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 Mw 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 (Mw 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.