Viscoelastic relaxation of asthenosphere can provide a reasonable ex planation for aftershocks' expansion of the May 1986 Hualien earthquake sequence. The relaxation time of asthenosphere in the Hualien region is very short which is determined by matching the aftershock seismicity and computed results. Such short relaxation time of asthenosphere explains the relatively short reoccurrence time (20-30 years) of magnitude 7 and larger earthquakes in the Hualien region. Besides this aftershock expansion phe nomena of the May 1986 earthquake, there are several major earthquakes drifting southeasterly in this region. The drifting direction of these earth quakes is opposite to the direction of plate convergence in East Taiwan. Stress redistribution after a major event plus tectonic force might be ad equate to account for phenomenon of earthquake migrating in the Hualien area of Taiwan.

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