Clustering and recurrence intervals of earthquakes: selected cases and analyses

Long-term seismic records show extreme heterogeneity in their internal structure ranging from almost periodic occurrences to super-cycles and chaotic patterns. Spatial and temporal clustering is an intrinsic property of seismic activity at both short- (seismic sequences) and long-timescales related to the structural properties of fault systems, stress gradients and rheology. Since the geological features are almost the same over the temporal intervals we are interested in, the average properties of seismicity are invariant. Moreover, at the spatial scales involved by seismicity from micro-events to mainshocks, the mechanical properties can be roughly considered the same since rheology gradients usually act at regional lengths. Then, the mechanism of stress accumulation and release can also be considered almost scale invariant by a seismological point of view. Therefore, where small magnitude events are characterized by a clustered behaviour, it is also likely that the same dynamics may be upscaled to major seismicity in the same area, even though no clear evidence can be found in instrumental catalogues. We analyse seismic activity in various regions around the world (e.g., Italy, New Zealand, Türkiye) during the last decades and compare it with major historical events above the completeness of available catalogues. We consider the local and global coefficients of variation, the scaling exponent of the Gutenberg–Richter law (b-value), the fractal dimension of epicentral series, and the number of events. We find that the largest earthquakes tend to occur where even relatively short records show locally Poissonian and globally clustered seismicity (Zaccagnino et al., 2023). At last, we investigate the relationship between structural features and the distribution of recurrence times of large and moderate seismic events. Reference Zaccagnino, D., Telesca, L., & Doglioni, C. (2023). Global versus local clustering of seismicity: Implications with earthquake prediction. Chaos, Solitons & Fractals, 170, 113419.