Co-seismic and co-diking crustal deformation along subaerial rift zones detected by satellite synthetic aperture radar : East Africa, Iceland, and Southwestern Japan

Crustal deformation along the plate boundaries is one of the important research interests for elucidating their mechanisms because earthquake activities and volcanic eruptions usually concentrate along plate boundaries due to high strain rates. Along the divergent plate boundaries which usually call rift zone, we can observe the processes of continental rifting evolution, such as generating new crust and splitting continental crust. Most of the divergent plate boundaries, however, are located beneath the sea, and the onshore rift zones are only less than 2 % of all the rift zones on the Earth. The accumulation of crustal deformation data along the subaerial rift zones improves our understanding of the rift systems and the geophysical phenomena. The aim of this thesis is to reveal detailed crustal deformation along the subaerial rift zones by processing satellite SAR data, one of the space-based geodetic methods. SAR data can detect surface movement with high spatial resolution without installing on-site observation instruments, like GNSS receivers. This thesis focuses on three dike intrusion episodes and one earthquake event along the subaerial rift zones: 1) the 2007 Natron dike intrusion episode, Tanzania, 2) the 2005-2010 Afar dike intrusion episode, Ethiopia, 3) the 2014-2015 Bárðarbunga dike intrusion episode, Iceland, 4) the 2016 Kumamoto earthquake sequence, Japan. By applying InSAR and pixel tracking approaches to PALSAR-1 data the horizontal displacements parallel to the rift axis on the graben subsidence were detected during the 2007 Natron and the 2005-2010 Afar dike intrusion episodes which occurred along the East African Rift. The focal mechanisms during each episode indicate normal faulting with few strike-slip components, these strikes nearly coincide with the rift axis direction. Our inversion model confirmed that the optimal elastic models need not only normal faulting but also strike-slip components to reproduce the horizontal displacement on the graben subsidence. Thus the along-rift horizontal displacements on the graben floor were implied to occur by aseismic processes. The 2014-2015 Bardarbunga dike intrusion episode occurred along the subaerial divergent plate boundary between the Northern American and the Eurasian plates. The overall description of crustal deformation associated with the dike intrusion remains poorly revealed because 80% of dike path located beneath the Vatnajökull icecap which is the largest ice in Europe. Although the conventional InSAR method needs to avoid decorrelation problems for detecting signal above ices and/or glaciers, previous InSAR data could not reveal any signal at the icecap due to the decorrelation problems. In this thesis, we applied pixel tracking approach, which is suitable for detecting the velocity of glacier flow, to satellite SAR data to reveal icecap surface movement associated with the subglacial dike intrusion. Because the co-diking signals contain the signal of both steady-state of icecap flow and crustal deformation, we corrected the icecap flow signal by subtracting the scaled pre-diking signal from the co-diking signal assuming that the pre-diking signals show only the steady-state of icecap flow signal. The ice-corrected signal implies the description of subglacial crustal deformation. We confirmed that the ice-corrected signal contributed to improve the dike opening/faulting model. The 2016 Kumamoto earthquake sequence is the series of earthquake, including Mw 7.0 of mainshock, along the Beppu-Shimabara graben system in Kyushu, Japan. One of the characteristics of the event is that the sequence contains two foreshocks greater than Mw6.0 28 hours before the mainshock and four aftershocks greater than 5.5 one week after the mainshock. The focal mechanism of mainshock from the Japan Meteorological Agency shows not only the dominant of strike-slip components with N-S extensional axis but also ineligible non-double couple component. The newsletter from Geospatial Information Authority of Japan has reported that InSAR results indicate data lacking near the surface ruptures due to the decorrelation problems. In this thesis, we applied the cross-correlation-based pixel tracking approach to PALSAR-2 data to avoid the decorrelation problems. Although the focal mechanism of the mainshock dominates strike-slip component, our pixel tracking data indicates not only right-lateral displacement with NE-SW striking but also over 2 m of subsidence at the northern half of displacement field. The complexity of crustal deformation associated with the earthquake sequence can be explained by the non-double couple component in the focal mechanism of mainshock and the slip-partitioning.