Systematic variations in recurrence interval and moment of repeating aftershocks

The recurrence intervals for 194 repeating clusters on the Calaveras fault follow a power-law decay relation with elapsed time after the 1984 M6.2 Morgan Hill, California, mainshock. The decay rates of repeating aftershocks in the immediate vicinity of a high-slip patch that failed during the mainshock systematically exceed those that are farther away. The trend between relative moment and recurrence interval, which is a measure of the fault-healing rate, varies systematically with depth and changes from negative to positive value as the distance between the repeating aftershock and the mainshock slip patch increases. We speculate that high strain rate in the early postseismic period may cause transient embrittlement and strengthening of the deep repeating clusters in the areas adjacent to the mainshock slip patch, resulting in large moments that decrease with time as the strain rate diminishes. Our observations suggest that systematic behavior of repeating aftershocks reflect variations in the fault zone rheology.