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A new seismic hazard map of Spain, based on Gumbel I distribution
Engineering Geology, 20 (1984) 187--191
187
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Short Communication
A NEW SEISMIC HAZARD MAP OF SPAIN, BASED ON GUMBEL I DISTRIBUTION
A.J. MART~N and J. SIERRA GOMEZ
Instituto Geogrdfico Nacional, Apartado 3007, Madrid (Spain) (Accepted for publication October 1, 1983)
INTRODUCTION
The last map of seismic hazard in Spain, included in the Spanish Building Code, was made in 1969 (Munuera, 1969). Afterwards, other researchers have mapped the seismic hazard of partial areas of the country (Roca and Udfas, 1976; Johnson et al., 1981). During these years the seismic catalogue of Spain has undergone a thorough revision of epicenter coordinates, magnitudes and other parameters and the isoseismal catalogue of Spain has been published, containing more than two hundred isoseismal maps of earthquakes which occurred since the fourteenth century (Mezcua, 1983). This revised catalogue provides the opportunity to obtain a new seismic hazard map; this was the purpose of the present communication. GENERAL OUTLINE OF COMPUTATIONAL PROCEDURE
Three main steps have been followed to calculate the seismic hazard at each site, (1) Computation of the attenuated intensity at the site for each earthquake which occurred within a specific time period. (2) Determination of the maximum value of the intensity for each time interval considered. (3) Computation of the expected hazard at a site by estimating the parameters of Gumbel I distribution from the obtained maxima (see Gumbel, 1958). The calculations were made by using the program RIEGUM (Sierra and Martfn, 1983), written in Fortran. The program calculates the hazard (intensity or acceleration) associated with different probabilities/return periods as well as the probability/return period associated to specific parameters (intensity or acceleration). An option of the program is the use of earthquakes with epicenter in the sea. The time period of data is 1405--1980 and the time interval is 25 years. 0013-7952]84/$03.00
© 1984 Elsevier Science Publishers B.V.
188
For a data interval w i t h o u t reported significant seismicity, the program takes the smallest of all other m a x i m u m values of intensity or acceleration. Computation is made for a grid of 15 min interspace and MSK-Intensity is taken as a continuous variable for ground shaking parameter. Using attenuation laws of the form: I o - - 1 = a - - b log (R + c) where Io = epicentral intensity, and R = distance to epicenter, the parameters a, b and c used are those obtained by Martfn (1983) as shown in Fig.1 and Table I. RESULTS
The map obtained is presented in Fig.2 with isolines corresponding to intensities for a return period of 500 years. There are certain coincidences between our map in Fig.2 and the m a x i m u m felt intensities map (Fig.3) (Martin, 1983), at least for the seismic areas; however, in the .northern part of the peninsula, the isolines are quite different, due to the Olot earthquake of 1428, On the other hand, for the lesser seismic areas, the isolines of the Lisbon earthquake of Nov. 1, 1755, the m a x i m u m felt intensities and the new map in Fig.3 show very similar features.
I
'< m
I
2
3
4
5
6
8
I0
20
40
60
IE)
IDO
200
400
I000
Km
F i g . 1 . A t t e n u a t i o n l a w s f o r different parts of Spain and t h e A z o r e s - - G i b r a l t a r z o n e : i = general (I > V I I I ) ; 2 = g e n e r a l (I < = V I I I ) ; 3 = s o u t h (I > V I I I ) ; 4 = s o u t h e a s t (I > V I I I ) ; 5 = s o u t h e a s t (I < = V I I I ) ; 6 = A z o r e s - - - G i b r a l t a r .
g
8
sv,, '/
7
6
,',. \...: ',,,,._
5
•
•
MELILLA
,v
"
IV lit
IALENCIA
L
~"
.,~-'xJ
,VII ,/
ARCELONA
c~
Fig.2. Seismic hazard map (distribution Gumbel-I, time interval 25 years) with calculated intensities. Return period 500 years, Data period 1405--1980.
