Similar seismic moment release process for shallow and deep earthquakes

The generation of earthquakes at depths exceeding 60 km remains debated, as rocks at such depths are anticipated to be ductile. Seismological investigations have revealed a variety of rupture characteristics that are distinguishable between shallow (0-60 km), intermediate-depth (60-300 km) and deep-focus (300-700 km) earthquakes, but it is unclear whether different physical mechanisms are controlling earthquake ruptures. Here, we apply machine learning classification to a global database of earthquakes with moderate to large moment magnitudes to show that depth-dependent elastic properties can explain the range of resulting rupture characteristics. We find that the rigidity of the surrounding medium is the primary control of the earthquakes' source characteristics and that all the analysed earthquakes shared similar moment release processes with the medium effect corrected. Thus, the rupture duration, rupture length and the associated drop in stress scale with depth due to the accompanying changes in rigidity. Our results support a constant strain drop hypothesis, in which the ratio of coseismic slip to the characteristic rupture length remains largely unchanged for earthquakes at all depths and regardless of the nucleation mechanisms. These results also suggest that medium-rigidity-corrected earthquake self-similarity holds for earthquakes of different depths and host-rock types. Differences in shallow- and deep-earthquake characteristics can be explained by Earth's depth-dependent rigidity instead of different rupture processes, according to machine learning classification of moderate to large earthquakes.