Exploratory Statistical Analysis of Precursors to Moderate Earthquakes in Japan

Modern statistical techniques allow quantitative characterization of seismic activity. Analysis of the 2011 Tohoku megathrust earthquake revealed clear precursory signals: shortened inter-event intervals, increased magnitude scale (σ), and a pronounced precur-sory swarm immediately before the mainshock. While unique to this magnitude 9 event, here I present subtler anomalies may precede magnitude 7-class events, especially when swarms occur. In such cases, magnitude distributions often differ from background seis-micity, frequently showing elevated location (μ) and scale (σ). Conversely, σ was some-times reduced, particularly in volcanic regions, where large earthquakes may occur with-out discernible swarms. Detection of swarm activity and analysis of magnitude parame-ters thus remain central to seismic risk assessment. If swarm characteristics resemble background levels, the likelihood of a major event is presumably low. However, the dis-tinct, immediate precursory swarm observed before the Tohoku earthquake was not repli-cated elsewhere. These findings indicate that statistical anomalies may signal elevated risk but are unlikely to enable precise temporal prediction of seismic events.