All 592 midnight starts had 300-second labels in the 2017-2025 PNSN tremor catalog

We audited the duration field near midnight in a public PNSN tremor catalog. All 460,042 retained rows for 2017-2025 had durations of 150 or 300 seconds. All 592 starts exactly at midnight had 300-second labels: 288 in 2017-2021 and 304 in 2022-2025. Nearby clock slots contained both labels. This exact-slot result was found after figure inspection. A planned broader contrast gave a null result. In 2022-2025, the 300-second share was 45.07% in the hour around midnight and 45.42% in adjacent periods, after omission of the empty final start slot. The difference was -0.36 percentage points, with a conditional 95% interval of -1.95 to 1.18 from 30-day blocks. No retained start plus its reported duration crossed midnight. These results identify clock structure in a catalog field. They do not measure physical event lengths, establish the cause of the labels, or explain the prior start-count deficit. One snapshot, implicit clock zone, region weighting and conditional uncertainty limit inference. We provide daily evidence, exact-slot counts, source hashes, code, an independent source audit, an offline repeat and a figure.

Cascadia · tectonic tremor · catalog audit · duration field · midnight · null result