Patterns of tectonic fault interactions captured through geostatistical analysis of microearthquakes

Historical earthquakes are often strongly clustered in space and time. This clustering has been attributed to static stress triggering associated with tectonic fault interactions and/or to fluid migration. Discrimination between these two models requires detailed information on the timing, location and size of earthquakes. The Matata earthquake sequence in the Taupo Rift, New Zealand, provides a unique opportunity to chart spatial and temporal patterns of earthquakes along individual faults and their relations to other faults in the system over timescales of days to years. This is possible because there are 2563 accurately relocated (within <+/- 100 m) earthquakes (1 < Mw < 4.7) that ruptured the rift's crust during 49 months of earthquake activity. These earthquakes define subparallel faults which were active over discrete periods of time, with earthquake activity migrating initially along and subsequently across the rift. Although the process of microearthquake generation on individual faults is highly complex, we show by means of semivariogram analysis that system-wide correlations extend over at least eight successive earthquakes. The system-wide coherence requires the interaction of earthquakes located on neighboring faults over timescales that span tens of days, and it is achieved when about 76% of the total faults in the system are included. This pattern is consistent with tectonic (as opposed to fluid-triggered) fault interactions that have been established for larger magnitude (e.g., Mw > 5.5) earthquakes that occur over thousand year timescales (e.g., < 60 kyr).