Hierarchy of Factors of Seismic Danger in Four Towns of Colima State, México

Transcription

1 Natural Hazards 33: , Kluwer Academic Publishers. Printed in the Netherlands. 427 Hierarchy of Factors of Seismic Danger in Four Towns of Colima State, México VYACHESLAV M. ZOBIN 1 and J. FRANCISCO VENTURA-RAMÍREZ 2 1 Observatorio Vulcanologico, Universidad de Colima, Colima, Col., 28045, México ( vzobin@cgic.ucol.mx); 2 Facultad de Ingeniería Civil, Universidad de Colima, Colima, Col., 28045, México ( fic@ucol.mx) (Received: 12 May 2003; accepted: 7 January 2004) Abstract. The RADIUS methodology is applied to study the hierarchy of the factors of seismic danger in four towns of Colima state, México. These towns are situated near the western coast of Pacific ocean, within zone of highest seismic risk. The 31 indicators and five factors (Hazard, Exposure, Vulnerability, External context and Emergency response and recovery capability) of seismic danger were calculated. Their comparative analysis showed that the social factors of seismic danger might compete and even prevail over the natural factors. The recent earthquake of 21 January 2003 (Intensity up to VII VIII MM in Colima state) was used to discuss some problems of application of RADIUS methodology in various locations, such as in Latin America. Key words: Seismic danger, México, Colima, RADIUS. 1. Introduction An earthquake lasts a few seconds or some minutes. It may kill people and destroy constructions during this time. But the danger produced by an earthquake does not end with the seismic vibrations. Intensity VIII to X according to the Modified Mercalli (MM) or the European Macroseismic (EMS) scales may interfere with social and economic activities of a city for a long time. The problem of seismic risk reduction is of great importance for our towns. The RADIUS (Risk Assessment Tools for Diagnosis of Urban Areas against Seismic Disasters) methodology was developed worldwide during to detect the weak elements in the social structure and help to plan the urban development. The unified methodology for calculation of factors of seismic danger allows comparing the seismic danger for a group of cities using a small group of specialists within a low-budget project. The following five factors of seismic danger were proposed: 1. Hazard (H), which measures the severity, extent and frequency of the geological trigger phenomena to which the city may be subjected. 2. Exposure (E), which measures the size of the city, the number of people and physical objects and the amount and type of activities they support.

2 428 V. M. ZOBIN AND J. F. VENTURA-RAMÍREZ Table I. Characteristics of the towns under study Town Population Annual growth Area (km 2 ) Status Colima 196, Capital of state Armería 15, Agricultural town Manzanillo 94, Oceanic port Tecomán 74, Agricultural town 3. Vulnerability (V), which measures how easily the exposed people, physical objects and activities, may be affected in the short or long term. 4. External context (C), which measures how impact within a city affects people and activities outside the city. 5. Emergency response and recovery (R), which measures how effectively and efficiently a city can reduce the impact of an earthquake through formal, organized efforts made specifically for that purpose (Davidson and Shah, 1998) In this paper, the RADIUSmethodology was applied to four towns in Colima state, México. The main factors of seismic danger estimated for each of these four towns are compared for purposes of diagnosis to study the hierarchy of the factors of seismic danger for the city. 2. Description of the Towns under Study Figure 1 shows Colima state and the four towns of Colima, Armería, Manzanillo and Tecomán that were selected for study. They are situated near the western Pacific coast, within the zone D of the highest seismic risk according to the map of seismic zoning of México (Manual de diseño por sismo, 1993). Two lithospheric oceanic plates (Rivera, R.P., and Cocos, C.P.), separated by the El Gordo graben, subduct along the Middle American trench (M.A.T.) beneath the continental North American plate (N.A.P.). During the last century, six destructive earthquakes from the Mexican subduction zone (magnitude Mw 7.4 to 8.0), that occurred in 1932 (Nos. 1 and 2), 1941 (No. 3), 1973 (No. 4), 1975 (No. 5) and 2003 (No. 6), damaged towns in Colima state (Zobin and Ventura-Ramírez, 1998, 2001; Table II) with intensity up to VII IX MM. The problem of seismic danger is of importance for these towns. The main characteristics of the towns are presented in Table I. They are rather small towns, from 15,000 to about 200,000 of population. The towns occupy areas from about 3 km 2 (Armería) to about 30 km 2 (Colima). The annual growth of population varies from 220 persons for Armería to more than 4000 for Colima. Armería and Tecomán, are agricultural centers; Manzanillo is an important ocean port; Colima is a cultural and industrial center, the capital of state.

