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Contributions were made (in alphabetical order) by J. A guirre 2, D. Almora 2, J. G.

Se presenta una comparación entre dos procedimientos, forma espectral normalizada y ley de atenuación espectral, para estimar el espectro de aceleraciones horizontales absolutas para una fracción del 5 por ciento del amortiguamiento crítico en la estación acelerográfica CU. Esta estación se localiza en la zona de terreno firme de la ciudad de México, específicamente en el Instituto de Ingeniería de la Universidad Nacional Autónoma de México. De la comparación, resulta que la ley de atenuación espectral es el mejor procedimiento para estimar el espectro de aceleraciones en la estación CU. También, se estudia la técnica que se basa en los cocientes espectrales de respuesta (CER) para estimar el espectro de aceleraciones en las estaciones de la zona de terreno blando de la ciudad. Del análisis de resultados, se concluye que la mayor fuente de incertidumbre se debe a la estimación del espectro en la estación CU. Finalmente, se presentan ejemplos de espectros de peligro uniforme para distintos periodos de retorno en la estación CU y en estaciones localizadas en la zona de terreno blando de la ciudad.

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El objetivo de este artículo es presentar los mapas de distribución de velocidad máxima del suelo de la Ciudad de México para cuatro eventos sísmicos de interés. Se describe el procedimiento de cálculo, el programa de computadora empleado y su calibración, la cual permite que los valores de velocidad calculados se aproximen a los medidos. Además, se propone un grupo de ecuaciones de ajuste, en función de la magnitud del evento, con el propósito de calcular mapas de velocidades para sismos postulados. Finalmente, se presentan mapas de velocidad máxima (componentes NS y EW) para un sismo postulado con magnitud igual a 8.4 y distancia focal de 300 km, como aplicación de las funciones de ajuste propuestas.

The Jabalpur earthquake (23.08° N, 80.06° E, H = 36 km) is the first event in the Indian Peninsular shield region to be well recorded by a newly installed, 10-station, broadband seismographic network. Using these data, we estimate Q of Lg wave in the Indian shield region as Q = 508ƒ0.48 (1 ≦ ƒ ≦ 20 Hz). The corrected source spectrum, with M0 = 5.4 × 1024 dyne-cm (reported in the Harvard CMT catalog) and an ω2-source model, requires a stress parameter, Δσp, of ∼420 bar to explain high-frequency spectral level. The computed seismic energy from the records is 7.4 × 1020 erg, which yields an apparent stress and Brune stress drop of 62 and 270 bars, respectively. The analysis of the Jabalpur earthquake provides some elements for the estimation of ground motions during future earthquakes in the Indian shield region. Based on the results of the Jabalpur earthquake and on studies of tectonically similar region of eastern North America, we assume that (1) the sources follow an ω2 model; (2) S waves dominate at R < 100 km, and Lg waves dominate at R ≧ 100 km; (3) Q(f) = 508ƒ0.48; (4) the ground motion is a bandlimited, finite-duration, Gaussian white noise; and (5) the effective duration of the ground motion equals ƒ−1c + 0.05R, where fc is the corner frequency. We apply random vibration theory (RVT) to compute various measures of ground motion, such as Amax and Vmax. At near-source distances, the source finiteness is approximately taken into account. The attenuation curves for 5.5 ≦ Mw ≦ 7 and for Δσp of 100-400 bar are presented. As expected, the predicted values (with Δσp ∼ 420 bar) agree reasonably well with the limited Jabalpur data. An Amax of ∼150 gal is predicted in the epicentral region of this earthquake. The predicted curves imply Δσp ≦ 100 bar for the Latur earthquake of 1993 (Mw = 6.1; H = 2.5 km) to explain Amax < 1 g and the reported isoseismal intensities in the epicentral region. For Koyna earthquake of 1967 (Mw 6.3), the inferred Amax and Vmax from isoseismal intensities and the recorded strong motions at the Koyna dam site are in agreement with the prediction curves for Δσp ∼ 100 bar. The RVT predictions seem reasonable but need validation from more strong-motion data, which is presently lacking.

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The description of the great earthquake of 19 June 1858 is unusual: damage and high intensities were reported both in the state of Michoacan and in Mexico City. Although a coastal epicenter for this earthquake cannot be ruled out, the reports agree better with an intermediate-depth (about 50 km), normal-faulting event in the subducted Cocos plate. A careful examination of the reports of this event and other normal-faulting events below the Mexican altiplano suggests that a likely location is 18.0 °N, 100.8 °W, near the epicenter of the 6 June 1964 (M7.3, H = 55 km) event. This location is 220 km SW of the city. The magnitude of the earthquake is estimated to be about 7.7. We synthesize expected ground motions in CU, a hill-zone site in the city, from an event similar to that of 1858, using records from the 23 May 1994 earthquake (18.0 °N, 100.6 °W, H = 50 km, M5.7) as an empirical Green's function and stress parameter, Δσ, of 50, 160, and 300 bar. The expected peak horizontal acceleration in CU of Δσ = 160 bar is about 30 gals. Similar acceleration was recorded in CU during the 1985, Michoacan earthquake (M8.0). We compute expected ground motions at many sites in Mexico City using empirical transfer functions and random vibration theory and compare these motions and the expected damage in the city with those from the 1985 Michoacan earthquake. Results show that the overall expected damage during the postulated earthquake is ⅔ and 1⅓ of that during the Michoacan earthquake for Δσ = 160 and 300 bar, respectively. A greater percentage of low-rise construction, which constitute about 80% of the total in the city, will be damaged during the postulated earthquake than during the Michoacan earthquake. The expected ground motions for Δσ = 50 bar are smaller at all periods than those from the Michoacan earthquake. As the present building code for Mexico City contemplates coastal earthquakes of magnitude greater than 8.0, the case of Δσ = 50 bar is not of interest in this article. This preliminary study suggests a need for a more careful evaluation of expected ground motion in the Valley of Mexico from the postulated earthquake and its impact on the current design spectra of the city.