Question about nonlinear site amplification logic in NGA-East implementation (reference Vs30 condition)

[Bruno Collaço]

Dear OpenQuake users,

I have a question regarding the implementation of the nonlinear site amplification term (Fnl) in the NGA-East GMPEs within OpenQuake.

According to Hashash et al. (2019) — particularly the section shown in the attached excerpt — the nonlinear amplification model (Eq. 2–4) defines the reference-rock condition as Vs30 = 3000 m/s. However, the text also states that the parameter Vref  in Eq. 3 is modified to 760 m/s for TOSC <0.4 s, and remains 3000 m/s for TOSC ≥0.4s, only if my original reference is 760 m/s (as far as I understood).

So, if I’m writing a code to calculate site effects, my understanding is that if the input intensity measure (Ir) is already defined for the 3000 m/s reference, then the conditional definition (as defined inside nga_east.py):

if period <= 0.4:

    vref = 760.

else:

    vref = 3000.

should not be necessary, since my model is already referenced to 3000 m/s across all periods.

Could you please confirm whether this conditional logic is indeed required only in the OpenQuake context, or if it could be bypassed when the input PGA (Ir) is already referenced to 3000 m/s?

Thank you very much in advance.

[Marco Pagani]

Dear Bruno,

Many thanks. Those few lines address this note in Hashash’s paper (see page 71).

“In addition, the Vref parameter in Equation 3 is modified to 760 m/s for T(osc) < 0.4, but is retained as 3000 m/s for T(osc) >= 0.4 s”

Hope this helps clarify things

Marco

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