3 FACTORS OF SEISMIC DANGER IN MEXICO 429 Figure 1. Geographic and seismo-tectonic position of Colima state. The state borders are shown by a slight dashed line, the towns are shown by the diamantes. The epicentres of six destructive earthquakes are shown by circles; their numbers correspond to the numbers in Table II. The coastal line of Mexico is shown by a heavy line. N.A.P. is the North American plate; R.P. is the Rivera plate, C.P. is the Cocos plate. The outline of El Gordo graben is shown by a heavy dashed line. The Middle American trench (M.A.T.) is shown by a dashed line parallel to the coast. Table II. List of destructive earthquakes of Colima state during last 100 years Date Latitude, N Longitude, W Mw Ms

4 430 V. M. ZOBIN AND J. F. VENTURA-RAMÍREZ Dominant construction is of one- or two-story masonry. They can be divided into three groups due to their design (Zobin and Ventura-Ramírez, 2000, 2001): (1) Type A, good quality. The buildings are designed with some lateral resistance to ground shaking. Columns are installed along the walls at a distance of 2 m and in the corners of the construction. (1) Type B, intermediate quality. The buildings are not designed to resist ground shaking. The walls have resistant elements only in the corners of the construction. (1) Type C, bad quality. Old constructions made out of adobe and cinder blocks. No resistant elements. The majority of old constructions (before 1980) are of types C and B; more recent constructions are of B and A types. The towns are different in their shape. The port of Manzanillo occupies a long narrow stretch along the waterfront (Figure 2a). The other towns are more compact (Figure 2b). 3. Methods and Data The RADIUS methodology (Davidson and Shah, 1998) provides a diagnosis of weak elements in the urban organism which may be dangerous for a city while the earthquake occurs. It provides an approach to calculate thirty-one Earthquake Disaster Risk Index (EDRI) indicators of five factors (Table III), the factors themselves (Table IV), and to estimate the mean values and standard deviations for each factor of a set of cities. The city whose values of any factor are above the standard deviation is assumed to be endangered. These quantitative characteristics allow assessing the city s seismic risk and the development of an earthquake scenario describing the effects of a probable earthquake on the city. All aspects of the calculations of the EDRI components are described in Davidson and Shah (1998). EDRI indices are firstly scaled with respect to the mean and standard deviation for a sample of cities to remove the influence of the units of measurement. To calculate the EDRI and the five factor indices H, E, V, C and R, the scaled indicators X ik were corrected by weights w ik proposed by Davidson and Shah (1998), as shown in formulas (1). Here i is the factor index and k is the number of the indicator of the factor. EDRI = w H H + w E E + w V V + w C C + w R R; H = w H1 X H1 + w H2 X H2 + w H3 X H3 + w H4 X H4 + w H5 X H5 + w H6 X H6 + w H7 X H7 ; E = w E1 X E1 + w E2 X E2 + w E3 X E3 + w E4 X E4 + w E5 X E5 + w E6 X E6 ; (1) V = w V1 X V1 + w V2 X V2 + w V3 X V3 + w V4 X V4 + w V5 X V5 + w V6 X V6 ; C = w C1 X C1 + w C2 X C2 + w C3 X C3 ;

5 FACTORS OF SEISMIC DANGER IN MEXICO 431 Table III. The indicators of seismic danger Indicator Colima Armería Manzanillo Tecomán XH1 exp(pga with a 100-year return period) 1,800 3,000 3,000 3,000 XH2 exp(pga with a 500-year return period) 4,900 8,100 8,100 8,100 XH3 Percentage of urbanized areas with soft soil XH4 Percentage of urbanized area with high liquefaction susceptibility XH5 Percentage of buildings that are wood XH6 Population density (people per square kilometer) 6,174 5,376 3,813 7,386 XH7 Tsunami potential indicator XE1 Population 196,318 15,384 94,893 74,106 XE2 Per capita GDP, in constant 2000 US dollars (1USD = 10 pesos) 4,472 4,343 11,922 10,591 XE3 Number of housing units 48,756 3,729 24,158 16,970 XE4 Urbanized land area (in square km) XE5 Population 196,318 15,384 94,893 74,106 XE6 Per capita GDP, in constant 2000 US dollars (1USD = 10 pesos) 4,472 4,343 11,922 10,591 XV1 Seismic code indicator XV2 City health indicator 1,087 1,233 1,172 1,167 XV3 City age indicator XV4 Population density (people per square kilometer) 6,174 5,376 3,813 7,386 XV5 City development speed indicator XV6 Percentage of population aged 0 4 or XC1 Economic context indicator 5,258 E6 32 E6 1,228 E6 749 E6 XC2 Political country context indicator XC3 Political world context indicator 3,259 E6 66 E6 1,131 E6 784 E6 XR1 Planning indicator XR2 Per capita GDP, in constant 2000 US dollars (1USD = 10 pesos) 4,472 4,343 11,922 10,591 XR3 Avg. annual real growth in per cap. GDP in prev. 10 yrs XR4 Housing vacancy rate XR5 Number of hospitals per 100,000 people XR6 Number of physicians per 100,000 people XR7 Extreme weather indicator XR8 Population density (people per square km) 6,174 5,376 3,813 7,386 XR9 City layout indicator

