= Solution
A \b[reverse or <thrust fault> earthquake] is the expected end member: a <hanging wall> moves upwards relative to its <footwall>, accommodating north–south <tectonic shortening>. A plausible <double-couple seismic source> <focal mechanism> has an approximately east–west <fault strike>, a moderately north- or south-dipping <fault plane>, and up-dip <fault slip>. For example, right-hand-rule strike $090^\circ$, southward <fault dip> about $32^\circ$, and <fault rake> $+90^\circ$ specify a pure thrust mechanism.
In a lower-hemisphere equal-area plot, with shaded regions denoting compressional P first motions, this <focal mechanism> has a shaded central region and two unshaded outer lobes. The P axis is approximately horizontal north–south and the T axis approximately vertical for a symmetric idealization; for a single $32^\circ$ plane their axes are slightly tilted. The P and T axes of a <double-couple seismic source> need not exactly equal the regional <principal stress> axes. The nodal curves represent both the <fault plane> and its auxiliary plane; additional evidence is needed to choose between them.<Focal mechanisms or beachballs by United States Geological Survey>[Lower-hemisphere compression shading and the ambiguity between the two nodal planes.]{ref}
\Image[/past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2024/paper-3-q3-failure-and-mechanism.png]
{title=Mohr-Coulomb tangency, conjugate thrust dips, and a plausible thrust focal mechanism}
{description=The stress scale is arbitrary. The Mohr circle touches a friction envelope of slope 0.5; the physical fault dip is 31.72 degrees. The original lower-hemisphere mechanism uses an east–west-striking reverse fault; dark shading denotes P-wave compression.}
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