Spatio-temporal clustering of successive earthquakes: analysis of a global CMT catalog.

Spatio-temporal clustering of seismicity features is an interesting phenomenon that is relevant for earthquake generation process and operational earthquake forecasting. We analyze successive earthquakes that closely occur in space and time in order to clarify how large earthquakes successively occur. We use the Global Centroid Moment Tensor catalog for the period from 1976 to 2016. Shallow earthquakes with a moment magnitude, Mw , of larger than or equal to 5.0 are analyzed. We first sort all of the earthquakes in time to select a master event from the beginning. Then, we group the earthquakes that occur within a horizontal distance ( D ) and a lapse time ( Ta ) from the master event into a cluster. Next master event is selected from the catalog in order, and the same procedure is repeated. We count the number of the clusters, which represent the successive earthquakes, for different D and Ta  To examine whether or not successive earthquakes randomly occur, we compare the results with simulations in which earthquakes are set to occur randomly in time but at the locations same with the estimated centroid. The results show that the cumulative numbers of clusters for the simulation more rapidly increase with the horizontal distance than those for real data at short distance ranges, and the formers approach to the latter at long distance range. The triggering distance, at which the cumulative numbers of real and simulation data merge, increases with increasing the magnitude of master event. The triggering distance becomes smaller as the lapse time increases, which implies that the seismic activity turns to become the normal condition in which the occurrence time intervals of large earthquakes obey a Poisson distribution. The triggering distance increases with being almost proportional to the 1/3 of the seismic moment of master earthquake, and the number of earthquakes occurring in the region with positive Coulomb stress change (ΔCFF) are more than 60-80% of the total number of the successive earthquakes. These results suggest that static stress change introduced by a master event is one of the triggering mechanism of successive earthquakes.

Table 1. Successive earthquakes within triggering distance.

Magnitude	Total number of earthquakes in the catalog	Number of successive earthquakes (%)
		days	days	days
5.0 ≤ Mw <5.5	25299	37.9 %	44.2 %	37.5 %
5.5 ≤ Mw <6.0	9186	23.3 %	26.6 %	23.4 %
6.0 ≤ Mw <6.5	2753	18.2 %	18.9 %	16.3 %
6.5 ≤ Mw <7	931	9.6 %	12.7 %	14.9 %
Mw ≥ 7	419	4.1 %	7.6 %	5.5 %