6 432 V. M. ZOBIN AND J. F. VENTURA-RAMÍREZ Figure 2. The plans of two towns (A, Manzanillo; B, Tecomán). Table IV. The indices EDRI for five factors of seismic danger for four towns of Colima state Factor Colima Armería Manzanillo Tecomán Hazard Exposure Vulnerability External context Emergency response and recovery capability EDRI

7 FACTORS OF SEISMIC DANGER IN MEXICO 433 Figure 2. Continued. R = w R1 X R1 + w R2 X R2 + w R3 X R3 + w R4 X R4 + w R5 X R5 + w R6 X R6 +w R7 X R7 + w R8 X R8 + w R9 X R9 ; For calculation of the EDRI components, we used maps of seismic hazard with a return period of 100 and 500 years (Tejeda, 2001); the catalog of tsunamis on the western coast of Mexico (Tsunamis, 1996); the statistical data for Colima state collected by Instituto Nacional de Estadística, Geografía e Informática (INEGI, 1980, 1990, 1995, 1997, 2001); the data about the financial situation (Banco de Mexico, 2001); the data about urban constructions (Secretaría de Industria y Comercio, 1973; Tabulados Basicos, 2000); and the maps of the towns. 4. Evaluation of EDRI Indices Table III shows the EDRI indicators as calculated for the four towns. These values were used in the following analysis. The indicators reflect possible weak points in the urban organization against seismic hazard, including the destructive effects produced by an earthquake and the ability of a city to survive during and after the earthquake. The indicators reflect the economical and political health of the city, and the ability of its habitants to resist natural hazards.

8 434 V. M. ZOBIN AND J. F. VENTURA-RAMÍREZ In the group of Hazard indicators, the port of Manzanillo is characterized by maximum values for five of seven indicators (the functions of the Peak Ground Acceleration (PGA) with different return period XH1 and XH2, the indicators XH3 and XH4, which are the functions of the bad quality of soil and the tsunami potential indicator XH7). It is the nearest town to the epicenters of destructive earthquakes in the subduction zone, and it is the closest to the ocean. In terms of Exposure factor, Colima is the most critical with the largest population (XE1 and XE5), the number of housing units (XE3) and the largest urbanized land area (XE4) but with the lowest indicators of per capita Gross Domestic Product (GDP) (XE2 and XE6). The Vulnerability indicators are more or less similar for all four towns. They reflect the common features of the society. The greater difference is observed for the indicators that characterize the External context of these four towns. The state capital Colima city has the largest values of the indicators of economical (XC1), political country (XC2) and political world (XC3) contexts. Finally, the indicators of the Emergency response and recovery show that Manzanillo is in the best situation, with the lowest density of population (XR8) and the largest planning indicator (XR1) and per capita GDP (XR2). Manzanillo has also the largest number of physicians per 100,000 people (XR6) and a good number of hospitals (XR5). It is characterized also by the lowest indicator of extreme weather (XR7). At the same time it has the worst layout indicator XR9 as a result of its narrow long shape (see Figure 2a) and the lowest annual growth in per capita GDP (XR3). Colima city is characterized by the lowest per capita number of hospitals (XR5), a low planning indicator and a low annual growth in per capita GDP (XR3) for the high density of population (XR8). It has good weather conditions and a good layout indicator. Thus, all four towns are characterized by positive and negative indicators of their situation against seismic danger. For better comparison of all indicators, they were scaled using the formulas and weights proposed by Davidson and Shah (1998). The scaled values of the indicators were used to calculate indices of the factors with formulas (1). Two types of indices were calculated (Table IV): the indices for each of the factors and the total EDRI index which is a sum of all five scaled factors. The comparison of the individual factors allows us to detect the weakest and strongest elements in the urban resistance against the earthquake for a city while the total EDRI index shows the position of a city among other towns under study. The largest total EDRI index was obtained for Manzanillo, nearest to the Mexican subduction zone. The seismic factor of hazard is the most powerful for this town. At the same time, the analysis of relative portion of each individual factor in seismic danger shows the importance of the social components. Figure 3 shows the relative values of the five factors for the four towns. These values are given in units of standard deviations (s.d.) from the mean value of the four towns as presented in Table IV. For example, the factor of hazard in Table IV has values of 1.084, 0.196, and for the four towns. Its mean value

