ADVANCES IN SEISMIC MICROZONING IN COLOMBIA

ANDRÉS JOSÉ ALFARO CASTILLO*

SUMMARY

In developing countries, like Colombia, there are many effects from natural events. They could affect the Gross National Product (GNP) and the amount of displaced people and refugees as well. Natural Risk reduction could be a priority responsibility for Colombian authorities. The Earthquake Engineering and Seismology Research Group, founded and directed by the author since 1999, is working in Seismic Microzonation for the following Colombian cities: Tunja, Barrancabermeja, Villavicencio, Ibagué, Neiva and Pasto. Additionally, the last two years the group has made a Bogotá Regional Seismic Hazard assessment. In Colombia it is necessary to improve the Seismic Hazard assessment, in both scales: regional and local. However, we have two significant limitations: economic and human resources to evaluate Seismic Hazard accurately and in a realistic way. On the other hand, we could use novel and inexpensive technologies, like a Geographical Information System Database and Microtremor Analysis in order to improve Colombian hazard assessment. INTRODUCTION The Earthquake Engineering and Seismology Research Group† is working on Local Seismic Microzonation for the following Colombian cities: Tunja [1], Barrancabermeja [2], Villavicencio [3], Neiva [4] and Pasto [5], we are using geological and geomorphic information generating 1:25.000 scale maps. Additionally, for the city of Ibagué [6] we used geotechnical information from logs and wells in order to improve the Local Seismic Hazard Maps. Finally, the group made the Javeriana University Campus Seismic Microzonation, which includes Bogotá Regional Seismic Hazard assessment [ 7]. It is necessary to reduce the natural hazard risk of Colombian cities. This topic was successfully developed in highest hazard zones, like San Francisco and Tokyo. In order to

* Pontificia Universidad Javeriana Earthquake Engineering and Seismology Research Group Researcher † The members of the Earthquake Engineering and Seismology Research Group are Rene Van Hissenhoven, MSc, PhD; Alexander Caneva, MSc, PhD; Pilar Monroy, MSc; Alfonso Ramos, MSc; Diego Garcia, MSc and Andres Alfaro, MSc

succeed we have to asses the seismic hazard accurately, and the vulnerability of buildings as well.

COLOMBIAN NATURAL DISASTERS IN THE LAST TWO DECADES (1980S – PRESENT)

This part summarize the most dramatic Natural disasters that have being happening in Colombia in the last twenty years, however, this summary does not include floods and landslides, because, unfortunately, they happen too many times every year in the whole country. Popayán Earthquake, March 31 1983. This earthquake affected Popayán City and surrounding towns. It resulted in 287 deaths, 7,248 injured people, 13,650 dwelling destroyed and a huge economic loss. Rebuilding cost approximately US$ 76 million. This amount does not include National Monuments or Ancient Buildings damaged or destroyed [8]. Ruiz Volcano Eruption, November 13, 1985. The Eruption caused a huge mud avalanche, which destroyed the city of Armero completely and the city of Chinchina partially, resulting in approximately 23,000 dead people and 200,000 hurt people [ 9]. Murindo Earthquake, 1990. The Murindo earthquake destroyed more than 200 dwellings in Chocó and Antioquia Provinces. Paez River Earthquake, June 6, 1994. This earthquake generated an avalanche in the Paez River basin causing 1,000 dead people and more than 30,000 hurt people. Pasto Earthquake, March 4, 1995. This earthquake caused 300 damaged dwelling and more than 1,000 people without home, a public hospital out of service, and 10% of the schools damaged. Earthquake in the Coffee Growing Region of Colombia, January 25, 1999. This earthquake caused 1,185 dead, more than 8,000 injured people, and more than 400,000 hurt people. The most significant destruction happened in the city of Armenia, and in the towns of Salento, Circasia and La Tebaida [10].

ADVANCES IN SEISMIC MICROZONING STUDIES IN COLOMBIA Most of Colombian people live in towns and cities, which are located in significant seismic hazard zones (pga more than 150 gals for 475 years), additionally at least 30 cities have

more than 100,000 inhabitants. Chart 1 summarizes some studies for Seismic Microzoning in Colombia.