.43
i-a oo r,D
190 TABLE I Parameters of the attenuation laws Zone
Intensity
a
South
(I0 > VIII) (I, < VIII)
11.23 5.23
7.138 5.089
25 5
Southeast
(I, 2 VIII) (I0 < VIII)
15.51 5.92
10.131 6.01
25 5
Azores-Gibraltar
(I, b VIII) (I, < VIII)
21.41 5.23
9.256 5.089
25 5
Rest of peninsula
(I, > VIII) (I, < VIII)
12.55 5.23
8.128 5.089
25 5
Fig.3, Maximum felt intensities from 1396 to 1979.
b
c!
191 REFERENCES
Gumbel, E.J., 1958. Statistics of Extremes. Columbia University Press, New York, N.Y., 375 pp. Johnson, W.I., Vaidia, N.R. and Michalopoulos, A.P., 1981. Probabflistic Assessment of seismic risk in the northwest Iberian Peninsula. Proc. Reunibn sobre contribuciSn a una posible revision de la Norma Sisrnorresistente Espafiola. Organizada por la AsociaciSn Espafiola de Ingenierfa Sfsmica. In press. Martfn, A.J., 1983. Riesgo sfsmico en la Penfnsula Ib~rica. Doct. Thesis. In press. Mezcua, J. (Editor), 1983. CatAlago General de Isosistas de la Penfnsula Ib~rica. Instituto Geogr~fico Nacional, Publ. n ° 202, Madrid. Munuera, J.M., 1969. Mapa de zonas sfsmicas generalizadas de la Penfnsula Ib~rica. (Generalized Seismic Zones Map of the Iberian Peninsule). Roca, A. and Udias, A., 1976. Sismicidad y riesgo sfsmico de la zone de Catalufia y Pirineos. Rev. Geoffs., XXXV, (3): 183--279. Sierra, J. and Martfn, A.J., 1983. Programa RIEGUM para el c~lculo del riesgo sfsmico mediante la distzibucibn de Gumbel. Proc. IV Asamblea Nacional de Geodesia y Geoffsica, Instituto Geogr~fico Nacional, Madrid.
187
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Short Communication
A NEW SEISMIC HAZARD MAP OF SPAIN, BASED ON GUMBEL I DISTRIBUTION
A.J. MART~N and J. SIERRA GOMEZ
Instituto Geogrdfico Nacional, Apartado 3007, Madrid (Spain) (Accepted for publication October 1, 1983)
INTRODUCTION
The last map of seismic hazard in Spain, included in the Spanish Building Code, was made in 1969 (Munuera, 1969). Afterwards, other researchers have mapped the seismic hazard of partial areas of the country (Roca and Udfas, 1976; Johnson et al., 1981). During these years the seismic catalogue of Spain has undergone a thorough revision of epicenter coordinates, magnitudes and other parameters and the isoseismal catalogue of Spain has been published, containing more than two hundred isoseismal maps of earthquakes which occurred since the fourteenth century (Mezcua, 1983). This revised catalogue provides the opportunity to obtain a new seismic hazard map; this was the purpose of the present communication. GENERAL OUTLINE OF COMPUTATIONAL PROCEDURE
Three main steps have been followed to calculate the seismic hazard at each site, (1) Computation of the attenuated intensity at the site for each earthquake which occurred within a specific time period. (2) Determination of the maximum value of the intensity for each time interval considered. (3) Computation of the expected hazard at a site by estimating the parameters of Gumbel I distribution from the obtained maxima (see Gumbel, 1958). The calculations were made by using the program RIEGUM (Sierra and Martfn, 1983), written in Fortran. The program calculates the hazard (intensity or acceleration) associated with different probabilities/return periods as well as the probability/return period associated to specific parameters (intensity or acceleration). An option of the program is the use of earthquakes with epicenter in the sea. The time period of data is 1405--1980 and the time interval is 25 years. 0013-7952]84/$03.00
© 1984 Elsevier Science Publishers B.V.