9 FACTORS OF SEISMIC DANGER IN MEXICO 435 Figure 3. The comparative values of five factors (hazard, H; Exposure, E, Vulnerability, V; External Context, C; and Emergency response and recovery, R) for the four towns of Colima state. S.d. is the standard deviation. is with a standard deviation of Thus the hazard values for Colima are 1.68, for Armería 0.303, for Manzanillo 1.04, and for Tecomán in units of standard deviation. Here Manzanillo has a factor of hazard above the standard deviation. Only four values of factors are above the standard deviation. They are the Exposure and External context for Colima; for Armería, Vulnerability; for Manzanillo, Hazard and Emergency response and recovery capability. The danger factors for Tecomán are within the range of standard deviations. The analysis of the most dangerous factors for each of the towns is presented below. Colima External context. This is the highest factor of seismic danger for Colima. This factor is driven by the following components of the city: economic and political

10 436 V. M. ZOBIN AND J. F. VENTURA-RAMÍREZ external contexts, and planning. All these indicators (XC1, XC2 and XC3, see Table II) are the largest for Colima as it is the capital of state and the largest town of state. Exposure. This factor is driven by the physical infrastructure, population, and economy. For Colima, the population (indicators XE1 and XE5), the number of housing units (XE3), and the area of urbanized land (XE4) are the largest of state towns (see Table III). At the same time, its per capita GDP (XE2 and XE6) that describes the economic flow in a city is only third of the four towns. This lack of balance between the population and economical exposures may increase the seismic danger. Armería Vulnerability. The high vulnerability in Armería is conditioned by the largest values of indicators XV3 (City age indicator), XV5 (City development speed indicator) and XV6 (Percentage of population aged 0 4 or 65+) and the smallest indicator XV1 (Seismic code indicator). It means that the vulnerability is driven by its relatively older age and the effects this age could have on its infrastructure (XV3). The fast development of population (XV5) as well as its high percentage of vulnerable population (17 percent) (XV6) in Armería is observed simultaneously with the lowest grade of seismic code imposition (XV1). Manzanillo Hazard. The highest value of hazard for this sample of towns is driven in Manzanillo by its high percentage (43 percent) of urbanized area with soft (XV3) and potentially able for liquefaction (XH4) soil. Manzanillo is also the only town with the high tsunami hazard (XH7). Emergency response and recovery. This factor is the highest for Manzanillo due its low level of recent economic performance (XR3) and its irregular city layout (XR9). Tecomán This town is characterized by danger factors that are close to the mean values for this group of towns. At the same time, the factor of Vulnerability is the most dangerous for the town.

