Spatial and temporal evolution of the b-value: A comparative analysis across different tectonic settings

The b-value of the Gutenberg-Richter law represents a fundamental parameter for characterizing earthquake size distributions and assessing crustal stress conditions.

Numerous studies have demonstrated a significant relationship between the b-value and the tectonic stress regime. While low b-values are often associated with stress accumulation and potential precursors to large earthquakes, their applicability as universal precursors remains debated.

This study investigates the spatial and temporal evolution of the b-value preceding large earthquakes (M≥5.5) in regions characterized by contrasting tectonic settings and fault kinematics. We examine geographically and tectonically different areas—such us Italy, China, New Zealand, and Myanmar—which encompass different stress regimes and plate boundary configurations. By systematically analyzing local seismic catalogs from these contrasting regions, we assess whether b-value variations constitute a universal feature of the seismic cycle, or are primarily modulated by region-specific crustal properties.

Each study area is selected according to the seismogenic structures responsible for the target earthquakes. The completeness magnitude (Mc), defined as the lowest magnitude threshold above which the catalog reliably records all or nearly all earthquakes in the region, is rigorously determined as a function of time according to well-established methodologies.

We estimate Mc using the most appropriate method for each region, including either the maximum curvature method  or the 90% goodness-of-fit criteria, to ensure robust results.

The b-value is subsequently estimated using the maximum likelihood approach over appropriate spatiotemporal windows preceding each mainshock.

We present preliminary results showing that systematic b-value drops are observed in most cases. Our findings support the hypothesis that such variations represent a robust indicator of progressive stress accumulation. This comparative approach suggests that b-value monitoring can provide valuable precursory signals for seismic hazard assessment across different tectonic contexts.