Ground Motion at the World Heritage Archaeological Site of Monte Albán, Oaxaca, Mexico during Recent Damaging Earthquakes

Abstract

Archaeological site of Monte Albán, 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 Coos plate below the continent are relatively frequent, it follows that the archeological zone has been subjected to severe ground motion innumerable number of times. To adequately strengthen the monument against damage during future events, it is most desirable to know the ground motion at Monte Albán 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 Albán with respect to OXLC then the recordings at OXLC can be used to estimate the ground motion at Monte Albán through the application of random vibration theory. A 12-channel strong-motion network was in operation intermittently between 2004 and 2019 at Monte Albán. 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 Albán 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)H during the Huatulaco earthquake of 23 June 2020 (Mw, 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 Albán seems to occur if (PGA)H exceeds ~ 120 cm/s2 at the base; such motion has an estimated return period of ~ 20 yrs. A rough 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 480 cm/s2; this motion is expected to exceed once every 100 yrs. Reports of damage to Monte Albán 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.