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Understanding the physical mechanism of major earthquakes is very important for earthquake prediction and its disaster reduction. The mainstream hypotheses on earthquake mechanism are reviewed in this paper. It is pointed out that the elastic rebound and stick-slip hypotheses, both of which are usually used to explain the mechanism of shallow earthquakes, have some serious drawbacks, and that such hypotheses explaining the mechanism of intermediate and deep earthquakes as the dehydration embrittlement, phase transition instability, shear melting and anticrack-associated faulting, are inconsistent with some observation data. Thus, it is necessary to establish a new hypothesis or theory of earthquake mechanism. The brittle failure theory of multiple locked patches in a seismogenic fault system developed by us since 2010 is introduced in the present study. It is stated by the theory that the progressive failures of locked patch result in the occurrence of earthquakes due to the fault movement. Hereafter, the significant earthquakes occurred at its volume expansion and peak strength points are referred to as characteristic ones. The earthquake cases indicate that the seismogenic processes of shallow, intermediate and deep characteristic earthquakes can be well explained by the theory. We emphasize that both the elastic rebound and stick-slip hypotheses contain the same implicit assumption that there exist locked patches in the seismogenic faults, and that the source body of deep earthquakes is with appropriate environment conditions leading to brittle failures. Moreover, some controversial issues, such as seismic stress drop much less than that of rock failure in the laboratory test, heat flow paradox and Self-Organized Criticality (SOC), are discussed and can be reasonably explained by the theory. The present study shows that the physical mechanism of major earthquakes is attributed to the brittle failures of locked patches.

Each of the seismogenic locked segments in a well-defined seismic zone can accumulate high strain energy to bring about a major earthquake. Hence, better understanding the physical implication of self-organized criticality in the locked segment (rock) failure process is crucial to achieving insights into such issues as earthquake predictability and so on. We point out that there exist two critical points in the locked segment fracturing process. The first critical point is volume expansion point, which is the starting point of self-organization, at which a rupture event with high energy occurs. It can be regarded as the only identified precursor to macroscopic rupture of locked segment. The second critical point is the peak strength point, namely, the instability point, at which a major earthquake which is able to generate obvious surface rupture zones takes place. According to our previous research on the theoretical relationship of strain ratio between the two points as well as the constrained expressions concerning earthquake magnitudes and elastic strain energy, also known as the theory about the brittle failure of multiple locked segments in a seismogenic fault system, we can predict some characteristic earthquakes occurring at the first and the second critical point of locked segment, e.g., the 2004 Sumatra-Andaman MW9.0 earthquake in Indonesia, the 2008 Sichuan MS8.1 earthquake in China, and the 2011 Tōhoku MW9.0 earthquake in Japan. This was obtained by retrospectively analyzing the earthquake cases in 62 seismic zones covering the circum-Pacific seismic belt and the Eurasia seismic belt. The present results show that the self-organized process before the locked segment (rock) instability must arise due to its heterogeneity; there exists a causal link between the self-organization and criticality; it is possible to predict some large earthquakes (e.g. characteristic earthquakes) just because of the existence of self-organized process. We emphasize here that the damage process between the two critical points is not transient behavior, usually a long-term process; the evolution of characteristic earthquakes follows a deterministic rule; there is no probability with which small earthquakes can cascade into a large event (e.g. characteristic earthquakes). In summary, this study can help to comprehend the evolutionary mechanism of characteristic earthquakes, provide a physical basis of understanding the generation process of earthquakes, and clarify such issues as the identification of earthquake types and predictability of earthquakes.