The Taiwan Chelungpu‐fault Drilling Project penetrated three fault zones as the Chelungpu fault system, which slipped during the 1999 Chi‐Chi earthquake, discovering disk‐shaped black material (BM disk) within the middle and lower fault zones in Hole B. The microscopic features of the BM disks indicated that they were pseudotachylytes, and they showed high magnetic susceptibility, possibly the result of intense shearing or high temperature conditions. Inorganic carbon content of the BM disks was low, possibly because of thermal decomposition of carbonate minerals. The high temperatures might be related to frictional heating during the earthquake, implying that the BM disks were produced under intense shearing with frictional heating that reached melting temperature. Because the disks, which provide the only evidence of melting, pre‐date the 1999 earthquake, we concluded that frictional melting did not occur during the earthquake.

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