Aftershocks of the 1999 Chi-Chi, Taiwan, earthquake: The first hour

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 nearfield, on-scale records from the Taiwan Strong-Motion Instrumentation Program (TSMIP), we determined a catalog of 296 aftershocks with M-L >= 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 aftershocks indicates that the catalog is complete above magnitude M-L 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 concentrate in the fringe area of the main rupture. During the first hour, the b-value as defined by events in the 4.3 < M-L < 6 magnitude range is about 1, the value of the background seismicity, and the frequency of larger events is higher.