HR: 1330h
AN: G32A-0732    [PDF]
TI: Asymptotic Theory for Calculating Geoid Change Caused by Dislocations Buried in a Spherical Earth
AU: * Sun, W
EM: sunw@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Space techniques, e.g., altimetry and gravity missions, become powerful tools in modern geodesy. They can be widely applied in geodetic, oceanographic and geodynamic studies. As a potential application, they provide us with powerful means to detect geoid (or sea level) change caused by earthquake due to their nearly continuous measurements of sea level and geoid over repeat period intervals. To investigate co-seismic geoid (sea level) change or to interpret observed geoid changes by altimetry or gravity missions, theoretical work on co-seismic deformation is also necessary. Dislocation problem for a half-space was studied by many scientists. They presented analytical expressions for calculating surface displacement, tilt, and strain due to various dislocations buried in semi-infinite medium. Especially, Okada (1985) summarized previous studies and presented a complete set of analytical formulae for calculating these geodetic deformations. Okubo (1991, 1992) proposed expressions in closed form to describe potential and gravity changes due to dislocations. Due to their mathematical simplicity, these dislocation theories have been widely applied up to the present day to study or invert seismic faults. However, validity of these theories is strictly limited to a near field because Earth's curvature and radial heterogeneity are ignored. Therefore, a dislocation theory for a more realistic earth model is demanded to interpret far field deformation. A homogeneous or stratified sphere should be considered for this purpose. A homogeneous sphere model is obviously superior to half-space since it includes earth curvature. Efforts to develop formulations for such an earth model were advanced through numerous studies. These studies revealed that earth's curvature effect is negligible for shallow events, while vertical layering may have considerable effects on deformation fields. However, Sun and Okubo's (2002) recent study indicates that both curvature and vertical layering have significant effects on co-seismic deformation. Compared to the half space and homogeneous sphere models, a stratified sphere is the most realistic; they reflect both sphericity and stratified structure of the earth. Sun and Okubo (1993, 1998) and Sun et al. (1996) presented theories to calculate co-seismic displacements and gravity changes in spherically symmetric earth models. Okubo (1993) proposed a reciprocity theorem for connecting solutions of dislocation and tidal, shear and load deformations. This theoretic result provides a useful tool for the current study. The above studies concerning different earth models indicate that different theories have different advantages and disadvantages. For the half-space earth model, corresponding theories are mathematically simple and can be used easily in an application. However, the disadvantage of the theories is that the earth model is physically too simple to reflect sphericity and stratified structure of the earth. On the other hand, theories for spherical earth models are physically better, but are mathematically tedious due to numerical integration and summations (Sun and Okubo 1993, 1998; Sun et al., 1996). Therefore, to overcome the disadvantages of the two cases, in this study, we present a new asymptotic theory as an approximation of the dislocation theory. The co-seismic geoid (sea level) change is investigated and a set of asymptotic expressions is presented for calculating potential and geoid changes caused by four independent seismic sources. This theory is valid in near and regional areas. Note that although geoid and sea level are different physical concepts, their co-seismic changes are considered to be the same.
DE: 1200 GEODESY AND GRAVITY
DE: 1214 Geopotential theory and determination
DE: 1242 Seismic deformations (7205)
DE: 1299 General or miscellaneous
DE: 7205 Continental crust (1242)
SC: Geodesy [G]
MN: 2003 Fall Meeting