Gravity survey in northwestern Shikoku, Japan, and subsurface structure of the median tectonic line.

We carried out a gravity survey by means of LaCoste and Romberg gravity meters in northwestern Shikoku, Japan, and presented a detailed Bouguer anomaly map. The most significant feature of the .map is the anomaly pattern along the Median Tectonic Line (MTL). We regarded it as a pattern due to the sharp density contrast between both sides of the MTL, and analyzed it assuming a simple step model of a uniform horizontal slab with a dipped end. We also measured densities of rocks that are distributed in the surveyed area. Optimum models obtained on three profiles are 2 km in thickness, 22° in dip, and 0.30 g/cm3 in density contrast approximately.