What Kind of Research are You Conducting?

Earth and planetary sciences include many disciplines called seismology, meteorology and so forth. My specialty is geodesy, which you probably have never heard before, right?

Geodesy used to be defined as a discipline that studies the Earth’s shape, rotation and gravity until around the 1980s. Since the measurement of the Earth’s radius by Eratosthenes, the history of geodesy has been a chronology of measurement precision and accuracy. As the quality and frequency of measurements have improved, we have been able to detect time-variability in the Earth’s shape, rotation and gravity. Due to slow and tiny amplitudes, it has been tough to directly measure the solid Earth beneath the surface. However, the surface motion revealed by geodesy reflects the activity of the Earth, and thus become useful to study earthquakes, volcanic eruptions, glaciers and ice sheets.

Fig 1: Modern geodesy provides us with data sets from variety of sensors on satellites, indicating not only crustal deformation but also atmosphere and hydrosphere.

Nowadays, the Global Navigation Satellite System (GNSS, Fig 1) allows us to measure the daily position of ground observation points with millimeter-level precision. This high-precision geodetic data became available only after the mid-1990s, when space geodesy technology developed. Moreover, because the GNSS relies on the propagation of microwaves from satellites through the atmosphere, the GNSS data sets turn out to be helpful for the study of the ionosphere and troposphere as well. Furthermore, the Gravity Recovery And Climate Experiment (GRACE, Fig 1) satellite allows us to detect the mass movement of water and ice through the measurements of time-variable gravity. Thus, modern geodesy is now an interdisciplinary tool for seismology, volcanology, glaciology, hydrology, meteorology, etc.

 

What Kinds of Devices are You Using in Your Research?

While routine monitoring is now carried out at many ground observation points in Japan, there are many places overseas and even in Japan where we cannot access and install such observation points, either. Such places are actually rather common on the planet Earth, and those are the real frontiers!

Fig. 2: The Tohoku-oki earthquake on Mar 11, 2011 caused not only large-scale deformation of the Japanese island but also local subsidence at volcanic areas．

Synthetic Aperture Radar (SAR) on board satellites made it possible to measure surface displacements without installing any ground observation points. SAR data can reveal the crustal deformation as an image. Besides the broad-scale east-west extending deformation of Honshu due to the 2011 Tohoku-Oki earthquake, it turned out that the volcanic areas in Figure 2 underwent subsidence. SAR allows us to render an image of surface displacement with unprecedented spatial resolution, which was impossible with conventional techniques.

Fig 3: Surface velocity distribution at West Kunkun Mountains.　Red indicates faster velocity．

Figure 3 shows the surface velocity distribution of the mountain glaciers in the NW Tibet, which was also derived from SAR. While we can observe a variety of velocity distributions, we should note that the altitude is higher than 5000 m above sea level, making it impossible to perform field observations. While glaciers and ice-sheets are under the threat of significant decline due to global warming, compounded by a subsequent rise in the sea level, the observations of glaciers and ice sheets are still seriously limited. Hence, there remains large uncertainties in the mechanisms of glacier/ice-sheet changes.

 

What Will be Your Next Goal?

Both the crustal deformation and the ice velocity distribution in Figures 2 and 3 were derived from the SAR onboard ALOS satellite launched by JAXA. The follow-on ALOS-2 was launched in May 2014 and is specifically designed for SAR (Figure 1). Several space agencies in the world have launched SAR satellites since ~2010, and there are also other future plans.

SAR has revealed the unexpected nature of Earth. New observation data is often surprising to us, and I do feel that many truths must still be hidden. Because other space geodetic tools also keep improving, they will further tell us the realities of the Earth that are remain undiscovered. Therefore, I am involved in many fields using space geodesy as a tool.

Incidentally, the earthquake faulting and glacier motion are never distinct from each other in terms of the physics from each generation, because water movement does control both phenomena. Geodesy can lead us to encounter this point of contact in what appear to be distinct subjects.