Stress field model around the fluid supply source associated with the regional stress state

The stress field around a fluid supply source, such as a magma chamber, can be qualitatively explained by superposing the local stress field of radial compression and the regional tectonic stress field. However, stress field models incorporating both influences have not yet been proposed. In this study, we propose a new stress field model around a fluid supply source that accounts for regional stress, verify its validity by comparing it with natural data, and develop a stress field inversion method based on the new model.

The existing stress field model around a fluid supply source (McTigue, 1987) assumes the crust to be a semi-infinite elastic medium and approximately derives the stress field induced by a spherical pressurized cavity. In the new model, based on the principle of superposition, McTigue’s stress field is combined with a regional stress whose differential stress increases proportionally with depth. This formulation allows representation of anisotropic stress trajectory in the horizontal section.

To validate the new model in nature, we collected orientation data of clastic dikes intruded into the Miocene Tanabe Group in southwestern Japan. Stress inversion (Yamaji & Sato, 2011) was applied to the orientation data within subareas of several tens to hundreds of meters, and the stress state acting on each subarea was estimated. The results suggest that the orientation distribution of clastic dikes reflects both local stress associated with a fluid supply source (a mud diapir) located in the southern part of the study area and regional stress with a NNE–SSW-trending maximum horizontal compressive axis.

Based on the stress states detected in each block and their spatial locations, we estimated the stress field at the time of dike intrusion. In the inversion, the misfit between observed and modeled stresses in each block was assumed to follow a Fisher distribution, and a Markov chain Monte Carlo method was employed. As a result, WNW–ESE tension normal faulting regional stress was detected. The inferred location of the fluid supply source in the southern part of the study area is consistent with qualitative geological interpretations.

The results of this study provide fundamental insights for practical applications, such as identifying volcanic activity centers from dike or microseismic data and predicting the spatial extent of volcanic influence when dikes are discovered, contributing to disaster prevention/mitigation and geological disposal projects.

This study was carried out as a part of a supporting program titled "Program to support research and investigation on important basic technologies related to radioactive waste (2023–2025 FY)" under the contract with the Ministry of Economy, Trade and Industry (METI).

McTigue, 1987, J. Geophys. Res. 92, 12,931–12,940. Yamaji & Sato, 2011, J. Struct. Geol. 33, 1,148–1,157.