Calculation of isostatic gravity anomalies and isostatic geoid heights using two-dimesional filtering : implications for structure in subduction zones

Abstract

Using radar altimeter data from the GEOS-3 satellite, a characteristic geoid anomaly has been identified which occurs over all major deep-sea trenches. As one approaches the trench from the seaward side, the altimeter geoid profiles are typified by a long geoidal gradual increase, a narrow low over the trench axis, and a geoidal high with variable slope landward of the trench. The long-wavelength regional trend in these geoidal profiles is similar to the regional increase in bathymetry profiles. Airy and Pratt models of isostatic compensation have been applied to bathymetry profiles to estimate the amount of geoidal variation due to isostatically-compensated ocean-floor topography. Two-dimensional models were used in order to separate out the topographic effect due to two-dimensional structures, which may not always be seen using a one-dimensional analysis. Results show that short-wavelength, high-amplitude variations in the geoid can be reasonably reproduced using a Pratt model of isostatic compensation as compared to an Airy model.By removing this topographic component in isostatic compensation from the geoid and gravity measurements, it is possible to construct a density model for the structure beneath subduction zones; in particular the Tonga-Kermadec Trench region was modeled. This model was used to calculate the geoid and gravity on a spherical Earth in agreement with those values with the topographic component in isostatic compensation removed. In this model, the uppermost mantle body landward of the trench is thinner and occurs at shallower depths than its counterpart on the seaward side of the trench. This dipolar type layered mass distribution is the fundamental cause for the relatively high geoid values landward of the trench and the gradual slope seaward of the trench. The outer bathymetric rise contributes to the local geoidal high that occurs on the seaward wall of the trench.