Abstract
Possible short-term precursors several days before the Taiwan Chi-Chi earthquake (M(subscript w) = 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.
Highlights
Monitoring of crustal deformation has for many years been considered one of the most promising methods for detecting earthquake precursors
Detailed examination of tidal crustal deformation from broadband seismic data in Taiwan shows that some anomalous crustal deformation occurred and was detected at two stations in central Taiwan several days before the 1999 Taiwan Chi-Chi earthquake
Delay in the detecting of anomalous crustal deformation between Stations SSLB and NACB indicates that the mechanism generating those anomalous crustal deformations is consistent in its temporary and spatial migration
Summary
Monitoring of crustal deformation has for many years been considered one of the most promising methods for detecting earthquake precursors. In addition to traditional observations such as strainmeters and water wells, broadband seismic stations might provide another opportunity to observe possible earthquake precursors from Earth’s tidal deformation (i.e., Houlié and Montagner 2007). Some irregular Earth tidal pulses were obtained at two broadband seismic stations nearby the epicenter of the mainshock several days before the 1999 Taiwan Chi-Chi earthquake. These anomalous crustal deformations might be interpreted as a sequence of small slow slip events nearby the epicenter, and is worth consideration as a possible earthquake precursor of the 1999 Chi-Chi earthquake
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