Journal of Physics of the Earth

Closed-form analytic expressions for the displacements and stresses at any point of a two-phase medium consisting of a homogeneous, isotropic, perfectly elastic half-space in welded contact with a homogeneous, orthotropic, perfectly elastic half-space caused by two-dimensional seismic sources located in the isotropic half-space are obtained. The method consists of first finding the integral expressions for two half-spaces in welded contact from the corresponding expressions for an unbounded medium by applying suitable boundary conditions at the interface and then evaluating the integrals analytically. Numerical computations indicate that the deformation field due to a source in an isotropic half-space in welded contact with an anisotropic half-space may differ substantially from the deformation field when both the half-spaces are isotropic.

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Giant clam Calyptogena soyoae colonies were discovered on the deep seafloor of Sagami Bay. The biological activity of the colony is considered to be kept by sulfur and carbon compounds in upwelling fluid at the site. The radioisotope ratio ¹⁴C/¹²C and stable isotope ratio ¹³C/¹²C in the giant clam shells were measured to confirm the fluid upwelling and to investigate the origin of the fluid.
The ratio ¹⁴C/¹²C was measured on specimens collected from two areas: Off-Hatsushima and the Sagami Knoll. The values were scattered over a wide range: normalized isotope ratio in per mil, δ¹⁴C= -347 to -159%o. Even the living clams have various δ¹⁴C values, and have not the highest δ¹⁴C in the colony. These data indicate that the δ¹⁴C value does not correspond to the age of the shell, but to the degree of contamination of the seawater around the shell with the upwelling fluid.
The ratios ¹³C/¹²C and ¹⁴C/¹²C were measured on specimens collected from the Sagami Knoll. The data showed linear relation between the values of ¹⁴C/¹²C and ¹³C/¹²C. We infer from the linear relation that the carbon in the shells came from two sources, ordinary bottom seawater (Δ¹⁴C = -220‰, δ¹³C= 0‰) and upwelling fluid (Δ¹⁴C = -1, 000‰, δ¹³C= -33‰). The δ¹³C value suggests that the carbon in the upwelling fluid has its origin in sedimentary rocks. In addition, the Δ¹⁴C value suggests that the sedimentary rocks are older than several tens of thousands of years.

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