Dynamics of the Earth's Interior
- Masao Nakada, Professor
- Shigeo Yoshida, Associate Professor
- We investigate the rheology and fluid dynamics inside the Earth through theoretical studies and data analyses. One of the main lines of research is large-scale viscoelastic deformation of the mantle. It aims at determining the viscosity profile of the mantle, and explaining sea-level changes. Another main line of research is core dynamics, including geomagnetic field variations, basic dynamo processes, and the flow in the inner core. Other research areas include dynamics of volcanic eruptions and hydrothermal circulation.
Earth's viscosity structure
Viscosity structure of Earth's mantle is a crucial quantity in discussing mantle dynamics. One of the methods for inferring viscosity structure is to use the observed relative sea level (RSL) variations for glacial isostatic adjustment (GIA) process due to the last deglaciation. On the other hand, the rotational variations due to the GIA processes are degree-two response of the Earth, and therefore have been used to infer the lower mantle viscosity. In particular, the rate of change of degree-two harmonics of Earth's geopotential, J2-dot, provides an important constraint on the lower mantle viscosity. However, the observationally derived J2-dot is affected by recent melting of glaciers and the Greenland and Antarctic ice sheets, and therefore we have to extract the recent melting component from the observation to estimate the GIA-induced J2-dot available for inferring the viscosity structure. Nakada et al. (2013, 2015) estimated the recent melting component using the data taken from the IPCC 2013 Report and obtained the GIA-induced J2-dot based on the observationally derived J2-dot. GIA-induced J2-dot is also highly sensitive to the Late Pleistocene melting histories of both polar ice sheets, particularly, to the meltwater volume since the Last Glacial Maximum (LGM). Recently, we have examined the GIA-induce J2-dot and LGM sea level changes at Barbados and Bonaparte Gulf, Australia, to infer the viscosity structure and the ESL component(Nakada et al. 2016). These results indicate the effective lower mantle viscosity of (5–10)×1022 Pa s, upper mantle viscosity of (1–3)×1020 Pa s and the preferred ESL component of ~130 m.
Recent Sea Level Rise and Vertical Tectonic Movement of Japanese Islands
Geological studies suggest that megathrust earthquakes, such as the 2011 earthquake off the Pacific coast of Tohoku, have occurred repeatedly along the Japan Trench with a recurrence interval of approximately 1000 years. It is therefore important to examine the tectonic crustal movements of the Japanese Islands based on the observed relative sea level (RSL) changes, which may be a societal demand after the 2011 earthquake off the Pacific coast of Tohoku. In tectonically active Japanese Islands, observed RSL changes during the late Quaternary are caused by change of ocean volume, spatially and temporally non-uniform tectonic crustal movement associated with the plate subduction and the GIA in response to the redistribution of ice and water loads. It is possible to infer average rates of tectonic crustal movement along the Japanese coastlines on three typical timescales of ~50 yr, ~6 and ~125 kyr based on tide gauge and Holocene RSL observations and the altitudes of marine terraces formed at the last interglacial (LIG) phase at ~125 kyr BP.The rates on a timescale of ~50 yr are derived from tide gauge data, thermosteric sea-level changes due to thermal expansion of the oceans and predictions due to the GIA processes and also recent melting of the mountain glaciers and Greenland and Antarctic ice sheets (Nakada et al. 2013). Those for ~6 kyr are inferred from many Holocene observations and the predictions by GIA modelling. Those for ~125 kyr are also inferred from many marine terraces formed at the LIG. Many marine terraces for the LIG are observed at altitudes higher than 40 m at sites facing the Pacific Ocean. However, terraces with an altitude higher than 10 m have not been reported at sites along the seismically inactive west coast of Kyushu. Thus, the long-term crustal deformation inferred from the altitude of LIG terraces appears to be very different along the various Japanese coastlines.Our studies based on these observations show that the rates of vertical tectonic crustal movements on a timescale of ~6 kyr are consistent with the rates for ~125 kyr and GIA-predictions in many sites, but inconsistent with those for ~50 yr in most sites, indicating that the rates on a timescale of ~50 yr are not representative of the tectonic crustal movements for timescales longer than ~6 kyr in most sites along the Japanese coastlines. However, the inferred rates on these timescales may be useful in discussing the recurrence of megathrust earthquake with its interval of ~1 kyr such as the 2011 earthquake off the Pacific coast of Tohoku.
Dynamics in the outer core
We investigate basic fluid dynamical processes in the outer core. The Earth's outer core is the origin of the geomagnetic field. (1) Part of the variations of the geomagnetic field may be a manifestation of magnetohydrodynamic (MHD) waves. We examine characteristics of MHD waves, including wave speeds and dispersion relations, and compare them with observations. In particular, we are now interested in the MAC wave, which is confined in the stratified layer in the uppermost outer core. (2) Dynamo processes may be classified into the alpha effect and the omega effect. We study the basic mechanism of the alpha effect, which is the twisting of the magnetic field by helical fluid motions (Fig.1). We have found a non-locality in the alpha effect. (3) Thermal interaction between the mantle and the outer core may be responsible for persistent non-dipole spatial patterns of the geomagnetic field. We investigate how thermal heterogeneities on the core-mantle boundary affect the geomagnetic field
Dynamics in the inner core
The inner core shows seismic anisotropy, with the P-wave velocity faster in the polar directions. This anisotropy is interpreted as a consequence of preferential alignment of iron crystals, which in turn results from a flow in the inner core. We model such flows in relation to the outer core dynamics (Fig.2).
Fluid dynamics of volcanic eruptions and hydrothermal circulation
We study fluid dynamics related to geothermal phenomena, including volcanic eruptions and hydrothermal circulations. These systems exhibit interesting interplay among flow, phase change, and other thermodynamical changes. (1) Volcanic eruptions show periodic temporal changes, which may be due to an instability of a bubbly flow in the volcanic conduit. We study such instabilities. (2) In hydrothermal circulations, a flow induces chemical precipitation, which affects the pattern of a flow field. We study such interplays by numerical simulations (Fig.3).