Recognizing the waveform of a foreshock

Abstract

The 2011 Mw9.1 Tohoku, Japan, earthquake is the paradigmatic example of an earthquake anticipated by a significant foreshock activity, with a Mw7.3 earthquake occurred two days before, within about 10 km 1. Recent results 2 show that statistically relevant changes can be found in the magnitude distribution after the Mw7.3 foreshock but the discrimination between normal and foreshock activity still remains a scientific challenge 3. Here we show that the envelope of the ground velocity recorded after the Mw7.3 foreshock presents an atypical sawtooth profile very different from the one observed after other earthquakes 4. We interpret this profile as the consequence of the locked state of the mainshock fault which reduces the possibility of the foreshock to trigger its own aftershocks. We find a similar sawtooth profile after other Mw6+ foreshocks followed within 10 days by a larger earthquake, as in the case of the 2014 Mw8.1 Iquique, Chile, sequence. This observation allows us to define a level of concern, simply extracted from the first 45 minutes of the recording waveform, associated to the occurrence of a larger earthquake. A test of the method for 47 Mw6+ worldwide earthquakes gives precise warning in time and space after all the 10 earthquakes followed by a larger one with only 2 false alerts.

Testing of the Seismic Gap Hypothesis in a Model With Realistic Earthquake Statistics

“…More generally, the presented model can be used to validate patterns that, because of instrumental uncertainties, emerge less clearly from real world data. For instance, the model exhibits (Petrillo et al., 2020) a decrease in the b‐value of the GR law during pre‐mainshock seismicity which has been also proposed as a distinct feature of instrumental foreshocks (Gulia & Wiemer, 2019; E. Lippiello, Petrillo, & Godano, 2021). A better understanding of these patterns in the physical model can be fundamental to better test this hypothesis in instrumental data.…”

Distribution related to all samples and extreme events in the ETAS cluster

“…More precisely, in , , Vere-Jones and Zhuang (2008) and Luo and Zhuang (2016), the distribution of the largest event in the critical ETAS model is related to the magnitude distribution of foreshocks, which are defined as background events that have at least one offspring, direct or indirect, with a larger magnitude. Although the difference between mainshocks and foreshocks is still not rigorously defined and accepted in the literature Lippiello et al, 2020;, the derivation of the probability of foreshocks under a clustering model can be used as a test to find distinguishing features between these kinds of events. The distribution of foreshocks is in fact considered essential in the earthquake prediction , as it allows us to evaluate the probability of occurrence of the largest event in a sequence.…”