A Major Earthquake in the Strong Crust: the 1999 Chi-chi Earthquake

The 1999 Chi-Chi earthquake (MW=7.6; ML=7.3) ruptured in an area considered unusual in that, historically, large earthquakes have been unheard of and the background seismicity are low. A detailed investigation of the crustal characteristics based on seismicity, subsurface structures and GPS surveys shows that crustal strength in the low-seismicity block was most likely stronger than in the surrounding area. The earthquake took place to release large energy that had been accumulating in a strong block with low-seismicity over a long period of time. Such a strong block is associated with a pre-Miocene basement high, Peikang High, which served as a tectonic dam or barrier during the Miocene. The occurrence of the 1999 Taiwan earthquake, like many other major earthquakes such as the 1857 Fort Tejon, the 1906 San Francisco and the 1923 Kanto earthquakes, suggests that major earthquakes (MW > 7.5) are more likely to take place in low-seismicity areas where the crust is stronger. On the other hand, moderately large earthquakes (MW < 7) are more inclined to occur in areas of high-seismicity.