Estimation of Peak Ground Acceleration (PGA) and Spectral Acceleration (Sa) Vertical Component for Interface Subduction Zone Earthquakes of Northeast India (NEI) and Adjacent Regions

This article presented ground motion model (GMM) for vertical peak ground acceleration (PGA) 6 and pseudo spectral acceleration (S a ) at 5 % damping for North-east India (NEI) and adjacent 7 regions at a time period of 0.01 to 5 s, and hypocentral distance 40 to 300 km. We used 8 combined point source (4.5 ≤ M w ≤ 6.5) and finite fault model (6.5 < M w ≤ 9 .5) (refer as 9 combined model) to develop GMM for vertical component of ground motion (VCGM) for the 10 region. The vertical GMM obtained is validated with the available recorded events in NEI and 11 adjacent regions for the interface subduction zone earthquakes. It is observed that peak ground 12 accelerations and spectral accelerations are 55 to 65% lesser than the horizontal components of 13 ground motions. VCGM parameters obtained in this study play an important role in designing 14 low rise buildings and linear superstructures such as bridges, silos and chimneys.


Introduction
This article is a companion article published by Rahman and Chhangte (2021). Rahman and 17 Chhangte (2021) developed ground motion model (GMM) for the horizontal component of peak 18 ground acceleration (PGA) and 5% damping spectral acceleration (Sa) for the interface 19 subduction earthquakes of NEI and adjacent regions (hereon refer as NEI). Seismo-tectonic 20 characteristics of the interface subduction zone earthquakes of NEI was presented in details in 21 the companion article (Rahman and Chhangte, 2021).  Goswami and Sharma, 1982). The vertical ground motions obtained from this high magnitude 28 earthquakes (Mw > 8.0) cannot be ignored in designing the engineering structures. The vertical 29 ground motion obtained from the subduction earthquakes (Mw 8.5) which occurred in 1897 in 30 NEI was estimated to be 1.1 g (Oldham, 1899). Rahman (2008)   This earthquake had also diverted the line of course of the river Brahamaputra from the original 42 mainstream. The vertical ground motions for this earthquake were estimated as 1.2 g 43 (Raghukanth, 2008;Rahman, 2008;Tiwari, 2002).

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Based on this study, it is observe that vertical component of earthquakes also contribute to the 45 destructive effect on structures as well. In the past, VCGM were totally ignored while designing 46 ordinary structures as it was commonly believed that vertical ground accelerations are 47 significantly lower than corresponding horizontal ground accelerations (Cagnan et al. 2017).

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In this article, we have simulated 50,000 vertical component of PGA and Sa for 4.5 ≤ Mw ≤ 6.5 49 using Boore's (1983;2003) point source seismological model. We have also simulated 50000 50 vertical component of ground motion for 6.5 < Mw ≤ 9.5 using finite fault model. We used 51 combined point source and finite fault model as point source have some limitation in ground 52 motion simulation for high magnitude earthquake (Rahman and Chhangte, 2021). 53 We used the same data and resources, path duration, geometrical spreading factor, stress drops 54 for the interface subduction earthquakes presented in our companion article (Rahman and 55 Chhangte, 2021). 56 We have estimated quality factor (Q) for VCGM which are further used in ground motion  (1) 76 with the standard deviation (σ1) values as (13.7 (Q0), 0.038 (η)).

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The boot-strap method is applied similar to our companion paper Rahman and Chhangte (2021).

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The operation is repeated 150 times and 150 sets of parameters are generated from which we can  Table 3.

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The ranges of seismic input parameters used for VCGM simulations of NEI interface 85 earthquakes are presented in Table 3   for NEI interface earthquakes which are presented in Table 6.

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The following functional GMM form used by Abrahamson et al. (2016) for global interface 104 subduction earthquakes is used in this study. The same mathematical form is also used in 105 Rahman and Chhangte (2021). The functional GMM form is as follow: Here, VS30 = 2800 m/s 120 The regression coefficients in Equation (5) are presented in Tables 7 and 8.

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The regression coefficients θ1 to θ11 are calculated using a Joyner and Boore (1981)  The sensitivity analysis performed in this study for different parameters is similar to the 132 procedure adopted by Rahman and Chhangte (2021). The sensitivity analysis results obtained 133 from our model are presented in Table 9. 134 It is observed that the spread of standard deviations for VCGM input parameters vary from 0.042 135 to 0.416 (Table 9). The standard deviations input parameters namely focal depth, cut-off 136 frequency, stress drop, radiation pattern, anelastic attenuation, geometric attenuation and time 137 duration are negligible (  also observed that these differences may also arises due to different value of VS30.