Simulated Seismicity as a Tool for Studying the Long-Term Seismogenic Process: An Italy–Japan Comparison

In this study, we aimed to assess the capacity of a physics-based earthquake simulator to
improve our understanding of the seismogenic process. In this respect, we applied a previously tested
earthquake simulator to two well-known and completely different seismogenic fault systems, namely
the Italian Apennines and the Nankai subduction in Japan, for which long historical records of strong
earthquakes are available. They are characterized by different fault mechanisms, fault sizes, and
slip rates. Because of the difference in slip rates, the time scale of the seismicity patterns is different
for the two systems (several hundreds of years for the Apennines and a few tens of years for the
Nankai Fault). The results of simulations that produced synthetic catalogues of 100,000 years show
these significant long-term seismicity patterns characterizing the seismic cycles for both seismogenic
areas as follows: The average stress and the occurrence rate of earthquakes increase in the long
term as the next major earthquake approaches; while the average stress increases uniformly, the
occurrence rate stops increasing well in advance of the mainshocks; the b-value exhibits a long-term
increase before major earthquakes and a fast decrease shortly before the mainshocks. Even if no
specific statistical tool was applied for the quantification of the similarities between the seismicity
patterns of the two seismic areas, such similarities are clearly justified by the large number of seismic
cycles included in the 100,000-year synthetic catalogues. The paper includes a discussion on the
capability of the simulation algorithm to reliably represent the real long-term seismogenic process.
This question is difficult to answer because the available historical observations are of too short a
duration to provide significant statistical results. In spite of the limitations characterizing the use of
earthquake simulators for time-dependent earthquake hazard assessment, and the lack of convincing
mechanistic explanations of the specific seismic patterns reproduced by our simulator algorithm, our
results encourage further investigations into the application of simulators for the development of
seismogenic models, including short-term features.