Recently, Professor Didier Sornette, Co-Dean of the Institute of Risk Analysis, Prediction, and Management (Risks-X) of the Southern University of Science and Technology (SUSTech) Academy for Advanced Interdisciplinary Studies and his group published an article, entitled “Is Seismicity Operating at a Critical Point?” in the top international journal, Physical Review Letters. The paper was selected as one of the highlighted articles.

Figure 1. Spatial density of the earthquakes identified as background events by the and models for the global catalog.

Seismicity and faulting within the Earth’s crust are characterized by many scaling laws that are usually interpreted as qualifying the existence of underlying physical mechanisms associated with some kind of criticality in the sense of phase transitions. Using an augmented epidemic-type aftershock sequence (ETAS) model that accounts for the spatial variability of the background rates μ (x, y), they present a direct quantitative test of criticality. They calibrate the model to the ANSS catalog of the entire globe, the region around California, and the Geonet catalog for the region around New Zealand using an extended expectation-maximization (EM) algorithm including the determination of μ (x, y).

They demonstrate that the criticality reported in previous studies is spurious and can be attributed to a systematic upward bias in the calibration of the branching ratio of the ETAS model, when not accounting correctly for spatial variability. They validate the version of the ETAS model that possesses a space varying background rate μ (x, y) by performing pseudoprospective forecasting tests. The non-criticality of seismicity has major implications for the prediction of large events.

Shyam Nandan from ETH Zurich is the first author of this paper, while Professor Didier Sornette of Risks-X of the SUSTech Academy for Advanced Interdisciplinary Studies is the corresponding author.

Article link: https://doi.org/10.1103/PhysRevLett.126.128501