Earthquake Repeat Time and Average Stress Drop

Existing data on source parameters of large crustal earthquakes (subduction events are not considered here) over a wide range of repeat imes indicate that, for a given magnitude (Ms or Mw), earthquakes with long repeat times have shorter fault lengths than those with short repeat imes. A shorter fault length for a given magnitude indicates a larger average stress drop which reflects the average strength of the fault zone. Our result therefore suggests that faults with longer repeat times are stronger than those with shorter repeat times. In terms of an asperity model in which the average strength of a fault zone is determined by the ratio, ra, of the total area of the asperities (strong spots on a fault plane) to the total area of the fault zone, the above result suggests hat ra is proportional to the repeat time. Our result provides a method to estimate seismic source spectra from the fault length and the repeat time of a potential causative fault.