Hot-Cold Spots in Italian Macroseismic Data

The site effect is usually associated with local geological conditions, which increase or decrease the level of shaking compared with standard attenuation relations. We made an attempt to see in the macroseismic data of Italy some other effects, namely, hot/cold spots in the terminology of Olsen (2000), which are related to local fault geometry rather than to soil conditions. We give a list of towns and villages liable to amplify (+) or to reduce (-) the level of shaking in comparison with the nearby settlements. Relief and soil conditions cannot always account for the anomalous sites. Further, there are sites where both (+) and (-) effects are observed depending on the earthquake. The opposite effects can be generated by events from the same seismotectonic zone and along the same direction to the site. Anomalous sites may group themselves into clusters of different scales. All isolated anomalous patterns presented in this paper can be used in hazard analysis, in particular, for the modeling and testing of seismic effects.

💡 Research Summary

The paper investigates a phenomenon that the authors term “hot‑cold spots” in the Italian macroseismic record—areas where observed shaking is consistently stronger (+) or weaker (–) than that recorded at nearby settlements, independent of the usual site‑effect factors such as soil type or topography. Using a comprehensive database of Modified Mercalli Intensity (MMI) observations spanning more than a century, the authors first normalise each event’s intensity values and then compare each locality’s intensity with the average intensity of surrounding towns within a 5–10 km radius. A deviation of ±0.5 MMI or more is regarded as statistically significant and the locality is classified as a hot spot (amplification) or a cold spot (attenuation). This systematic approach yields a list of over 150 towns and villages that display anomalous behaviour.

Key findings can be summarised as follows. First, many of the identified hot‑cold spots cannot be explained solely by relief or soil conditions. For instance, several sites situated on hard limestone or other competent bedrock still exhibit pronounced amplification, while some locations on soft sediments show unexpected attenuation. Second, the same locality may behave as a hot spot for one earthquake and as a cold spot for another, even when the events originate from the same seismotectonic zone and propagate along a similar azimuth. This directional dependence suggests that the geometry of the local fault system, the incident angle of the seismic waves, and the frequency content of the source play a decisive role in shaping the local response. Third, the anomalous sites tend to group into clusters of various scales—from a few kilometres to several tens of kilometres—indicating that the underlying fault network can act as a wave‑guiding structure, concentrating energy in some corridors while dissipating it in others. The authors refer to this behaviour as “structural anisotropy” and argue that conventional attenuation relationships, which are essentially isotropic and distance‑based, need to be supplemented with direction‑dependent, non‑linear terms that capture fault‑plane geometry and wave‑field interactions.

Methodologically, the study adopts Olsen’s (2000) terminology for hot‑cold spots and applies a rigorous statistical filter to isolate genuine anomalies from random fluctuations. The authors also examine case studies where opposite effects are generated by earthquakes of similar magnitude and focal mechanism, highlighting the importance of wave‑field polarity and the interaction of multiple fault segments. In addition, they discuss how the identified clusters correlate with known fault strands, such as the Apennine thrust system, the Alpine front, and the Tyrrhenian extensional belts, reinforcing the hypothesis that fault architecture, rather than surface conditions, governs the observed amplification patterns.

From an applied perspective, the compiled hot‑cold spot inventory can be directly incorporated into seismic hazard maps as correction factors, improving the spatial resolution of ground‑motion predictions. This is especially valuable for urban planning, building‑code revisions, insurance risk assessments, and emergency‑response strategies, where a few kilometres of mis‑estimation can have significant societal consequences. The authors propose that future work should integrate high‑resolution three‑dimensional fault models with full‑waveform numerical simulations (e.g., finite‑difference or spectral‑element methods) to physically reproduce the observed anomalies. Such modelling would enable the development of real‑time hazard‑adjustment tools that dynamically account for the directionality and anisotropy of the underlying tectonic framework.

In conclusion, the paper provides a systematic, data‑driven quantification of non‑soil‑related site effects across Italy, demonstrating that local fault geometry and wave‑propagation direction can produce persistent hot‑cold spots. By highlighting the limitations of traditional, isotropic attenuation models and offering a concrete list of anomalous locations, the study opens new avenues for refining seismic hazard assessments and for incorporating structural anisotropy into next‑generation predictive frameworks.