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1854 | Geof%u00edsica Internacional (2026) 65-1Figure 4. Spectral ratios at three sites of MAPN with respect to OXLC. Cyan curves exhibit ratios of individual events. Median and %u00b1 one standard deviation curves are shown in red. (a) From events with depth H %u2264 35 km. Thick blue curve is the observed spectral ratio between mid-height, MAPN and OXLC during the 2020 Huatulco earthquake; at f > 3 Hz observed ratio is lower than the estimated ratio likely due to nonlinear behavior of the structure. (b) From events with depth H > 35 km.mid-height of MAPN are 15.1, 16.7, and 27.9 s (Table 4). Thus, the effective duration of a given component at OXLC is about the same as at mid-height, MAPN. In the calculations we assume the same Te at both sites. The results are shown in Figure 5c. Estimated and observed Sa for the two horizontal components are very similar at period T > 0.5 s but differ by as much as a factor of 2 at some shorter periods. Sa for the vertical component follows a similar trend. The main cause of the discrepancy is an unexpectedly large difference between the median spectral amplification and the observed spectral amplification during the 2020 earthquake alone (Figure 4a, middle frames). This difference points to a possible nonlinear behavior at MAPN middle-height site. Horizontal Sa recomputed using the observed spectral amplification (blue curve in Figure 5c) is much closer to the observed Sa (blue dashed curve); however, the difference is somewhat greater for the vertical Sa. The test suggests that sites on the MAPN are likely to suffer nonlinear behavior under intense ground motion. The results obtained here using median spectral amplification provides needed input ground motion to compute nonlinear response.4.3 Estimated Ground Motions at Monte Alb%u00e1n during Recent Large EarthquakesIn estimating Sa at MAPN sites from the recordings at OXLC, we assume that Te at the three MAPN sites is the same as at OXLC. This is justified from the effective duration observed during the 2020 earthquake. Te values at OXLC are compiled in Table 4.Figure 6 shows the observed Sa at OXLC and the estimated Sa at the base, mid-height and top of MAPN for all six events El H < 35 km, Horizontal H < 35 km, Z m H > 35 km, Horizontal H > 35 km, Z Base Base Base 101 101 101 101 :3 :3 Pc: \Pc: \\\b 10%u00b0 b 100 10%u00b0 () () \\[/) [/) 10- 1 10- 1 10- 1 10- 1 T 10- 1 10%u00b0 101 10- 1 10%u00b0 101 10- 1 100 101 10- 1 10%u00b0 101 101 101 !iid-heigh 101 101 Mid-heigh :3 :3 Pc: \Pc: \\\b 10%u00b0 10%u00b0 b 10%u00b0 10%u00b0 () () \\[/) [/) 10- 1 10- 1 10- 1 10- 1 T 10-1 10%u00b0 101 10-1 10%u00b0 101 10-1 10%u00b0 101 10- 1 100 101 101 101 101 101 Top :3 :3 Pc: \Pc: \\\b 10%u00b0 100 b 10%u00b0 () () \\[/) [/) ' 10- 1 10- 1 10- 1 10- 1 10- 1 100 101 10-1 100 101 10- 1 10%u00b0 101 10-1 100 101 Hz Hz Hz Hz