[M-IS13] New frontiers in geology

[MIS13-16] Lithological and thermal structures of Honshu arc

*Masahiro Ishikawa¹, Takumi Matsumoto² (1.Graduate School of Environment and Information Sciences, Yokohama National University, 2.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Crust, Honshu, Thermal structure

In understanding the crustal deformation of the island arc, the lithological composition of the crust and the thermal structure are important information. By interpreting the seismic wave velocity structure from the elastic wave velocity measurement experiment of rocks at high temperature and high pressure, a heterogeneous structure model of the lower crust of the Honshu arc has been proposed. Thermal structure becomes important when estimating the lower limit depth of the seismogenic layer in the crust, but the temperature structure inside the crust is not yet clear and has been a topic of discussion. According to the elastic wave velocity measurement experiment of rocks and minerals under temperature and pressure conditions equivalent to the deep crust, P-wave velocity (Vp) and S-wave velocity (Vs) have a clear temperature dependency, while Vp/Vs has a very small temperature dependency and greatly depends on the rock and mineral species. In other words, if the relationship between P-wave velocity, S-wave velocity and temperature can be determined with high precision, it is possible to convert the seismic wave velocity structure to the temperature structure by extracting the seismic wave velocity of the same rock species (a certain range of Vp/Vs) from seismic tomography. Another method for estimating the temperature structure inside the crust is to use temperature measurements from boreholes. For example, in Tanaka et al. (2004), the geothermal gradient in northeastern Japan exceeds 50 °C/km in a wide range (the lower crust exceeds the solidas and lithadas temperatures in a wide area), which was in conflict with the distribution of actual active volcanoes. However, recent studies (Matsumoto et al., 2022) using temperature measurements from the deep boreholes have shown that the temperature structure in northeastern Japan does not generate such a contradiction. In this presentation, we will compare the thermal structures estimated from seismic tomography and temperature measurements from boreholes, and discuss the thermal structure of the Honshu arc.