We have investigated the causal relationship between two recent Taiwan events, the 1998 M 6.2 Ruey-Li and the 1999 M 7.6 Chi-Chi earthquakes. It is found that both earthquakes are generated by ruptures essentially along nearby sections of a large through-going fault zone along the Western Foothills of Taiwan. The rapid termination (in 10 days) of the M 6.2 Ruey-Li sequence is interpreted by the presence of an asperity in terms of the Peikang basement high, which dominates the tectonic deformation in central western Taiwan. We have carried out the relocation of the aftershocks, their fault-plane solutions, plus stress inversion, waveform source slip inversion, Coulomb stress change calculations, and regional strain energy calculations for the Ruey-Li sequence, so as to examine its causal relationship from different angles. We have concluded that as both events are results of the same regional stress accumulation due to the plate convergence, the Ruey-Li earthquake sequence would likely have escalated into a much larger rupture including the Chelungpu fault had it not been for the presence of the Peikang basement high.

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