3-D density model of the crust of southern and central Finland obtained from joint interpretation of the SVEKALAPKO crustal P-wave velocity models and gravity data

Abstract

The paper presents a three-dimensional (3-D) density model of the crust of southern and central Finland. The large SVEKALAPKO seismic array experiment was carried out in this area during 1998–1999. One of the intermediate objectives of the experiment was compilation of a 3-D P-wave velocity model (Vp model) of the crust to provide crustal corrections for teleseismic P-wave traveltimes. The model was constructed from the results of previous controlled-source seismic (CSS) experiments in the area and was composed of two main crustal layers, an upper crust and a high-velocity lower crust with Vp > 7.0 km s−1. The thickness of the crust in the model varies from 64 to 38 km and includes three pronounced troughs. We used this model and the results of petrophysical studies of bedrock density in Finland as a priori information to construct a 3-D density model of the crust in the SVEKALAPKO area. The initial results of gravity modelling demonstrated, however, that the model lacks information about lateral velocity variations in the upper and middle crust. To improve the fit of the model to the observed Bouguer anomaly, the model was corrected by introducing two additional layers, called the lower crust and the middle crust, with 6.8 < Vp < 7.0 km s−1 and 6.4 < Vp < 6.8 km s−1, respectively. The depth to the upper boundaries of these layers was retrieved from the results of previous seismic profiles in the area and the values of density and velocity in the upper crust were constrained using the information on bedrock densities in Finland, the Bouguer anomaly and new data about the velocity distribution within the upper crust obtained from local event studies of the SVEKALAPKO seismic experiment. The corrected model was used as a starting model for inversion of the observed Bouguer anomaly. The resulting 3-D density model agrees well with the observed Bouguer anomaly and explains the sources of large-scale Bouguer anomalies in the region. The model demonstrates that there is no correlation between the observed Bouguer anomaly and Moho depth. Rather, the Moho depressions in the region mark the boundaries of crustal blocks with different types of density distribution and are associated with the presence of additional compensating masses within the crust. The high-velocity lower crust of density 3.1–3.25 × 103 kg m−3 does not completely compensate the Moho depressions and the compensation is mostly a result of the presence of additional dense material in the upper and middle crust. Thus, the Moho depressions in central and southern Finland are fully compensated, or even overcompensated. On the other hand, the Moho depression in the area of the Gulf of Bothnia is compensated only in its southern part, resulting in a regional-scale minimum of the Bouguer anomaly in the northern part of the depression. The varying degree of compensation may result from variation in the origin and age of the present-day Moho boundary in the region.