= Solution
<Effective stress> and frictional strength generally increase with depth through the shallow crust undergoing <brittle fracture>. At $1\ \mathrm{km}$, small overburden and often weak, fractured near-surface materials limit the <shear stress> and <elastic energy> that can accumulate. Some shallow materials also undergo stable sliding rather than unstable <earthquake> nucleation. Near $15\ \mathrm{km}$, a locked <fault> prone to <brittle fracture> can sustain a much larger <differential stress> before frictional instability releases stored <elastic energy>.
This depth is plausibly near the lower, strong part of the seismogenic layer: below it, increasing <temperature> eventually promotes <material creep> across the <brittle-ductile transition>. Once nucleated at depth, a dynamic <finite-source rupture> can propagate into shallow regions that could not independently nucleate the same event. Depth alone does not determine the nucleation point: local <pore pressure>, frictional stability and <stress> heterogeneity also matter.
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