Repeated foreshock sequences in the thrust faulting environment of eastern Taiwan

[1] Foreshock sequences have repeatedly been observed in the thrust faulting system of the Chengkung area in eastern Taiwan. Examinations of earthquakes since 1992 in this area show all six of the larger earthquakes (ML R 5) had foreshocks at a distance of only a few kilometres within just a few days before the main shocks. The largest earthquake (ML = 6.5) on December 10, 2003 was preceded by two significant foreshocks about four days and 6 minutes respectively prior to the main shock. This earthquake sequence shows the tectonic boundary can most probably be described as a variable-dip reverse fault. The second largest earthquake occurred on May 28, 1992 (ML = 5.4) was similar with two clear foreshocks. Repeated foreshock sequences in this reverse-faulting environment are primarily associated with a higher degree of small-scale heterogeneity in the crust and might very well be considered as potential predictors of large earthquakes in the future.