Power Law Random Behaviour and Deterministic Earthquake Seasonality of Northeast India

The Northeast Indian region (NEI) is characterised by frequent earthquakes associated with swarms and quiescence activities. To gain physical insight about its origin, inter-event time series of earthquakes M≥3 and 5 has been analysed using rank-order statistics and non-linear perspective. The rank-order statistics of inter-event time series intlicates "mixed response" with distinct breaks in slope suggesting unstable, quasi-deterministic and random recurrence time that is sensitive to earthquake dynamics. The break in "slope" possibly suggests that the long term (of recurrence time > 400 days) earthquake dynamics in this region is unstable. The middle segment representing the time range of approximately 30-400 days can be described by Power Law function indicating the possible element of determinism on this time scale. Lastly, the existence of "roll off" towards higher rank-order, however, seerns to indicate the transition in the nature of structure. In order to constrain these results, we further compute the second order Kolmogorov-Sinai entropy (K₂), which reveals a coherent time structure with predictive time limit of about 12 to 16 months, indicating a quasi-deterministic dynamics that apparently is also evident (Power Law) in rank-order statistics. This predictive time limit seems to be too short to be associated with major global tectonics. We therefore, propose an alternative possible physical mechanism, which may coexist with regional tectonics. In the light of present results, we suggest that surface/subsurface water infiltration into the main fractures and faults during a favourable season could increase the pore pressure thereby triggering frequent earthquakes, particularly in critically unstable tectonic regions.