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S.K. Singh et al. | 1857(Table 1). Sa for the vertical component at different sites on the platform is nearly the same as at OXLC with few exceptions. This is also true for Sa of the horizontal component at periods T %u2265 1 s, i.e., at long periods Sa is not affected by the location of Monte Alb%u00e1n on a hill. At short periods, T < 1 s, Sa at sites on the Platform differs from that at OXLC so that: (Sa)Top > (Sa)Mid-height > (Sa)Base > (Sa)OXLC.At very small T, Sa equals PGA which is compiled in Table 5. Figure 7 gives ratio of PGA at sites on the MANP with respect to its corresponding value at OXLC; we denote this ratio the PGA amplification. We note that there is little or no PGA amplification for Z component. In fact, a de-amplification occurs during some events. PGA of the horizontal components at sites at the platform are amplified which increases with the height of the sensor location.Average amplification of PGA at MAPN with respect to OXLC may be summarized as follows (Table 6). For Intraslab events the PGA at the base, mid-height, and top are amplified by a factor of 1.72, 2.29, and 3.73 on the horizontal components, and 0.83, 0.87, and 0.90, respectively on the Z component. For the subduction thrust events the amplification of the horizontal components at the base, mid-height, and top, are 1.79, 1.96, and 3.05; the corresponding values on the Z component are 0.93, 0.78, and 0.94.The estimated (PGA)H at the base of MANP during the earthquakes of 30/09/1999, 08/09/2017, and 23/06/2020 are 137, 129, and 151 cm/s2; the corresponding observed values at OXLC are 72, 83, and 78 cm/s2, respectively. Here (PGA)H= [{(PGA)NS2 + (PGA)EW2}/2]1/2. There are two problems with the estimated values. (1) The observed (PGA)H at mid-height Figure 7. Amplification of PGA at the three sites of the MAPN with respect to OXLC. Note that the horizontal components at the base, mid-height, and top of MAPN are amplified with respect to OXLC. No such amplification occurs on the Z component; in fact, significant de-amplification is seen for events 3 and 4.Table 5. PGA at Monte Alb%u00e1n estimated from OXLC Recordings.Date PGA at OXLC, cm/s2 Estimated PGA at Monte Alb%u00e1n, cm/s2Base Mid-height TopNS EW Z NS EW Z NS EW Z NS EW Z15/06/1999 23.5 18.3 13.0 35.0 33.6 12.8 46.7 45.1 12.4 78.2 74.5 12.830/09/1999 70.1 74.7 48.3 132.2 142.2 50.8 175.4 188.8 53.5 293.1 315.9 55.120/03/2012 37.2 27.7 32.9 66.9 40.4 20.2 69.2 42.9 17.1 92.4 56.7 19.908/09/2017 58.6 102.3 67.5 109.6 145.5 37.8 145.2 194.3 40.1 231.7 298.4 41.019/09/2017 22.5 21.8 12.1 35.2 38.4 9.5 47.0 51.2 10.1 77.6 80.4 10.423/06/2020 72.5 82.9 30.4 149.7 152.5 37.9 169.7 173.0 31.4 244.3 248.7 38.3- - - 133.7+ 138.0+ 31.6+ - - -- - - 128.1* 113.0* 56.0 - - -+Computed with observed spectral amplification between mid-height, MAPN and OXLC during the 23/06/2020 earthquake *Observed PGA at mid-height site during the 2020 earthquakePGA amplification relative to OXLC at MAPN 5-------------- - - -----~ Blue: Base , 4 0 ljl %u25a1 0 Red: Mig - heigh +Green: 1,op Circle: NS, Square: EW Cross: Z L %u25a1 0 r 0 %u25a1 0 --s- 0 ~ 0--~g---+ ~ -l r t -f 2 3 4 5 6 Event Number 7