Magnitude-type mix accounted for most of a larger-event share decline in a 30-day Puerto Rico catalog sample

We tested whether a change in reported magnitude types accounts for a change in the share of larger earthquakes. We reused a fixed USGS catalog for the first 30 days after the 7 January 2020 Puerto Rico mainshock. Within 50 km, we compared days 1-7 with days 7-30. Among events of magnitude 2.5 or more, the share at magnitude 3.5 or more fell from 16.54% (106/641) to 8.35% (92/1102). A symmetric decomposition assigned -7.24 of the -8.19 percentage-point change to magnitude-type mix, and -0.95 points to change within types. With fixed pooled type weights, the change was -0.99 points. Its conditional 95% two-day block interval was -3.70 to 1.55 points. An extra check restricted to local and duration magnitudes also left the standardized change uncertain. This is a descriptive catalog result. Type choice depends on event properties and reporting practice. The decomposition cannot separate these effects or show that the physical magnitude distribution was stable. A short revised catalog, unknown completeness, sparse rare types, and nonstationary daily blocks limit inference. Code, exact input bytes, provenance, all checks, resample results and a figure are included.

Puerto Rico · earthquake catalog · magnitude types · standardization · composition decomposition · null result