188
For a data interval w i t h o u t reported significant seismicity, the program takes the smallest of all other m a x i m u m values of intensity or acceleration. Computation is made for a grid of 15 min interspace and MSK-Intensity is taken as a continuous variable for ground shaking parameter. Using attenuation laws of the form: I o - - 1 = a - - b log (R + c) where Io = epicentral intensity, and R = distance to epicenter, the parameters a, b and c used are those obtained by Martfn (1983) as shown in Fig.1 and Table I. RESULTS
The map obtained is presented in Fig.2 with isolines corresponding to intensities for a return period of 500 years. There are certain coincidences between our map in Fig.2 and the m a x i m u m felt intensities map (Fig.3) (Martin, 1983), at least for the seismic areas; however, in the .northern part of the peninsula, the isolines are quite different, due to the Olot earthquake of 1428, On the other hand, for the lesser seismic areas, the isolines of the Lisbon earthquake of Nov. 1, 1755, the m a x i m u m felt intensities and the new map in Fig.3 show very similar features.
I
'< m
I
2
3
4
5
6
8
I0
20
40
60
IE)
IDO
200
400
I000
Km
F i g . 1 . A t t e n u a t i o n l a w s f o r different parts of Spain and t h e A z o r e s - - G i b r a l t a r z o n e : i = general (I > V I I I ) ; 2 = g e n e r a l (I < = V I I I ) ; 3 = s o u t h (I > V I I I ) ; 4 = s o u t h e a s t (I > V I I I ) ; 5 = s o u t h e a s t (I < = V I I I ) ; 6 = A z o r e s - - - G i b r a l t a r .
g
8
sv,, '/
7
6
,',. \...: ',,,,._
5
•
•
MELILLA
,v
"
IV lit
IALENCIA
L
~"
.,~-'xJ
,VII ,/
ARCELONA
c~
Fig.2. Seismic hazard map (distribution Gumbel-I, time interval 25 years) with calculated intensities. Return period 500 years, Data period 1405--1980.
.43
i-a oo r,D
190 TABLE I Parameters of the attenuation laws Zone
Intensity
a
South
(I0 > VIII) (I, < VIII)
11.23 5.23
7.138 5.089
25 5
Southeast
(I, 2 VIII) (I0 < VIII)
15.51 5.92
10.131 6.01
25 5
Azores-Gibraltar
(I, b VIII) (I, < VIII)
21.41 5.23
9.256 5.089
25 5
Rest of peninsula
(I, > VIII) (I, < VIII)
12.55 5.23
8.128 5.089
25 5
Fig.3, Maximum felt intensities from 1396 to 1979.
b
c!
191 REFERENCES
Gumbel, E.J., 1958. Statistics of Extremes. Columbia University Press, New York, N.Y., 375 pp. Johnson, W.I., Vaidia, N.R. and Michalopoulos, A.P., 1981. Probabflistic Assessment of seismic risk in the northwest Iberian Peninsula. Proc. Reunibn sobre contribuciSn a una posible revision de la Norma Sisrnorresistente Espafiola. Organizada por la AsociaciSn Espafiola de Ingenierfa Sfsmica. In press. Martfn, A.J., 1983. Riesgo sfsmico en la Penfnsula Ib~rica. Doct. Thesis. In press. Mezcua, J. (Editor), 1983. CatAlago General de Isosistas de la Penfnsula Ib~rica. Instituto Geogr~fico Nacional, Publ. n ° 202, Madrid. Munuera, J.M., 1969. Mapa de zonas sfsmicas generalizadas de la Penfnsula Ib~rica. (Generalized Seismic Zones Map of the Iberian Peninsule). Roca, A. and Udias, A., 1976. Sismicidad y riesgo sfsmico de la zone de Catalufia y Pirineos. Rev. Geoffs., XXXV, (3): 183--279. Sierra, J. and Martfn, A.J., 1983. Programa RIEGUM para el c~lculo del riesgo sfsmico mediante la distzibucibn de Gumbel. Proc. IV Asamblea Nacional de Geodesia y Geoffsica, Instituto Geogr~fico Nacional, Madrid.