Shallow intraslab modelling

[Fatahillah Surya Yudha]

Hi everyone,

I have a question about how GEM / OpenQuake workflows handle shallow intraslab (or shallow slab) earthquakes in subduction hazard models.

From reading several GEM regional model reports and the OQ-MBTK subduction documentation, my understanding is that shallow slab events are earthquakes located just below the megathrust interface (outside the interface buffer) but still considered intraslab in terms of mechanism.

What I am still unsure about is how these events are usually handled in practice when building models using MBTK.

Specifically:

• Are shallow slab events normally treated as part of the general intraslab source, or are they sometimes modelled as a separate sub-domain?

• If they are merged into the intraslab model, are there any recommended rules of thumb for defining the shallow slab boundary (for example using distance from the interface or depth thresholds)?

• Do GEM models ever treat shallow slab differently in terms of MFD estimation, segmentation, or GMPE selection compared to deeper slab seismicity?

• When generating intraslab virtual ruptures using OQ-MBTK, is there any special treatment typically applied to shallow slab events?

If there are any examples, guidance documents, or past models that show how this is usually handled, I would really appreciate being pointed to them.

Thanks in advance!