The precursor phenomena of the 1999 Mw 7.6 chi-chi, Taiwan earthquake: The seismicity aspect

The Chi-Chi earthquake with Mw 7.6 struck central Taiwan on 21 September 1999 (or at UTC 17:47 20 September), and was the largest inland earthquake in Taiwan in the 20th century. A set of precursor phenomena related to the Chi-Chi earthquake, viz., the seismic activation, seismic quiescence and seismic reversal have been reviewed in this chapter. By means of the region-time-length (RTL) and the pattern informatics (PI) algorithms, the epicenter of the Chi-Chi main shock was found to exhibit signatures of anomalous activity related to the seismic activation and quiescence in the Taiwan region over a time span of about 6 years before the main shock. Also, a map showing the standard normal deviate Z-value for the variations in seismicity in the Taiwan region strikingly reveals the seismic reversal pattern associated with the Chi-Chi earthquake. The self-organizing spinodal model could explain observations concerning these precursory phenomena in seismicity. In the critical point theory for earthquakes, main shocks, their foreshocks and aftershocks are all associated with the formation of a correlated, cooperative spatial region with high stress, which is embedded in a characteristic earthquake cycle.