How the choice of magnitude-conversion regression changes earthquake activity rates: evidence from the ISC Bulletin for Türkiye, 2007–2025

Earthquake hazard catalogues convert local and other magnitudes to moment magnitude (Mw) with empirical regressions, but the choice of regression is rarely tested for its effect on activity rates. This preprint measures that effect for Türkiye using 57,392 events from the ISC Bulletin (35–43° N, 25–45° E, 2007–2025). Main findings AFAD’s catalogued Mw is 0.13 units below Global CMT Mw (95 % CI 0.11–0.14; n = 363), independent of magnitude. Error variances measured from inter-agency replicates are 0.14 for ML and 0.055 for GCMT Mw, far from the equal-variance assumption commonly used. A structural likelihood model with a Gutenberg–Richter latent magnitude and a truncated reference sample (EIV-exp) recovers slopes, b-values and rates in 1,500 synthetic catalogues, while ordinary least squares overstates the rate of Mw ≥ 4.3 by 1.8–2.8 times. For KOERI ML against GCMT Mw, error-aware estimators agree on Mw = 0.10 + 0.99 ML (paired data range ML 4.2–7.5). Converting the 2007–2025 KOERI ML catalogue, the declustered annual rate of Mw ≥ 4.3 is about 32 with error-aware estimators, 42 with ordinary least squares, and 73 with the ML relation used for the 2018 national hazard catalogue. Files in this record Ozpolat_Dagli_2026_magnitude_conversion_preprint_v1.pdf: the preprint. Licence: CC BY 4.0. mag-conversion-code.zip: analysis code and derived data (pair tables, fitted conversion relations, rate tables, simulation results, figures). Licence: MIT for the code, CC BY 4.0 for the derived data. isc-raw-snapshot.zip: an unmodified extract of the ISC Bulletin, downloaded on 21 October 2026 and archived for reproducibility, with SHA-256 checksums. These data belong to the International Seismological Centre and its contributing agencies. They are not covered by the licences above and must be cited as: International Seismological Centre (2026), On-line Bulletin, https://doi.org/10.31905/D808B830. How to citeÖzpolat, E., & Dağlı, Y. O. (2026). How the choice of magnitude-conversion regression changes earthquake activity rates: evidence from the ISC Bulletin for Türkiye, 2007–2025 (Preprint, Version 1.0). Zenodo. https://doi.org/10.5281/zenodo.23536211