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Ground Motion at the World Heritage Archaeological Site of Monte Alb%u00e1n, Oaxaca, Mexico during Recent Damaging EarthquakesS.K. Singh1 , A. Iglesias1 , D. Arroyo2 , M. Ordaz3 , L. Alc%u00e1ntara3 and D. Almora3AbstractArchaeological site of Monte Alb%u00e1n, Oaxaca, a testimony to the grandeur of Zapotec civilization, suffered significant damage during the earthquakes of 30 September 1999 (Mw 7.5, R = 130 km) near Puerto Escondido, and 8 September 2017 (Mw 8.2, R = 383 km) offshore Chiapas. As large subduction thrust earthquakes along the Pacific coast of Oaxaca and intraslab events in the subducted Cocos plate below the continent are relatively frequent, it follows that the archeological zone has been subjected to severe ground motion a large number of times. To adequately strengthen the monument against damage during future events, it is most desirable to know the ground motion at Monte Alb%u00e1n during previous large earthquakes, especially those of 1999 and 2017. Unfortunately, there is no on-site recording of these events. There is, however, an accelerographic station, OXLC, 7 km from the site, which recorded six large earthquakes of interest since 1999. If we know the spectral amplification of ground motion at sites in Monte Alb%u00e1n with respect to OXLC then the recordings at OXLC can be used to estimate the ground motion at Monte Alb%u00e1n through the application of random vibration theory.A 12-channel strong-motion network was in operation intermittently between 2004 and 2019 at Monte Alb%u00e1n. The sensors were installed at the base, mid-height, and top of North Platform. Many moderate earthquakes were simultaneously recorded by the network and at OXLC. Spectral ratios of the motions at different levels of the North Platform and at OXLC provide the desired amplifications in the linear range. We use these spectral ratios to estimate ground motions at Monte Alb%u00e1n during the six large earthquakes. The estimated peak horizontal ground accelerations, (PGA)H, at the base of the platform during the 1999 and 2017 events are 137 and 129 cm/s2, respectively. Estimated (PGA)Hduring the Huatulaco earthquake of 23 June 2020 (Mw 7.4, R=154 km) is about the same as during the 1999 events; in fact, peak velocity (PGV), and displacement (PGD) values are greater. Yet there is no report of damage during 2020. A likely explanation is that the recovery and conservation efforts following the 2017 earthquake had adequately strengthened the vulnerable structures so as not to suffer damage during 2020.The damage at Monte Alb%u00e1n seems to occur if (PGA)H exceeds ~ 120 cm/s2 at the base; such motion has an estimated return period of ~20 yrs. Arough estimate of (PGA)H at the base during the 15 January 1931 (M 7.8) earthquake, which devastated the city and state of Oaxaca, is 480cm/s2; this motion is expected to exceed once every 100 yrs. Reports of damage to Monte Alb%u00e1n in 1931 are scarce because much of the monument was little more than rubble. Extrapolation of the limited period of observation, which, therefore, should be taken with caution, suggests that acceleration of 1 g at the base may exceed every 1000 yrs. This may have happened during the great, interface, Oaxaca earthquake of 1787.ResumenEl sitio arqueol%u00f3gico de Monte Alb%u00e1n, Oaxaca, testimonio de la grandeza de la civilizaci%u00f3n zapoteca, sufri%u00f3 da%u00f1os significativos durante los sismos del 30 de septiembre de 1999 (Mw 7.5, R = 130 km), cerca de Puerto Escondido, y del 8 de septiembre de 2017 (Mw 8.2, R = 383 km), frente a las costas de Chiapas. Dado que los grandes sismos de subducci%u00f3n a lo largo de la costa del Pac%u00edfico de Oaxaca y los eventos intraslab en la placa de Cocos subducida bajo el continente son relativamente frecuentes, se deduce que la zona arqueol%u00f3gica ha sido sometida a movimientos fuertes del suelo en numerosas ocasiones. Para