Crustal deformation due to a Pressure source with directivity

Abstract

   Since the mid-1970's marked crustal upheavals have been observed continuously in the Izu Peninsula. several models constructed on the basis of tectonic, volcanological, and other geophysical consideration have been proposed to explain these movements. However, since small eruptions took place off the east coast of the Izu Peninsula in 1989, most people may have accepted the idea that there exists volcanic background behind, these unusual events.
   When we see the annual variations in contours of these vertical ground movements, we notice some characteristic features: (1) The center of upheavals moves annually and (2) an area of small-amplitude depressions exists adjacent to this large-displacement upheaval area.
   In this paper, we introduce a pressure source with directivity and investigate whether the above characteristic features of crustal movement can be explained qualitatively using this source model. Our computation is based on the three-dimensional finite element method.
   We find that our pressure source with directivity can explain changes in the contour patterns of observed crustal movements if we assume that the direction of the pressure axis changes annually. Our source is preferable to a familiar equi-pressure source because in the latter case we must assume at least two sources with different intensity distributed just under the peak positions of upheaval and depression and their locations have to move annually but, in our case, one source at a fixed position is required.
   Our other conclusion is that a pressure source with directivity gives rise to the depression area adjacent to the upheaval area more efficiently than the inclined tensile fault even though both sources cause similar contour patterns of crustal movements.
   The Appendix discusses elastic deformation of a semi-infinite medium and a layered medium (a half-space with one superficial layer) caused by a buried spherical equi-pressure source. The former example examines the validity of our computational method by comparing numerical results with analytical solutions. From the computation for the latter case, we show that the existence of a low-velocity superficial layer increases horizontal surface displacements as much as 30% compared to that without it.