Cascadia tremor days did not raise earthquake rates near the tremor zone more than 40-100 km away in PNSN tremor and ComCat data for 2009-2026 (M1.5+, US margin)

Question: does episodic tremor and slip (ETS) increase regular earthquake rates or change magnitudes near the Cascadia tremor zone? Data: the PNSN tremor catalog (791,720 windows, 2009-08 to 2026-11) and USGS ComCat (101,648 earthquakes, 39.5-50.5 N). Method: we compared daily M1.5+ counts on tremor-active and quiet days in 0.25-degree latitude bands, with band x calendar-month strata. Earthquakes 40-100 km from the tremor zone were the spatial control. A null distribution came from 1,000 circular time shifts of the tremor record. Results: on days with 10 or more tremor windows, near earthquakes (within 20 km) were 1.30 times more frequent (95% interval 1.12-1.53). But far earthquakes were also 1.34 times more frequent. The near/far ratio was 0.97 (0.76-1.30). The excess did not occur on strong tremor days (50 or more windows: 1.04, 0.82-1.36), around 1,118 major tremor arrivals (days 0-13: 0.91, placebo 0.86-1.16), or when tremor was moved 111 km or more along strike. When we removed about 4% of tremor windows on earthquake days, the excess decreased to 1.10-1.13 and was not significant. This occurred both at the earthquake times and 2-8 hours away. The b-value (0.86 during ETS and the next 14 days, 0.89 outside) and the M2.5+ fraction (12.5% and 12.2%) did not change. Conclusion: the same-day tremor-earthquake link is a shared detection or catalog effect, not ETS triggering. A positive hint for deep M1.5+ events on strong tremor days in Washington (1.51, p = 0.04) does not survive multiple tests. Limitations: completeness near M1.5, most events are in Washington and near Mendocino, no declustering or slab model, and the near/far control was added after a first pass.

Cascadia · episodic tremor and slip · slow slip · earthquake triggering · tectonic tremor · PNSN · ComCat · seismicity rate · b-value · detection bias · placebo test