|講 題||地層欲斷從震起 – 談台灣的斷層與地震|
|講 者||張中白 國立中央大學地球科學系教授|
|日 期||2013/11/21||長 度||00:33:29||人 氣||1,203 次|
|An earthquake is the vibration of Earth produced by the rapid release of energy. And most often earthquakes are caused by slippage along a fault in Earth’s crust. |
The tremendous energy released by atomic explosions or by eruptions can produce an earthquake, but these events are relatively weak and infrequent. What mechanism produces a destructive earthquake? Ample evidence exists that Earth is not a static planet. We know that Earth’s crust has been uplifted at times, because we have found numerous ancient wave-cut benches many meters above the level of the highest tides. Others regions exhibit evidence of extensive subsidence. In addition to these vertical displacements, offsets in fence lines, roads, and other structures indicate that horizontal movement is common. These movements are associated with large fractures in Earth’s crust, called faults.
The earthquake focus is the point where an earthquake starts to release the elastic strain of surrounding rock. And fault slippage also begins at the focus and spreads across a fault surface in the rupture front. Although no part of Earth’s surface is exempt from faults and earthquakes, plate boundaries are most subject to frequent earthquakes. And subduction zones tend to have the largest quakes. The circum-Pacific belt, where about 80 percent of all recorded earthquakes originate, follows the subduction zones on the Pacific coast of the Americas from Cape Horn to Alaska, and crosses to Asia where it extends southward down the coast, through Japan, Taiwan, the Philippines, New Guinea, and Fiji, where it loops around Tonga far southward to New Zealand. Next in prominence, giving rise to 15 percent of all earthquakes, is the Mediterranean-Himalayan belt, extending from Gibraltar to Southeast Asia along convergent plate boundaries, mostly continental in character, between Eurasia and the plates that border it.
Situated in the circum-Pacific belt, the Taiwan Island is under an intense compressional tectonic regime, there have been more than 10 disastrous earthquakes reported in the past century. The most recent one, the Chichi earthquake, occurred in central Taiwan in 1999. Studies of the Chichi earthquake show that the main fault plane dips 29° eastward down to roughly 15 km in depth. During this earthquake, an approximately 80 km long upthrust unit underwent displacement of several meters upward and westward along the Chelungpu thrust fault. A picture of the co-seismic displacement of the Chelungpu fault derived from near-field GPS surveys (Yu et al. 2001) showed that the amount of slip gradually increased from about 2 m in the south to about 8 m in the north, in agreement with field studies along the fault trace.
The Chelungpu fault is not the only active faults in Taiwan. After the report of Central Geological Survey, there are more than 30 active faults and most are very close to the population. For example, west to the Chelungpu fault, the sigmoid shape of the present-day Taiwan deformation front in the Pakuashan-Changhua fold-thrust area is probably the result of Quaternary tectonic activity corresponded to the western propagation of convergent stresses within an arc-continent collision zone. In order to mitigate the potential geological hazards associated with the earthquakes, monitoring the distribution and surface deformation in and around these active faults should be the top priority.