SYMPTOMS OF INSTABILITY IN A SYSTEM OF EARTHQUAKE-PRONE FAULTS

The major features of the dynamics of seismicity are summarized in ord er to provide a phenomenological background for its theoretical modeli ng. The earthquake-generating part of-the solid Earth presents a hiera rchical nonlinear dissipative system. It consists of a hierarchy of bl ocks, separated by fractured boundary zones where, due to the relative movement of the blocks, earthquakes are generated. This system shows partial self-similarity, fractality, and self-organisation; it remains in a subcritical state even after a large discharge of energy and it probably has an intermediate number of degrees of freedom. The approac h of a strong earthquake is reflected in the transient characteristics of earthquake flow; their scaling is indicated. Phenomenology and num erical modeling of these characteristics suggest that a wide variety o f the systems of interacting elements share the following hypothetical symptoms of approaching critical transition: the response of the syst em to excitation increases and the background activity (static) of the system becomes more clustered, intense, irregular and synergetic.