[S-SS05] Crustal Deformation

[SSS05-P09] Modification of computer program GOTIC2 for calculation of the ocean tidal loading effect
- Application of Green’s function to calculate theoretical strain at depth of borehole observation near coasts -

Osamu Kamigaichi^1,2, *Norio Matsumoto², Fuyuki Hirose³ (1.Japan Meteorological Business Support Center, 2.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 3.Seismology and Tsunami Research Department, Meteorological Research Institute)

Keywords:GOTIC2, oceanic tidal loading, Green’s function, strain, subsurface observation

We modified computer software GOTIC2 (Matsumoto et al., 2001) for estimating the crustal deformation caused by ocean tidal loading to enable calculation of the loading effect not only at the surface but also in subsurface observations using the PREM Earth model. To modify the software, we (i) revised the eigenfunctions using the PREM Earth model and (ii) calculated Green’s functions of displacements and strains caused by a surface load in surface and subsurface observations by applying the eigenfunction. Green’s functions of displacements and strains at depth became different from those at the surface for a range of angular distance less than 0.01 – 0.1° . These Green’s functions closely agree with the corresponding Boussinesq approximations for tidal loading at angular distances of less than about 0.01 – 0.001° , which supports the adequacy of these Green’s functions. We examined the difference between the ocean tidal loading effect estimated by Green’s functions at the surface and the effect for subsurface observations based on very simple and realistic conditions, and found a non-negligible difference when the ratio of deployment depth to distance between the coast and observatory is larger than 0.05. We compared amplitudes and phase shifts of areal and shear strains in M₂ and O₁ tidal constituents observed by borehole strainmeters at 11 observatories with the theoretical strains using the Green’s functions at the deployment depth and those at the surface. The theoretical strains using Green’s function at the deployment depth are much closer to the observed strains than theoretical strains at the surface at four observatories where the ratios of deployment depth to distance from the coast to the observatories are large. These results suggest that Green’s functions at the deployment depth are needed to estimate theoretical strains precisely near the coast. The computer program modified GOTIC2 with the Green’s functions of gravity, displacement, tilt, and strain at depth was published in GSJ Open-File Report (https://www.gsj.jp/publications/pub/openfile/openfile0705.html).