reforzar adecuadamente el monumento ante futuros eventos, es deseable conocer el movimiento del suelo en Monte Alb%u00e1n durante grandes sismos pasados, especialmente los de 1999 y 2017. Desafortunadamente no existen registros in situ de estos eventos. Sin embargo, existe una estaci%u00f3n acelerogr%u00e1fica, OXLC, a 7 km del sitio, que ha registrado seis sismos de magnitud importante desde 1999. Si se conoce la amplificaci%u00f3n espectral del movimiento del suelo en Monte Alb%u00e1n con respecto a OXLC, entonces los registros en OXLC pueden utilizarse para estimar el movimiento del suelo en Monte Alb%u00e1n mediante la aplicaci%u00f3n de la teor%u00eda de vibraciones aleatorias.Una red de movimientos fuertes de 12 canales oper%u00f3 de forma intermitente entre 2004 y 2019 en Monte Alb%u00e1n. Los sensores fueron instalados en la base, a media altura y en la cima de la Plataforma Norte. Muchos sismos moderados fueron registrados simult%u00e1neamente por la red y la estaci%u00f3n OXLC. Los cocientes espectrales de los movimientos en diferentes niveles de la Plataforma Norte con respecto a los registros de OXLC proporcionan las amplificaciones deseadas en el rango lineal. Usamos estos cocientes espectrales para estimar los movimientos del suelo en Monte Alb%u00e1n durante los seis sismos mencionados anteriormente. Las aceleraciones horizontales m%u00e1ximas estimadas del suelo, (PGA)H, en la base de la plataforma durante los eventos de 1999 y 2017 son de 137 y 129 cm/s2, respectivamente. La (PGA)H estimada durante el sismo de Huatulco del 23 de junio de 2020 (Mw7.4, R = 154 km) es aproximadamente la misma que durante el evento de 1999; de hecho, los valores de velocidad m%u00e1xima (PGV) y desplazamiento m%u00e1ximo (PGD) son mayores. Sin embargo, no hay reportes de da%u00f1os durante 2020. Una posible explicaci%u00f3n es que los esfuerzos de recuperaci%u00f3n y conservaci%u00f3n posteriores al sismo de 2017 fortalecieron adecuadamente las estructuras vulnerables, evitando da%u00f1os durante 2020.Los da%u00f1os en Monte Alb%u00e1n parecen ocurrir si la (PGA)H excede aproximadamente los 120 cm/s2 en la base; tal movimiento tiene un periodo de retorno estimado de unos 20 a%u00f1os. Una estimaci%u00f3n aproximada de la (PGA)H en la base durante el sismo del 15 de enero de 1931 (M 7.8), que devast%u00f3 la ciudad y el estado de Oaxaca, es de 480 cm/s2; se espera que este tipo de movimiento se exceda una vez cada 100 a%u00f1os. Los reportes de da%u00f1os a Monte Alb%u00e1n en 1931 son escasos porque gran parte del monumento no era m%u00e1s que escombros. La extrapolaci%u00f3n del limitado periodo de observaci%u00f3n, que por lo tanto debe tomarse con cautela, sugiere que una aceleraci%u00f3n de 1 g en la base podr%u00eda excederse una vez cada 1000 a%u00f1os. Esto podr%u00eda haber ocurrido durante el gran sismo interplaca de Oaxaca en 1787.Geof%u00edsica Internacional (2026) 65-1: 1847- 1861Received: June 15, 2025; Accepted: November 25, 2025; Published on-line: January 1, 2026.Editorial responsibility: Dr. Luis Rivera* Corresponding author: Arturo Iglesias arturoi@sismologico.unam.mx 1 Universidad Nacional Aut%u00f3noma de M%u00e9xico, Instituto de Geof%u00edsica, Mexico City, Mexico2 Universidad Aut%u00f3noma Metropolitana Azcapotzalco, Departamento de Materiales, Mexico City, Mexico3 Universidad Nacional Aut%u00f3noma de M%u00e9xico, Instituto de Ingenier%u00eda, Mexico City, MexicoS.K. Singh, Arturo Iglesias, Danny Arroyo, Mario Ordaz, L. Alc%u00e1ntara, D. Almorahttps://doi.org/10.22201/igeof.2954436xe.2026.65.1.1895Key words: ground motion amplification, archaeological heritage preservation, spectral ratios, strong-motion records, seismic hazard assessment.Palabras clave: amplificaci%u00f3n del movimiento del suelo, preservaci%u00f3n del patrimonio arqueol%u00f3gico, cocientes espectrales, registros de movimiento fuerte, evaluaci%u00f3n del%u00a0peligro%u00a0s%u00edsmico.-E> -E> -E> -E> -E>