= Solution
Assuming pure <dip-slip faulting>, use an east–west striking, $65^\circ$ north-dipping <normal fault>, with the northern <hanging wall> moving down dip. In a right-hand-rule strike convention this is \b[strike $270^\circ$, dip $65^\circ$, rake $-90^\circ$]. The auxiliary <nodal plane of a double couple>[nodal plane] strikes east–west and dips $25^\circ$ south. The <focal mechanism> has a predominantly vertical P axis and a predominantly north–south horizontal T axis; the steep central direction is dilatational.
\Image[/past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2022/paper-1-normal-focal-mechanism.png]
{title=Original lower-hemisphere normal-fault <focal mechanism>: black compression, white dilatation, and two alternative <nodal plane of a double couple>[nodal planes]}
The sketch is a double-couple <seismic moment tensor> calculation, not a claim that first motions uniquely identify which <nodal plane of a double couple>[nodal plane] slipped. The mapped north-dipping surface supplies that additional geological choice.
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