In shallow Global CMT earthquakes of Mw 5.6 and larger (1977-2026), the faulting-style b-value difference came from differences between regions, not from faulting style within regions

Many studies report that the Gutenberg-Richter b-value is highest for normal faults and lowest for thrust faults, and use b as a stress meter. We tested this in 10,194 shallow (depth 40 km or less) Global CMT earthquakes of Mw 5.6 or more from 1977 to mid-2026, with a plan written before the analysis. For all years, b was 1.14 for normal, 1.06 for strike-slip and 0.98 for thrust events (thrust/normal ratio 0.86, 95% cluster-bootstrap CI 0.80-0.92). The full order was present in 1977-2004, but not in 2005-2017, and in the new 2018-2026 data the ratio (0.88, 0.74-1.02) did not exclude 1. When we compared events only inside the same Flinn-Engdahl region (exact exponential likelihood with region fixed effects), the ratio was 0.98 (0.91-1.06): only 13% of the raw log contrast remained. Mc 5.8 or 6.0, depth to 70 km and plunge-based classes gave the same result; 10-degree cells and a declustered catalog kept a small part (ratios 0.93 and 0.91). The difference is not a corner-magnitude effect: it stayed in Mw 5.6-6.5 (ratio 0.87, 0.78-0.96) and in tapered fits with class-specific corners. An exploratory plate-boundary split (PB2002) showed the cause: normal events at oceanic spreading ridges had b = 2.06 and at oceanic transforms 1.50, while subduction normal and thrust events both had b near 1. Inside subduction-zone regions, thrust b was 15% higher than normal b, the opposite of the stress-meter order. Limitations: only Mw 5.6+ global data, so small-earthquake regional results are not tested; Flinn-Engdahl regions are large and mix stress environments; completeness and the 2004 GCMT method change can differ by region and period; the setting split is post hoc.

b-value · Gutenberg-Richter · faulting style · rake · stress meter · Global CMT · tapered Gutenberg-Richter · corner magnitude · fixed effects · composition effect · out-of-sample test · plate boundary · mid-ocean ridge · subduction zone · replication