CHART 1

COLOMBIAN CITIES WITH MORE THAN 100,000 INHABITANTS WITH SIGNIFICANT SEISMIC HAZARD

City Population

Last National Census 1993

Hazard PGA (475 years)

References

Figure

Armenia 211,000 >200 gals [11]-[12] Barrancabermeja 136,000 100-200 gals [13][2]. Figure 1 Bello 260,000 100-200 gals [14] Bogotá 100-200 gals [15] Bucaramanga 403,000 >200 gals [16] Cali 1.636,000 >200 gals [17] Dosquebradas 115,000 >200 gals [18] Envigado 110,000 100-200 gals [14] Ibagué 336,000 100-200 gals [6] Figure 2 Itagui 168,000 100-200 gals [14] Manizales 341,000 >200 gals [19][20] Medellin 100-200 gals [21] Neiva 223,000 >200 gals [4][22] Figure 3 Pasto 244,000 >200 gals [23][5] Figure 4 Pereira 329,000 >200 gals [18] Popayán 175,000 >200 gals [24] Tunja 102,000 100-200 gals [25][26] [27] Figure 5 Villavicencio 190,000 >200 gals [28][3] Figure 6 Chart 1 summarizes some of the most important Microzoning studies developed in the last years in Colombia. This means that Universities like Nacional de Colombia, Los Andes and Javeriana and Government agencies like Ingeominas, have been done remarkable contributions to assess the seismic local hazard. Nevertheless, it is necessary to improve most of these studies and, additionally, to contuct studies for cities like Cucuta (population 462,000 inhabitants), which was destroyed by an earthquake in 1875 [29]. Additionally, cities like Tunja, Neiva, Pasto, Barrancabermeja and Ibagué need to improve their Local Seismic Hazard Assessment, including geophysical, geological, seismological and geotechnical surveys, because these studies are in a rudimentary level based on a compilation and an interpretation of available data. Analysis done included interpretation of aerial photographs and morphological analysis [1], [21], [5], [2] and [6].

DISCUSSION In spite of the fact that in the last ten years a remarkable effort has been done in order to develop Seismic Microzoning studies of the biggest cities in Colombia, like Bogotá [15], Armenia [12], Medellín [21], Tunja [25], Barrancabermeja [2], Villavicencio [3], Neiva [4], Pasto [5], Ibagué [2], Bucaramanga [16], Manizales [20], etc. There are more than 10 cities bigger than 100,000 inhabitants and located in hazardous regions that need Seismic Microzoning studies (e.g.: Monteria, Cucuta, Palmira, Buenaventura, Tumaco, Quibdo, etc). Geological mapping for cities like Tunja, Ibagué, Barrancabermeja, Pasto and Neiva need to be complemented by cross-sections and one or several block diagrams in order to provide a three dimensional model of these cities. Also, it is advisable to carry out geophysical seismic refraction and reflection studies. Additionally it is necessary to carry out neotectonic studies for seismogenetic fault identification, using the most known general methods, e.g: microseismicity analysis, statigraphical analysis, evaluation of Quaternary deformation, the digging of reconnaissance trenches and the drilling of boreholes. On the other hand, in order to refine Microzonation studies it is necessary to evaluate soil parameters, in the city of Ibagué [6] some of the parameters (Vs, Vp, damping, etc.) were estimated using empirical correlations [30]. Sometimes, geological and geotechnical available data is not enough, in these cases, Microtremors could offer valuable and inexpensive information about several soil materials (Figure 7) [31]. Summarizing, one of the most significant challenges in Colombia is to improve and to assess Seismic Microzoning Studies.

Figure 1. Microzonation in Barrancabermeja (Without scale – Original scale 1:25.000) [2]

Figure 2. Soils in Ibagué – Blue dots indicate logs and boreholes location (Without scale

– Original scale 1:25.000) [6]

Figure 3. Microzonation in Neiva (Without scale – Original scale 1:25.000) [22]

975500 m E 976500 m E 977500 m E 978500 m E 979500 m E 980500 m E

Coordenadas EW

975500 m E 976500 m E 977500 m E 978500 m E 979500 m E 980500 m E

Coordenadas EW

623000 m N

623500 m N

624000 m N

624500 m N

625000 m N

625500 m N

626000 m N

626500 m N

627000 m N

627500 m N

628000 m N

Coo

rden

adas

NS

623000 m N

623500 m N

624000 m N

624500 m N

625000 m N

625500 m N

626000 m N

626500 m N

627000 m N

627500 m N

628000 m N

Coo

rden

adas

NS

Velocidad de Ondas de Corte estimada 65 m/s a 85 m/s 85 m/s a 100 m/s 100 m/s a 115 m/s 115 m/s a 130 m/s 130 m/s a 145 m/s 145 m/s a 160 m/s

Figure 4. City of Pasto . Shear Wave Velocity from Soil Indexes [30] Without scale [5].

