Revealing the spatiotemporal complexity of the magnitude distribution and b-value dur- ing an earthquake sequence

The Magnitude–Frequency-Distribution (MFD) of earthquakes is typically
modeled with the (tapered) Gutenberg–Richter relation. The main parameter
of this relation, the b-value, controls the relative rate of small and large
earthquakes. Resolving spatiotemporal variations of the b-value is critical to
understanding the earthquake occurrence process and improving earthquake
forecasting. However, this variation is not well understood. Here we present
remarkable MFD variability during the complex 2016/17 central Italy sequence
using a high-resolution earthquake catalog. Isolating seismically active
volumes (‘clusters’) reveals that the MFD differed in nearby clusters, varied or
remained constant in time depending on the cluster, and increased in b-value
in the cluster where the largest earthquake eventually occurred. These findings
suggest that the fault system’s heterogeneity and complexity influence the
MFD. Our findings raise the question “b-value of what?”: interpreting and using
MFD variability needs a spatiotemporal scale that is physically meaningful, like
the one proposed here.