Tectonic signatures in coseismic gravitational energy change

Abstract

We argue that the commonly ignored first order terms in displacement in the energetics of earthquakes may have tectonic significance. Chao et al. (1995) demonstrated this point in the context of global energy budget, but we demonstrate its significance in regional tectonics. The two first order terms are elastic strain energy under hydrostatic stress and gravitational energy. Because the two terms cancel, they are independently conserved and do not bear any significance to earthquake energy budget which is based on balancing of second order terms. Nevertheless, balance of first order terms represent large quantities of energy transformation between one type of energy (elastic strain energy) and the other (gravitational energy). Nature of this energy transformation can be examined by computing gravitational energy, generated coseismically by earthquakes. We evaluate this gravitational energy for the period from 1977 to 1998 by using the Harvard moment tensor catalogue and discuss three aspects of the gravitational energy change which carry tectonic implications: (1) Regions of extensional tectonics show systematic gravitational energy loss and regions of compressional tectonics typically show energy gain. For example, regions of gravitational collapse in the crust of Tibet and the Andes are clearly identified in the maps. (2) Regions with complicated patterns are found in some subduction zones such as the western and the southern Pacific. They indicate the existence of short wavelength stress heterogeneities and temporal variations in the crustal gravitational energy. The latter implies that the crust is going up and down over a time interval of one to a few years. (3) Energy gain/loss patterns for depths between 400 and 1000 km imply strong resistance of the lower mantle to sinking slabs. A notable exception is found near Japan, where energy loss is seen throughout this depth range, suggesting a smoothly sinking slab in this region.