| dc.description.abstract | Plate boundaries generate enormous earthquakes. The co-seismic change of gravitational potential energy is caused by a mass redistribution due to the earthquake faulting. Generally, the crustal gravitational potential energy is increasing in compressive areas, while the crustal gravitational potential energy is decreasing in extensional areas. The change of gravitational potential energy distribution shows that most of the subduction zones exhibits compressive stress while the spreading centers/backarc riftings undergo extensional stress. However, there is difference of gravitational potential energy change on both sides of the trench: the crustal gravitational potential energy at fore-trench area shows the energy loss while at the back-trench distributes the energy gain. The decrease of gravitational potential energy may be caused by the outer rise of the subducted slab and; oppositely, the increase of gravitational potential energy was induced by the slab subduction during plate convergence. This phenomenon appears at most of subduction zones in East Asia.
The Philippine Sea Plate is converging against the Eurasian Plate with a velocity of 7-8 cm/yr near Taiwan, which has caused the Taiwan orogenesis and induced abundant earthquakes. We have examined the corresponding gravitational potential energy change by using 757 earthquakes from the earthquake catalogue of the Broadband Array in Taiwan for Seismology (BATS) from July 1995 to December 2003. Our results show that the variation of the crustal gravitational potential energy strongly correlates with different stages of the orogenesis. Except for the western Okinawa Trough and the southern Taiwan, most of the colliding regions in Taiwan exhibit a gain of crustal gravitational potential energy. In contrast, the lithospheric gravitational potential energy change in the Taiwan region exhibits a reverse pattern of the crustal gravitational potential energy change. For the whole Taiwan region, the earthquake-induced crustal gravitational potential energy change and the lithospheric gravitational potential energy change during the observation period are +1.03E+17 joules and -1.15E+17 joules, respectively. The average rate of the whole gravitational potential energy change in the Taiwan region is very intense and is equal to -2.07E+10 watts, corresponding to about one per cent of the global GPE loss induced by earthquakes. During the past century, the largest and disaster earthquake, the Chi-Chi earthquake, has occurred. The Chi-Chi earthquake sequence provides most of the gravitational energy gain in the middle central Taiwan, instead of the western foothill where the Chi-Chi main shock locates. It implies that the Chi-Chi main shock may trigger the release of the cumulative strain in the crust of middle Taiwan.
Finally, the analysis of the global divergent mid-ocean ridge distribution of gravitational potential energy change shows that the cumulative gravitational potential energy change has strong nonrandom tendency with the ridge spreading rate. The fast spreading ridge shows the relatively low gravitational potential energy change. In total, the gravitational potential energy change caused from the earthquake at global mid-ocean ridge is equal to 2.1E+19 joules from 1976 to 2004. | en_US |