11 FACTORS OF SEISMIC DANGER IN MEXICO Discussion and Conclusion Our study shows that social factors of seismic danger may be comparable with the natural factors and even prevail over them. The infrastructure of a city may have a great influence during the earthquake and post-earthquake period. These results are in good agreement with the results of RADIUS evaluation for the following ten large cities: Lima, Peru; San Francisco, San Luis and Boston, USA; Tokyo, Japan, Jakarta, Indonesia, Istanbul, Turkey, Mexico-City, Mexico; Manila, Philippines; and Santiago, Chile (Davidson and Shah, 1998). For most of these cities the most dangerous factors are the social factors. Exposure is the most dangerous factor for Boston and Mexico-City; Vulnerability, for St. Louis; External context, for Tokyo; and Emergency response and recovery, for Istanbul, Jakarta, Manila and Santiago. Only for Lima and San Francisco, the Hazard factor prevails. The RADIUSproject (RADIUS, 2000) was also applied to Antofagasta, Chile; Guayaquil, Ecuador and Tijuana, Mexico. These three Latin American cities make up a diverse group. Antofagasta is a relatively small town of 220,000 inhabitants, which depends on mining. Antofagasta has experienced a destructive earthquake (Ms 7.3) in Guayaquil is a large city of 2.1 million inhabitants that contributes 2 percent of Ecuador s total GNP. It has experienced a destructive earthquake (Ms 7.9) in Tijuana is a relatively young city, intermediate in size (1.25 million inhabitants), that has not experienced a destructive earthquake since its foundation approximately a century ago. There are several differences among these three cities, but they experience similar problems as the four towns of Colima state. These problems include significant amounts of traditional construction, modern construction without enforcement of building codes, vulnerable critical facilities (schools, hospitals, etc.), lack of earthquake awareness within the community, and little support from local government for risk management activities. Therefore, it appears that the problems of seismic danger that were noted for the towns of Colima state may be of general significance for Latin America. The recent large earthquake that occurred in Colima state on 21 January at 20 hrs of local time 2003 (22 January, UT; No. 6 in Table II and Figure 1) was felt with intensity VI to VIII in the four towns but was not enough strong to reveal the social aspects of danger. At the same time, it demonstrated a strong influence of the hazard factor on the dangerous effects of the earthquake, larger than expected from the factor values. Thousands of houses of C and B-types were destroyed in the towns, mainly in Colima. Most of the destruction was observed for adobe constructions. For better-quality materials, the destruction occurred due to the absence of an adequate seismic code in the state up to 1991 and due to soft soil and liquefaction. The relatively low values of hazard factor and its indicators for Colima in Tables III and IV may be attributed to the absence of the indicator of the percentage of the buildings that were adobe constructions. For the Latin America cities the indicator XH5 (percentage of buildings that are wood) should perhaps

12 438 V. M. ZOBIN AND J. F. VENTURA-RAMÍREZ be changed to an indicator for traditional (e.g., adobe) construction. The lack of information about soil properties in areas of new constructions during the project evaluation significantly decreased the indicator XH4. This study suggests that the RADIUS methodology may be a useful tool for preliminary diagnosis of preparedness of a city (or group of cities) against seismic danger, and the construction of a hierarchy of the danger factors. The indicators proposed by RADIUS should be adapted to local conditions. The results may be used by municipal governments for planning of the development of cities. Acknowledgements We thank C. Lomnitz, T. Murty and an anonymous reviewer for useful comments. This study was supported by Grant No of CONACyT-SIMORELOS. References Banco de México: 2001, The INTERNET site < ce/oct/ci-e.csv>. Davidson, R. and Shah, H. C.: 1998, The Earthquake Disaster Risk Index: A Holistic Comparison of Earthquake Risk in Cities Worldwide, Stanford University, Stanford, 51 pp. INEGI: 1980, X Censo General de Población y Vivienda. Integración Territorial, Estado de Colima. México, D.F., pp INEGI: 1990, XI Censo General de Población y Vivienda. Resultados Definitivos, Datos por Localidad (Integración Territorial), Colima. México, D.F., pp INEGI: 1995, Conteo de Población y Vivienda. Resultados Definitivos, Tabulados Básicos, Colima. México, D.F., pp INEGI: 1997, Gobierno del Estado de Colima. Anuario Estadístico del Estado de Colima. Colima, México, pp. 7, 171, 172. INEGI: 2001, The INTERNET site < initivos/iter/initer06.pdf>. Manual de diseño por sismo: 1993, Comisión Federal de Electricidad, D.F. RADIUS: 2000, UNESCO, Geneva, 38 pp. Secretaría de Industria y Comercio: 1973, IX Censo General de Población 1970, 28 de Enero de Dirección General de Estadística, Vol. I, Aguascalientes-Guerrero, Colima. México, D.F., pp Tabulados Básicos: 2000, Colima. XII Censo General de Población y Vivienda, pp , 399, 400, Tejeda, J. C.: 2001, Estudio de Peligro Sísmico para el Occidente de México, Tesis, M.C., Universidad de Colima, 56 pp. Tsunamis: 1996, CENAPRED, D.F. Fasc. No. 12, 24 pp. Zobin, V. M. and Ventura-Ramírez, J. F.: 1998, The macroseismic field generated by the Mw 8.0 Jalisco, México, earthquake of 9 October 1995, Bull. Seismol. Soc. Amer. 88(3), Zobin, V. M. and Ventura-Ramírez, J. F.: 2000, The vulnerability of urban constructions in Colima, Mexico, In Proc. 6th Inter. Conf. on Seismic Zonation, Palm Springs, USA, pp Zobin, V. M. y Ventura-Ramírez, J. F.: 2001, Los Efectos Macrosísmicos y la Vulnerabilidad Sísmica de Construcciones en el Estado de Colima, Universidad de Colima, Colima, 85 pp.