A new approach to monitoring stochastic features of earthquake series
was applied to track the preparatory process of the 2017 Tehuantepec,
Mexico, Mw8.4 earthquake. The seismicity was parameterized by elapsed
times and epicentral distances from the directly preceding earthquakes
and by earthquake magnitudes. The transformation to equivalent
dimensions ensured the comparability of these parameters. We were
calculating the average distance between earthquakes in the transformed
parameter space in moving in time data windows, each consisting of 100
events. The average distance exhibited a consistent upward trend from
ten to two years before the main shock. Then, it declined until the main
shock. This precursory up-down signal was highly significant
statistically. We showed that the detected time changes of the average
distance resulted from the evolution of the earthquake clustering in the
considered parameters' equivalent dimensions space.