[lispandfound]

Summary

Replaces the tectonic type rules in source_modelling/focal_mechanism.py with the probabilistic upper-plate / interface / intraslab rule of Haynie et al. (2026), behind the existing public API.

Within the Slab2 footprint:

• On-interface probability: exp(-Δz² / 2σ²) with σ = 10 km, where Δz is the vertical distance to the interface.
• Subduction slip probability: exp(-K² / 2σ²) with σ = 30°, where K is the minimum Kagan angle between the event and slip on the interface. The interface slip is either (a) pure reverse or (b) oblique reverse along the Pacific–Australia relative plate motion (NUVEL-1A, DeMets et al. 1994).
• P_interface = on-interface × subduction slip × (depth ≤ 70 km).
• The remainder is crustal (upper plate) above the interface and intraslab below it.
• Outside the footprint, events are crustal.

A fixed depth drops the on-interface factor, and a missing focal mechanism drops the slip factor.

API

Stable. CMTClassifier.tectonic_type(centroid, nodal_plane_1, nodal_plane_2) keeps the call that workflow's gcmt-to-realisation (and so the realtime simulation workflow) makes, and TectonicType is unchanged. The planes are now optional, and there is a new fixed_depth keyword. The Kagan angle is the minimum over both planes, so plane order doesn't matter.

New: kagan_angle, plate_motion_azimuth, subduction_slip_planes.

Removed:

• The NSHM 2022 rule: the old module-level tectonic_type_probabilities(depth_below_slab, interface_like, normal_like, sigma), _is_interface_like and its constants.
• The sigma keyword of CMTClassifier.tectonic_type_probabilities.
• The NGA-SUB rule: nga_sub_tectonic_type(_probabilities), SlabZone, SlabZoneQuery and SlabModel.zone.

Behaviour change

The 2003 Fiordland event (GeoNet 2103645; centroid −45.19, 166.83, 22 km) moves from interface to intraslab, at about 0.54 slab vs 0.46 interface. It sits 9 km below Slab2, and the plate-motion rake at Puysegur is very oblique (~167°). workflow's test_gcmt_to_realisation_tectonic_type expects subduction_interface for this event and will need updating. A companion workflow PR is being prepared for that.

validation_small_magnitude/scripts/compare_sources.py calls tectonic_type_probabilities(..., sigma=0.0) and will need the sigma argument dropped.

Test plan

• New tests: Kagan angle identities and the ≤ 120° bound, plate motion azimuths at Hikurangi and Puysegur, horizontal slip of the oblique reference plane parallel to plate motion, the rule's probabilities, plane-order invariance, and the no-mechanism / fixed-depth paths
• Full suite on top of main: 862 passed

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