Figure 5. Microzonation in Tunja (Without scale – Original scale 1:25.000) [26]

Figure 6. Microzonation in Villavicencio (Without scale – Original scale 1:25.000) [3]

1.991.88

1.811.73

1.731.71

1.71

1.671.671.67

1.67

1.67

1.67

1.67

1.67

1.54

1.54

1.43

1.43

1.43

1.43

1.43

1.30

1.25

1.25

1.251.25

1.25

1.25

1.251.25

1.251.25

1.25

1.25

1.25

1.25

1.15

1.11

1.111.11

1.11

1.11

1.11

1.11

1.10

1.10

1.06

1.001.00

1.00 1.00

1.00

1.00

1.001.00

1.001.00

1.00

1.00

1.00

0.91

0.91 0.91

0.91

0.91

0.91

0.91

0.91

0.90

0.90

0.83

0.83

0.83

0.83

0.83

0.83

0.77

0.770.77

0.770.77

0.77

0.77

0.77

0.77

0.77

0.77

0.71 0.71

0.70

0.67

0.67

0.67

0.67

0.67

0.67

0.67

0.63

0.63

0.63

0.60

0.59

0.59

0.56

0.56

0.56

0.56

0.53

0.50

0.50

0.50

0.500.50

0.50

0.50

0.50

0.50

0.50

0.480.48

0.45

0.45

0.43

0.43

0.40

0.40

0.40

0.38

0.34

0.33

0.33

0.33

0.33

0.32

0.30

0.30

0.290.29

0.29

0.29

0.280.28

0.27 0.27

0.26

0.25

0.25

0.250.250.250.25

0.25

0.24

0.23

0.230.22

0.21

0.210.20

0.17

0.17

0.17

0.16

0.15

0.15

0.15

0.140.13

0.13

0.10

0.09

0.08

0.070.06

0.06

0.06

0.06

0.06

0.060.06

0.06

0.00

0.00

0.00

0.00

0.00

0.000.000.00

0.000.00

0.00

0.00

0.00

26000 28000 30000 32000 34000EAST

26000 28000 30000 32000 34000EAST

76000

78000

80000

82000

84000

86000

88000

90000

NORT

H

76000

78000

80000

82000

84000

86000

88000

90000

NORT

H

Figure 7. Soil’s predominant periods of Barcelona, from microtremors [31]

REFERENCES

1. Jaramillo, M., G. Pabon, O. Ramirez and J. Guacaneme (2000) Seismic and Geotechnical Hazard in Tunja. Proc. 8Th Colombian Cong. Geotechnical Eng. Bogotá. (in Spanish) 2. González C. and A. Alfaro. 2001. Seismic Risk for the refugee in Barrancabermeja City – Colombia. Proc. Second Iberoamerican Cong. Earthq. Eng. Madrid. 714-720. (In Spanish with English Abstract) 3. Ramos A. and A. Alfaro. (2001) Seismogenetic Sources and Landslides in Villavicencio for Seismic Microzoning. Revista Ingeniería y Universidad. Vol. 4, Número 2, 42 - 51. (In Spanish with English Abstract) 4. Pabón G., M. Jaramillo, C. Calpa and R. Pinto (2000) Geological and Geotechnical for Neiva. Proc. 8Th Colombian Cong. Geotechnical Eng. Bogotá. (in Spanish) 5. Calpa, C., J. Pantoja, A. Alfaro and R. Van Hissenhoven (2001) Evidencias de Efectos Locales y Mapa de Susceptibilidad Sísmica en la Ciudad de Pasto. Memorias XI Jornadas Geotécnicas Ingeniería de Colombia, Bogotá. 253-273. 6. Alfaro, A., A Díaz- Granados, P. Escobar and L. Martínez (2001) Aportes para la Microzonificación Sísmica de Ibagué. Red Sismológica Regional Eje Cafetero Viejo Caldas y Tolima Vol. 6 No 1, Manizales. 32-37. 7. Caneva, A. E. Salcedo, R. Van Hissenhoven and A. Alfaro (2003) Análisis de la amenaza sísmica y de la magnitud representativa para Bogotá, Revista Ingeniería y Universidad. Vol. 7, Número 2, 133 - 149. (In Spanish with English Abstract) 8. Gros, C. (1985) Popayán: deux ans ápres. Autopsie dún désastre. Revue Trimestreill de Sciences Sociales, Centre de Recherche sur l`Ámerique Latine et le Tires Monde, No 23. Paris 9. D`Ercole, R. (1989) La catástrofe del Nevado del Ruiz: una enseñanza para el Ecuador. El caso del Cotopaxi. Estudios de Geografía. Riesgos Naturales. 10. PNUD-CEPAL (1999) El terremoto de Enero de 1999 en Colombia. Impacto Socioeconómico del desastre en la zona cafetera. México. 89 p. 11. Cano, L. (1996) El método de las microtrepidaciones como aporte a la microzonificación sísmica de ciudades caso: Armenia (Q). Universidad de los Andes. Bogotá.

