Satellite pictures released yesterday by the Council of Agriculture indicate there are more than 1,800 locations in central Taiwan affected by mountain displacements, landslides and other geological disruptions, producing dramatic and permanent changes to a total area of over 7,000 hectares, roughly 0.2 percent of the island's area.
But many people have been asking whether the earthquake centered under Chichi was the "big one" or whether the worst is yet to come.
According to seismologists, that is a hard question to answer.
"It will take some time for (seismic) energy to build up in the areas already hit, so it is unlikely there will be a bigger one there. However, there are other faults located in the vicinity, so we still don't know," said Tsai Yi-ben (蔡義本), professor at the College of Earth Science under National Central University (國立中央大學).
Tsai made the comments yesterday during a briefing on "Earthquakes in the Taiwan Area," organized by the Government Information Office and the Central Weather Bureau.
The reason why Taiwan has so many earthquakes, Tsai said, is that the island is located in an active earthquake zone.
"The eastern side of the island is situated on the Philippine Sea Plate, while the Central Mountain zone and western Taiwan is located on the Eurasian Plate. When these two plates collide, earthquakes happen," Tsai said.
He said the Philippine Sea Plate moves northwest about seven to eight centimeters per year, and the impact as it crosses a fault causes resistance which reduces its momentum by half. He added that it usually takes a long time for the faults to build up enough energy for a jolt the size Taiwan experienced last month.
The two plates have created the unique geological formation of Taiwan, with the Central Mountain Range rising high up in the center of the island, and with lower mountains and hills surrounding it. The 921 earthquake, however, has changed all that.
The quake at Chichi took place along the Chelungpu (車龍埔) fault. It ruptured the earth's surface for over 80 kilometers, roughly from north to south, from Shihgang, Taichung County, to Tungtou in Nantou.
The fault, say seismologists, dips eastward at a 30-degree angle. The quake created a thrust westward and upward consistently and continuously along the whole length, Tsai said.
The amount of uplift along the fault, that is, the amount the earth raised, ranged from one meter to two meters in the south, to seven meters to eight meters in the north. These movements were accompanied by simultaneous left lateral slips of similar lengths at many locations.
The rupture front was recorded to move north and south at a speed of about 2.5 kilometers per second, causing powerful waves at the north end, said Tsai.
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