GNSS postseismic displacement grew later in the first year than aftershock counts after 24 large shallow earthquakes (M7.1-9.1, 2002-2025), so one count-to-displacement ratio does not fit

Question: is the cumulative aftershock count proportional to postseismic surface displacement in the first year, as Perfettini and Avouac (2004) found for the Chi-Chi earthquake? Approach: we used ComCat mainshocks of M6.5 or more (depth 60 km or less, 2000-2025) and daily Nevada Geodetic Laboratory GNSS positions. For each event, we removed the extrapolated pre-event trend and seasonal terms at stations within one rupture length, and counted ComCat aftershocks above completeness in the same circle minus the background rate. We compared the normalized first-year curves. Result: 24 events passed the planned filters. The share of the first-year signal after day 30 had a median of 0.66 for GNSS displacement and 0.36 for aftershocks (paired difference 0.30, 95% CI 0.23-0.37; larger for GNSS in 23 of 24 events). Aftershocks per mm of displacement after day 30 were a median 0.27 of the value in days 1-30. The aftershock rate decayed with a median exponent p = 1.05; GNSS velocity decayed with p = 0.65 (smaller in 24 of 24 events). Nearest-station, half-rupture-length, higher-magnitude-threshold and no-large-later-event subsets gave the same result. A placebo date 450 days earlier showed trend errors of about 4% of the signal. Limitations: the filters keep mostly M7.1+ events because ComCat completeness is near M4.5 outside the US; we compared aftershocks with total surface displacement, not modeled afterslip; aftershock circles and station areas are not the same. The data do not separate non-seismogenic postseismic processes from a decrease of aftershocks per unit afterslip.

aftershocks · afterslip · postseismic deformation · GNSS · Omori law · ComCat · Nevada Geodetic Laboratory · earthquake recovery · aftershock forecasting