We have developed a new method for the simultaneous determination of the earthquake source parameters: seismic moment, focal mechanism, rupture length, and rupture velocity, using far-field P-waves. The pseudo radiation patterns of P-waves are obtained from each station, accomplished through a grid search of the source time function and simple inversion. The observed waveforms are corrected for instrument response, geometrical spreading and surface effects. These corrections are made for the corresponding ray parameter of each station based on IASP91 velocity model. To obtain the best solution for the focal mechanism, pseudo radiation patterns are compared with the theoretical radiation patterns calculated for the given focal mechanisms obtained through the grid search. The source time functions obtained from all available stations for an earthquake are used to estimate the rupture length and rupture velocity based on rupture directivity. We have applied this method to three moderate-to large-sized global earthquakes. The results agree well with those of previous studies, suggesting that this method is applicable for seismic hazard assessment. A moderate Taiwan earthquake is analyzed to demonstrate that the method can indeed be used to rapidly determine the source parameters of earthquakes in Taiwan.

Abstract. Unusual decreases of water levels were consistently observed in 78 % (= 42/54) of wells in the Choshuichi Alluvial Fan of central Taiwan about 250 days before the Chi-Chi earthquake (M = 7.6 on 20 September 1999) while possible factors of barometric pressure, earth tides, precipitation as well as artificial pumping were removed. Variations in groundwater levels measured on anomalous wells from 1 August 1997 to 19 September 1999, which covers the 250 day unusual decreases, were transferred into the frequency domain to unveil frequency characteristics associated with the Chi-Chi earthquake. Analytical results show that amplitudes at the frequency band between 0.02 day −1 and 0.04 day −1 generally maintained at the low stage and were apparently enhanced a few weeks before the Chi-Chi earthquake. Variations of amplitude at this particular frequency band were further examined along with other Taiwan earthquakes (M > 6) from 1 August 1997 to 31 December 2009. Features of the enhanced amplitudes at the frequency band are consistently observed prior to the other two earthquakes (the Rei-Li and Ming-Jian earthquakes) during the 12.5 yr study period. In addition, surface displacements recorded from GPS, which provides insights into understanding stress status in subsurface during the Chi-Chi earthquake, are also inspected. The result confirms that abnormal rise and fall changes in groundwater level yield an agreement with forward and backward surface displacements around the epicentre prior to the Chi-Chi earthquake.

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We systematically investigated precursory seismic patterns using the pattern informatics (PI) method and suggest an operable procedure for making PI maps for all seasons, in the context of earthquake forecasting. We examined the PI patterns before several inland earthquakes with magnitudes larger than 6, which occurred between 2001 and 2010 in Taiwan. We fixed a cutoff magnitude and a change interval, which is the time span used to calculate the seismicity change. Our results show that locations with high PI anomalies are typically associated with large earthquakes when the cutoff magnitude is 3.2 and the change interval is 4 years. Therefore, the PI method can be utilized as a routine forecasting tool with regular updates, such performing the PI calculation every season. We also conducted random tests, the results of which indicate a significant difference between large events and random, hypothetical events.

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