Static Stress Transfer and Aftershock Triggering by the 1999 Chi-Chi Earthquake in Taiwan

The static stress changes caused by the Chi-Chi mainshock with M L =7.3 were analyzed with an elastic dislocation model in a homogenous half-space. The results show that most aftershocks in the fold-and-thrust belt might best be interpreted as the re-activation of pre-existing thrust faults triggered by the mainshock. Strike-slip motions near the terminations of the Chelungpu fault are also likely to have been enhanced by the static stress transfer. The Chukou fault, on the other hand, fell in a stress shadow; as a result, few aftershocks occurred there. The Chia-Yi earthquake sequence which occurred a month after the Chi-Chi earthquake turns out to have been an exception in this study. No evidence of static stress enhancement was found in that area. Unless the large aftershocks which followed the Chi-Chi earthquake altered the stress field and promoted failure, the time lag of the Chia- Yi earthquake might well be attributed to the influence of a stress shadow.