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1860 | Geof%u00edsica Internacional (2026) 65-1the base of North Platform of Monte Alb%u00e1n during the 30/09/1999, 08/09/2017, and 23/06/2020 earthquakes are 137, 129, 105cm/s2, respectively; the corresponding values of (PGV)H and (PGD)H are 5.0, 8.4, 9.3 cm/s and 2.4, 12.0, 4.1 cm. Damage to the archaeological site during the first two events is well documented. However, in spite of relatively high peak values during the earthquake of 2020, the report by Robles Garc%u00eda et al. (2022) does not mention any damage during this event. Lesser damage during 2020 than in 1999 may have been due to successful recovery and conservation efforts following the earthquakes of 1999 and 2017 which had adequately strengthened the vulnerable structures.For simplicity we assume that damage at Monte Alb%u00e1n occurs if the (PGA)H at the base of North Platform exceeds 120 cm/s2(70 cm/s2 at the reference station OXLC). From Figure 8, we find that such motion, on average, is expected every ~20 to 25 years. This implies that, on average, a (PGA)H value of 120 cm/s2is expected to be exceeded once every 20 to 25 years. Based on a Poisson probability model, the likelihood of exceeding this threshold within a 25 years observation period is estimated to be around 67%. Thus, since 500 BC when Monte Alb%u00e1n was constructed, the archaeological site has been subjected to such (PGA)H more than 100 times. The estimated ground motion of 480 cm/s2 during 1931 earthquake is exceeded every 100 years, which implies ~ 25 such events in the life span of the site. It is not possible to check the veracity of these estimations as there is no report of damage at the abandoned ruin of Monte Alb%u00e1n until 1928. First reports of damage come from the earthquake sequence of 1928, and the event of 1931 (Table 7) when the walls restored by Batres (1902) at the corner of the sculptures of Dancers (Danzantes) fell down (Robles Garc%u00eda et al., 2004). Alfonso Caso was highly critical of the restoration work carried out by Batres, especially because the restoration did not withstand the 1931 earthquake. Considering the devastation that the earthquake caused to the adjacent City of Oaxaca and the high estimated (PGA)H during the earthquake at Monte Alb%u00e1n, it may seem puzzling that the damage to the monument was not much greater in 1931 than in 1999 and 2017. In fact, in 1931 Alfonso Caso had just begun the explorations. Except for a small area restored by Batres, the rest of Monte Alb%u00e1n was a mountain of rubble. Alfonso Caso does not mention extensive damage because there was little which could be damaged (Nelly Robles Garc%u00eda, personal communication, 2025).In view of the cultural, social, historical, and touristic importance of this World Heritage Archaeological Site, it is of vital importance to install and operate seismic sensors in the zone. In the meantime, the recordings at the nearby site of OXLC may be gainfully used since velocity and displacement at Monte Alb%u00e1n and OXLC are very similar. The acceleration, which differs, may be roughly estimated from OXLC recording and the known spectral ratios through the application of random vibration theory. The results of this paper may be useful in making decision on how to adequately strengthen the monument against damage during future earthquakes.The importance of periodic geotechnical, geodetic, and geophysical monitoring of the monument can hardly be overemphasized. These methods were successfully used in the recovery and restoration efforts following the 2017 earthquake (Robles Garc%u00eda et al., 2022).7. AcknowledgementWe are greatly indebted to Nelly Robles Garc%u00eda and Salvador Aceves for their generous help and orientation; authors of this paper, with little knowledge of archaeology, would have made many more errors without their guidance. In 2004 Roberto Meli brought to our attention the damage at Monte Alb%u00e1n during the 1999 Puerto Escondido earthquake and to the accelerometric network which had begun operating at the site. For some reason, now forgotten, the research was abandoned by us after an initial exploration. The interest was revived during a recent visit to the monument by one of the authors as a tourist.Acquisition of the Monte Alb%u00e1n accelerographic network was funded by the Government of Japan. Maintenance, partially funded by INAH, was carried out by Instituto de Ingenier%u00eda (II), UNAM. OXLC is an II station. We thank Leonardo Ramirez Guzm%u00e1n, Citlali Perez Ya%u00f1ez, M. del Rosario Delgado Diance and Ana Laura Ruiz Gordillo for making the accelerograms recorded at Monte Alb%u00e1n available to us. Thanks to the Instrumentation Group of II; in particular to Juan Manuel Velasco Miranda, Mauricio Ayala Hern%u00e1ndez, Israel Molina %u00c1vila, Luis Alberto Aguilar Calder%u00f3n, Ricardo V%u00e1zquez Larquet, Alejandro Mora Contreras, H%u00e9ctor Sandoval G%u00f3mez, Marco Antonio Mac%u00edas Castillo, and Gerardo Castro Parra.The research was partially supported by DGAPA UNAM Project IN114426.8. ReferencesAbrahamson, N. A., Silva W. J. (1997). Empirical response spectral attenuation relations for shallow crustal earthquakes. Seismological research letters, 68(1), 94-127. doi: https://doi.org/10.1785/gssrl.68.1.9Arroyo, D., Garc%u00eda, D., Ordaz, M., Mora, M. A., & Singh, S. K. (2010). Strong ground-motion relations for Mexican interplate earthquakes. Journal of Seismology, 14(4), 769%u2013785. doi: https://doi.org/10.1007/s10950-010-9200-0Batres, L. (1902). Explorations of Mount Alban, Oaxaca, Mexico: Year 1902. Gante St. Press, Mexico.