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The First World Catalog of Earthquake‐Rotated Objects (EROs), Part II: Statistical Analyses and Related Interpretations

The article presents the results of the quantitative statistical analyses of the first world catalog of earthquake‐rotated objects (EROs), presented in Part I of the study (Cucci et al. , 2016). We searched for possible relations between the epicentral distance of EROs occurrence and a number of customary seismological observables, such as magnitude, intensity, focal mechanism, etc. The reliability of results is quantitatively checked by means of some suitable statistical tests. We found strong evidence of a clear log–linear dependence of the epicentral distance, to which an ERO can be observed, on the magnitude M w of the source event. We note that the probability of observing EROs near the epicentral area ( D <10  km) inversely decreases with the earthquake magnitude and that, for large earthquakes ( M w 8+), this probability remains significant (around 30%) beyond 100 km from the epicenter. Unexpectedly, we did not find significant relations between EROs occurrence and epicentral intensity, possibly because of high dispersion of intensity values.

The data analyzed in the present study identifies intensity 6 as the lowest intensity for rotation occurrence; this is different from the main macroseismic scales, which indicate the EROs as a diagnostic of larger intensity degrees. This outcome could suggest the need for revision and for updating the diagnostics indicated in the intensity scales. As for the focal mechanisms of the seismic events, we found higher probabilities of observing rotations beyond 10 km distance from the epicenter of a thrust‐faulting earthquake than for a normal‐faulting earthquake. This probability reverses beyond 50 km distance, especially for high‐magnitude seismic events. Our results indicate interesting insights to potential end users of the EROs catalog in the fields of historical seismology and earthquake engineering.