Three-dimensional Analysis of Gravity Data in the Ulleung Basin of the East Sea (Japan Sea)

Abstract

Multi-channel seismic and gravity data were three-dimensionally analyzed to obtain a deep crustal gravity effect and to interpret it in terms of crustal structure, crustal boundary, and crustal characteristics by referring to seismic refraction data by the OBS(Ocean bottom Seismometer) experiment in the Ulleung Basin. Four sedimentary layers and acoustic basement were defined from muti-channel seismic reflection profiles and a depth to Moho was roughly estimated from the OBS data. Densities applied to calculate the gravity effect were converted using velocity-density relationship from velocities in reflection data for the shallow part and velocities in refraction data for the deep part. The calculated gravity effects are relatively large in sea water-sediment and lower sediment layer-basement interfaces where a density contrast between neighbouring layers is high. The mantle Bouguer anomalies were estimated by subtracting the crustal Bouguer anomalies and the gravity contribution due to the crust-mantle interfaces by assuming a constant crustal thickness of 9 km from the free-air anomalies. The mantle Bouguer anomalies are distinctly high over a rectangular zone with a width of 90 km trending northeast-southwest in the central Ulleung basin. Its crustal boundary coincides with that in seismic refraction profiles. This pattern implies that the crustal characteristics between inside and outside of the central rectangular zone are different, and the mantle beneath the crust has been cooled since an extension of the crust and upwelling of the mantle to compensate a decrease of loading by the crust, and the Ulleung basin was symmetrically opened in NW-SE direction.