Near-Source Recordings of Small and Large Earthquakes: Magnitude Predictability only for Medium and Small Events

Abstract:

The feasibility of Earthquake Early Warning (EEW) applications has revived the discussion on whether earthquake rupture development follows deterministic principles or not. If it does, it may be possible to predict final earthquake magnitudes while the rupture is still developing. EEW magnitude estimation schemes, most of which are based on 3-4 seconds of near-source p-wave data, have been shown to work well for small to moderate size earthquakes. In this magnitude range, the used time window is larger than the source durations of the events. Whether the magnitude estimation schemes also work for events in which the source duration exceeds the estimation time window, however, remains debated.

In our study we have compiled an extensive high-quality data set of near-source seismic recordings. We search for waveform features that could be diagnostic of final event magnitudes in a predictive sense. We find that the onsets of large (M7+) events are statistically indistinguishable from those of medium sized events (M5.5-M7). Significant differences arise only once the medium size events terminate. This observation suggests that EEW relevant magnitude estimates are largely observational, rather than predictive, and that whether a medium size event becomes a large one is not determined at the rupture onset. As a consequence, early magnitude estimates for large events are minimum estimates, a fact that has to be taken into account in EEW alert messaging and response design.