure adopted for the construction characteristics. Furthermore, the

spatial distribution of the damaged buildings verifies the entirety of the design method-

ology applied for the earthquake risk map. As presented in Fig. 13, the majority of the

seriously (class 4) and moderately (class 3) damaged buildings were located within the

limits of the first, high risk, zone (the old town, the coastal zone and the small neigh-

borhood in the inner central section of the city). The rest of the damaged buildings were

located in blocks, of the second zone, evaluated as relatively high to very high risk

sections, mainly because of the increased percentage of old houses within their limits.

After that, the unfortunate incidence of the Kythira earthquake verified successfully the

accuracy of the earthquake risk (total risk) map and the applied procedure.

8 Conclusions—proposals

The produced earthquake risk map can provide substantial information for the develop-

ment of the city, the land planning design of future infrastructure, the planning of crises

confrontation procedures from the public protection services of the state, etc.

For example, the seismic risk map can provide information concerning the selection of

the proper location for the construction of infrastructure vital during a crises situation (e.g.

a hospital or a fire brigade). Considering that, the modern construction techniques can

provide solid anti-seismic buildings; one of the most important issues in case of selecting

the location for the construction of a new infrastructure is to reassure unconfined access

during a crises situation period. Based on the information provided from the risk map of

Chania, such locations can be selected at the perimeter of the city and along main roads

entering the city center without crossing the high risk sections. Note that the old hospital

(Fig. 10) is located next to the coastline high risk section and most of the main access roads

are crossing through high risk sections. Thus, in case of a strong earthquake some of the

main roads could be blocked from the collapsed buildings.

Additionally, the risk map can be used, from the public protection services, for the

definition of the proper concentration points’ distribution in order to cover correctly the high

risk sections. Currently, the selected concentration points cover sufficiently the perimeters

of the high risk sections but their distribution is not uniform at the rest of the city.

The above-mentioned examples and the numerous other applications of the seismic risk

map’s information appoint their importance for the protection of the cities against earth-

quakes and they justify the obligation of the state to provide them for every city.

Acknowledgments This project was carried out under the framework of the EU Community InitiativeProgramme, INTERREG IIIB ARCHIMED, ‘‘Advanced Techniques for Seismic RISK reduction in Med-iterranean Archipelago Regions’’.

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