12. INGEOMINAS (2000) Microzonificación Sísmica de Armenia. Bogotá. 13. Flórez, H., (1993) Zonificación sísmica preliminar de Barrancabermeja. Universidad de los Andes. Bogotá. 14. Universidad EAFIT, Integral, Ingeominas and Universidad Nacional de Colombia (2004) Microzonificación Sismica del Area Metropolitana de Medellín. In process 15. Ingeominas and Universidad de Los Andes (1997) Microzonificación Sísmica de Santa Fe de Bogotá. Bogotá. 16. INGEOMINAS (2001) Zonificación Sismogeotécnica Indicativa del Area Metropolitana de Bucaramanga. Bogotá. 17. Meyer, H. (1990) Proyecto Integral para la Mitigación del Riesgo Sísmico de Cali. Santiago de Cali. 18. CARDER - Universidad de los Andes (1999) Exploración Geotécnica, Investigación de Laboratorio y Microzonificación Sísmica de los Municipios de Pereira, Dosquebradas y Santa Rosa de Cabal 19. OMPAD – Universidad de los Andes (1998) Estudios de Amenaza Sísmica y Evaluación Sismogeotécnica preliminar para la Zonificación Sísmica de Manizales 20. CIMOC – CEDERI – Universidad de Los Andes (2002) Microzonificación Sísmica de Manizales. Informe Técnico. Universidad de los Andes. Bogota. 21. Universidad EAFIT, Integral, Ingeominas and Universidad Nacional de Colombia (2000) Instrumentación y Microzonificación Sísmica del Area Urbana de Medellín. 135 p. 22. Instituto Geofísico Universidad Javeriana and Consultoría Colombiana S.A. (2000). Microzonificación Sísmica Preliminar de Neiva. Bogotá. 23. Coral, H. (1993) Zonificación geotécnica urbana preliminar para Pasto. Universidad Nacional de Colombia. Bogotá. 24. INGEOMINAS-BRGM-ADK-KS, 1992. Microzonificación Sismogeotécnica de Popayán 25. Ramírez, O. (1990) Zonificación Geotécnica de Tunja. Universidad Nacional de Colombia. Bogotá. 26. Instituto Geofísico Universidad Javeriana and Consultoría Colombiana S.A. (2000). Microzonificación Sísmica Preliminar de Tunja. Bogotá.

27. Guacaneme J. (2001) Zonificación Morfodinámica de Tunja. Memorias XI Jornadas Geotécnicas de la Ingeniería Colombiana. Bogotá. 28. Instituto Geofísico Universidad Javeriana and Consultoría Colombiana S.A. (2000). Microzonificación Sísmica Preliminar de Villavicencio. Bogotá. 29. Ramírez, J. E. (1975) Cúcuta cien años después.18 de mayo de 1874. Instituto Geofísico de los Andes Colombianos. Bogotá. 30. Ohta, Y. and N. Goto (1978) Empirical Shear Wave Velocity Equations in Terms of Characteristics Soil Indexes, Earthq. Eng. Struc. Dyn., 6, 167-187. 31. Alfaro, L.G. Pujades, X. Goula, T. Susagna, M. Navarro, J. Sánchez and J.A. Canas. (2001) Preliminary Map of Soil’s Predominant Periods in Barcelona using Microtremors. Pure and Applied Geophysics. 158(2001) 2499-2511.