Structural effects of the crust on the geoid modelled using deep seismic sounding interpretations

Abstract

According to the theory of isostasy, the Earth has a tendency to deform its surface in order to reach an equilibrium state. The land‐uplift phenomenon in the area of the Fennoscandian Shield is thought to be a process of this kind. The geoid, as an equipotential surface of the Earth’s gravity field, contains information on how much the Earth’s surface departs from the equilibrium state. In order to study the isostatic process through geoidal undulations, the structural effects of the crust on the geoid have to be investigated. The structure of the crust of the Fennoscandian Shield has been extensively explored by means of deep seismic sounding (DSS). The data obtained from DSS are used to construct a 3‐D seismic‐velocity structure model of the area’s crust. The velocity model is converted to a 3‐D density model using the empirical relationship that holds between seismic velocities and crustal mass densities. Structural effects are then estimated from the 3‐D density model. The structural effects computed from the crustal model show that the mass deficiency of the crust in Fennoscandia has caused a geoidal depression twice as deep as that observed from the gravimetric geoid. It proves again that the crust has been isostatically compensated by the upper mantle. In other words, an anomalously high‐density upper mantle must exist beneath Fennoscandia.

Glacial isostatic adjustment in the static gravity field of Fennoscandia

“…With the increasing number of seismic measurements in the area, it was possible to construct an improved crustal density model. With this improved crustal model, Kakkuri and Wang [1998] concluded that the negative gravity anomaly produced by crustal effects was actually twice as large as the observed gravity anomaly and the density variations in the upper mantle made a dominant contribution to the observed gravity field. A more recent study [Kaban et al, 2010], with improved crustal models shows a positive residual gravity anomaly in the Bay of Bothnia after the removal of crustal effects, leaving no contribution for the GIA gravity signal.…”

“…The largest limitation of most of these studies was the use of a homogeneous crust [Kakkuri and Wang, 1998]. With the increasing number of seismic measurements in the area, it was possible to construct an improved crustal density model.…”

Effect of the GRACE satellite mission on gravity field studies in Fennoscandia and the Baltic Sea region

“…This phenomenon may thus overshadow the possible effect of Moho topography variations. It is notable, however, that the extreme of the depression is not exactly coincident with the site of the land uplift maximum (at N is mainly associated with the post-glacial rebound, and may also be influenced by the structure of the crust and subcrustal processes (Kakkuri & Wang 1998). It should be noted, however, that the residual geoid plot might slightly change when choosing different upper or lower limits of the spectral window.…”