Summer concentration fell more for tremor detections than for active days in northern Cascadia during 2010-2024

We tested a reported seasonal change in northern Cascadia tremor at 47-50 N. We used a fixed public PNSN API v3.0 snapshot. The July-September detection fraction fell from 56.25% in 2010-2015 to 24.45% in 2016-2024. The active-day fraction fell from 37.65% to 24.26%. Summer detections per active day were 2.13 times the rate on other active days in the early period, but 1.01 times in the late period. Thus detection intensity contributes to the larger count change. An exact primary test used all 133,225 independent calendar phase rotations of the two fixed clustered histories. Its two-sided p-value was 0.05446. Secondary active-day tests gave smaller p-values without a multiple-test correction. The descriptive change does not establish its cause. Catalog updates, unknown uptime, short periods, and the specific independent-phase null limit inference. The package gives daily evidence, source hashes, code, exact phase curves, bootstrap draws, a separate scalar audit, and a figure.

Cascadia · tectonic tremor · seasonality · active days · calendar phase · clustered events · PNSN · hydrological forcing