Incorporate subduction zones into regionalisation
The first regionalisation includes all tectonic zones except subduction regions. To select the most suitable GMPEs for subduction regions we need to classify the earthquakes as interface, in-slab, outer rise or upper mantle - with interface and in-slab being the most important. In addition, for most events classified as subduction interface or inslab, the geometry of the slab itself should be used to disambiguate the fault plane from the auxilliary plane in the moment tensor.
For modelling the subduction zone geometry, we have available the comprehensive Slab 2.0 global model from the USGS and the recent Hellenic/Cypriot and Calabrian arc models from the 2020 European Seismic Hazard Model.
The regionalisation should work in the following steps:
Simple point in polygon to determine if the earthquake is within the surface projection (plus a buffer) of a specific subduction zone
If yes, then initial assignments can be made based on depth data (e.g. if depth > 50 km then in-slab)
For events in the 0 - 50 km depth range then further information is needed to determine if in-slab or interface. Use focal mechanism if available and proximity to the interface.
For interface (and some slab events) update the earthquake rupture plane to align it with the interface.
Return the GMPE selection and re-calculate the distances using the revised rupture plane
Further revisions to this system using a more probabilistic approach are also anticipated in the future.