Abstract Within the hour immediately after the 1999 Chi-Chi mainshock, only

40 aftershocks were located by the Taiwan Central Weather Bureau Seismic Network (CWBSN) because of the power outage in more than half of the island and the limited dynamic range of the CWBSN high-gain instruments. Here, by analyzing 20 near-field, on-scale records from the Taiwan Strong-Motion Instrumentation Program (TSMIP), we determined a catalog of 296 aftershocks with ML 3.4 within the first hour after the Chi-Chi mainshock. Focal mechanisms were also determined for 24 of these aftershocks. The frequency-magnitude relation obtained from the 296 after-shocks indicates that the catalog is complete above magnitude ML 4.3. Most of the aftershocks occurred in a small-slip region of the mainshock immediately to the north of the mainshock epicenter. Spatially, the aftershocks appear to migrate downward from the mainshock hypocenter. Later in the hour, the aftershocks began to concen-trate in the fringe area of the main rupture. During the first hour, the b-value as defined by events in the 4.3 ML 6 magnitude range is about 1, the value of the background seismicity, and the frequency of larger events is higher