The Possible Observation of Slow Slip Events Prior to the Occurrence of the 1999 Chi-Chi Earthquake


Possible short-term precursors several days before the Taiwan Chi-Chi earthquake (Mw = 7.6) on 20 September 1999 were observed by examining crustal deformation that were directly integrated from broadband velocity seismograms. Significant deviations of the vertical displacement from a normal Earth tidal pattern on 15 - 19 September show some tiny surface crustal deformation having taken place several days in advance of the earthquake on 20 September. A series of slow slip events on the nearly horizontal plane (or decollement) at depths between 10 and 12 km provide a possible explanation for generating the anomalous crustal deformations before the earthquake. Although those slow slip events are not well constrained owing to limited observations at only two broadband seismic stations, their possibility is acceptable from a geological standpoint if the decollement beneath central Taiwan can be evidenced from both geological and geodetic studies. However, no matter what the exact mechanism was which generated the irregular earth tidal deformation several days before the 1999 Taiwan Chi-Chi earthquake, these anomalous crustal deformations might be considered to be possible short-term earthquake precursors.

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