Page 12 - Demo

1858 | Geof%u00edsica Internacional (2026) 65-1during 2020 was 121 cm/s2 (Table 5). We expect a smaller (PGA)H than this at the base of MAPN. The estimated value, 151 cm/s2, is higher, no doubt, because median spectral amplification differs greatly from the measured amplification for this event. Henceforth, we will assume that (PGA)H at the base of MAPN was ~ 115 cm/s2, slightly less than the observed value at mid-height. (2) The damage was greater in 2017 than in 1999 and 2020. Yet the (PGA)H was highest during 1999. As mentioned earlier, it is possible that PGV and PGD played an important role in the damage; these values, especially PGD, were much higher during the great 2017 Chiapas (Mw8.2) earthquake (Figure 2, Table 1b).If significant damage at Monte Alb%u00e1n occurred only during the earthquakes of 30/09/1999 and 08/09/2017, and, perhaps, also during the 2020 earthquake, then it is reasonable to state that the (PGA)H should exceed ~ 120 cm/s2 at the base of the North Platform for this to happen in the future. The exceedance rate of such (and higher) level of ground motion is a critical issue which is addressed next.Table 6. PGA amplification at sites at MAPN with respect to OXLC assuming linear behavior.Site AmplificationIntraslab Earthquake Interface EarthquakeH Z H ZBase 1.72 0.83 1.79 0.93Mid-height 2.29 0.87 1.96 0.78Top 3.73 0.90 3.05 0.945. Exceedance Rate of Damaging EarthquakesFrom the recordings at OXLC during the period 1999-2024, we computed the empirical exceedance rate curve of (PGA)Hshown in Figure 8 by counting the number of times that a given value of PGA was exceeded in the recordings and normalizing by the period of observation. As the data is for only 25 years, the empirical curve is likely to misrepresent the long-term exceedance rate. To partially remedy this shortcoming, we prepared a catalog of earthquakes for the period 1900 %u2013 2024 (Table 7) which may have produced (PGA)H >15 cm/s2 at OXLC. (PGA)H of earthquakes in the period 1999-2024 are, of course, the recorded values at OXLC, except for the 2018 event. For this event and all others that occurred before 1999, (PGA)Hwere computed from GMPE derived by Garc%u00eda et al. (2005) for intraslab events and Arroyo et al. (2010) for interface events (Table 7). Note that the highest estimated (PGA)H at OXLC (hence, most likely, also, at Monte Alb%u00e1n) in the last 124 years occurred during the normal-faulting, intraslab earthquake of 15 Figure 8. Exceedance rate curves of (PGA)H. Blue: from recorded data 1999-2014 at OXLC. Green: based on data 1900-2024 (recorded at OXLC: 1999-2024, estimated: 1900-1998). Green dash: curve for the base of MAPN (green curve shifted by a factor of 1.66 to account for amplification assuming intraslab events).101---------------------~ I.... co Q) -2:' 100 rl Q) u C co al 10-1 Q) u X Q) '+-0 10-2 Q) ...., co I.... - OXLC, PSHA - OXLC, empirical, 1999-2024 - OXLC, empirical, 1900-2024 Base Montealban, empirical, 1900-2024 10- 3 .....,_ _____________ .......,__~-------' 101 102 103 PGA, cm/s\