Scale-dependent spatial coherence between historical and instrumental earthquake catalogues at the global scale

Abstract. Historical earthquake catalogues extend seismic observations back by several centuries and are widely used in seismic hazard and tectonic studies, yet their global-scale informational content remains difficult to quantify due to strong spatial, temporal, and magnitude-dependent reporting biases. In this study, we present a quantitative, spatially explicit assessment of the consistency between global historical (1600–1899) and early instrumental (1900–1950, ) earthquake catalogues. Rather than relying on magnitude-based comparisons, we represent earthquake occurrence as spatial probability density fields obtained through Gaussian smoothing and define a scale-dependent spatial coherence metric based on the overlap between historical and instrumental distributions. This approach allows us to isolate large-scale tectonic signal from localized reporting artefacts and to systematically explore the role of spatial scale. Our results show that spatial coherence between historical and instrumental seismicity is low at small scales and increases monotonically with smoothing length, reaching moderate values only at regional to continental scales. Even at the largest scales considered, coherence remains well below unity, indicating that only a limited fraction of the instrumental spatial pattern is recoverable from historical data. Decomposition by tectonic domain reveals that subduction zones dominate the historical–instrumental agreement, while continental collision belts and intraplate regions contribute substantially less. These findings demonstrate that global historical earthquake catalogues contain a detectable but intrinsically limited imprint of tectonic structure. Meaningful use of historical seismicity at the global scale therefore requires explicit consideration of spatial scale and tectonic context. The framework proposed here provides a transparent and reproducible basis for evaluating the reliability of historical earthquake data in seismic hazard and global seismotectonic applications.