A High Frequency View of 1999 Chi-Chi, Taiwan, Source Rupture and Fault Mechanics

High-frequency band-pass filtering of broadband strong-motion seismograms recorded immediately adjacent to the fault plane of the 1999 Chi-Chi, Taiwan earthquake reveals a sequence of distinct bursts, each of which can be considered as a sub-event from an asperity source of the Chi-Chi mainshock. These bursts collectively make up the entire mainshock accelerogram. Each burst may have released a significant portion of the total energy release of the Chi-Chi mainshock. Many of these bursts contain quasi-periodic sub-bursts with periods on the order of a few tenths of a second. Most bursts occur well behind the propagating rupture front. Detailed pictures of these asperity sources do not appear in conventional slip-map studies, presumably because of the low-pass filtering used in these waveform inversions. We directly used the high-frequency data to determine the origin times, locations and magnitudes of these sub-events. The first asperities to rupture in a given location follow the Chelungpu rupture propagation history at a velocity of about 2.0 km/s. Later asperity events at a given location can be interpreted as aftershocks that begin before the Chi-Chi rupture has terminated. Spatially these asperity sources appear in groups, most of which are located at shallow depth along the Chelungpu surface rupture and are consistent with the large asperities presented in source inversion studies. Asperities located at great depth suggest a non-planer rupture surface with dip increasing to the east. The frequency-magnitude distribution of these sub-events has b-value equal to 1.0. In space, the larger sub-events are located at greater depth, while the small sub-events are only located at shallower depths.