Abstract To understand what governs earthquake size, we investigated the 2021 Mw 7.0 Japan Trench earthquake, which began near a nest of repeating earthquakes. High‐precision hypocenter relocation and P‐waveform correlation show that the mainshock initiated nearly identically to Mw 3.8 repeaters. Waveform deconvolution reveals that, after breaking the Mw 3.8 patch within 0.1 s, the mainshock rupture cascaded through Mw 5–6 patches within 2 s and into the main slip area at 3–6 s. Four preceding Mw 5.1–6.1 events, some with higher initial moment rates, did not rupture this main area, likely because the locked region remained too far from failure. Their repeated occurrence may reflect progressive unlocking of this region. In 2021, a start nearly identical to Mw 3.8 repeaters cascaded hierarchically into an Mw 7.0 rupture, suggesting that earthquake size depends less on how rupture begins than on whether surrounding locked regions are ready to fail.