M9 Basin Term for US 2023 Model #10118
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This PR adds the M9 Basin adjustment (as described on pp. 1178 of Moschetti et al. 2024 EQ spectra article on US 2023 NSHMP GMC). The M9 basin adjustment is an additive factor to the mean ground-motion for long period spectral accelerations (T > 1.9 s) of + log(2.0) for all sites with z2pt5 greater than 6 km.
The PR adds the following:
A separate ModifiableGMPE class (as exists for example for CY14 site term and NRCAN15 site terms). Having a separate ModifiableGMPE class with importable basin adjustment function will be required for when we create the "M9" class of each US 2023 subduction interface GMM (AbrahamsonGulerce2020, KuehnEtAl2020, ParkerEtAl2020, AtkinsonMacias2009, ZhaoEtAl2006).
The ability to specify the M9 basin adjustment as a kwarg when instantiating a ModifiableGMPE from a GSIM LT XML (or programmatically), like any other ModifiableGMPE capability such as the Al Atik 2015 sigma model.
Some simple unit tests checking that the basin adjustment is only applied for long period ground-motions at sites with z2pt5 > 6 km, with an expected different of log(2.0) being obtained at these sites compared to the values obtained at the same sites with the unmodified version of the same GMM.
T > 1.9 s threshold is confirmed by 1) - usgs java code (line 205 of https://code.usgs.gov/ghsc/nshmp/nshmp-lib/-/blob/main/src/main/java/gov/usgs/earthquake/nshmp/gmm/AtkinsonMacias_2009.java?ref_type=heads) and 2) the discussion in OQ issues of how to implement these adjustments - #9894