Geophysical Journal | 2001 volume 23 3

Patterns of Western Taiwan's Worst Earthquakes: 1711-1999

© Lewis Charles

he western half of Taiwan has experienced at least twenty - four moderate - major earthquakes in the past 289 years, including the devastating Chi-Chi quake of September 21, 1999 that resulted in more than 2,300 human casualties and more than 10,000 injuries. These «worst» quakes, ranging in magnitude from approximately 6.0 to 7.6 Mw, have collectively caused more than ten thousand deaths and billions of dollars of economic losses. From 1711-1999, these twenty - four shocks have reverberated from south to north and north to south in somewhat limited regions associated with up-thrusts and strike slip faults, both of which are linked to the convergence between the Philippine Plate and the Eurasia Plate. About half of the twenty - four quakes are associated with blind (concealed) faults like the Chelungpu fault, which was responsible for the Chi-Chi quake. Geographically, these twenty-four large quakes cluster into groups, with seismicity (number of earthquakes) increasing southward. In terms of the large quakes in this study, the two aseismic zones between the groups of quakes might partially result either from the presence of the Kuanyin and Peikang structural highs, which are basement uplifts with tectonic influences that extend to the western coastal areas of Taiwan, or from prior convergent history. The average recurrence rate of western Taiwan earthquakes >6.0 in magnitude for the period 1711 to 1999 is 12 years, whereas, the average recurrence rate of those quakes with magnitudes >7.0 for the same period is 50 years. The highest magnitude (>7.5) quake zone in western Taiwan is in the central part. Overall frequency of occurrence of these past earthquakes indicates that western Taiwan might be entering a phase of increased earthquake activity in which quakes greater than magnitude 6.0 will occur about every 2-10 years for the next 25-50 years.

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