日本地震学会学, 日本測地学会学長

GRACE detected regional-scale coseismic and postseismic gravity changes after recent great earthquakes, including 2004 Sumatra-Andaman, 2005 Nias, 2006/2007 Kuril, 2007 Bengkulu, 2009 Samoa-Toga, 2010 Maule, 2011 Tohoku-Oki, 2012 Wharton Basin (Indian Ocean), and 2013 Okhotsk earthquakes. Those earthquakes caused abrupt changes in the Earth's gravity field and triggered gradual postseismic adjustment expected to continue for years to decades by viscoelastic relaxation. Significant postseismic gravity changes were recorded in GRACE by not only megathrust ruptures (as large as Mw9.2), but also earthquakes (as small as Mw8.1) with very different mechanisms, such as strike-slip earthquakes and normal faulting events. The cumulative postseismic gravity changes can be even larger than the coseismic changes depending on the rupture mechanism and the Earth's rheological structure around the region. For example, the results from the GRACE Level-2 data found that the combined coseismic gravimetric signal from Mw8.3 Kuril thrust and Mw8.1 normal faulting events (doublet) was too small to be distinguished by GRACE, but it produced substantial postseismic gravity change, indicating the prominent influence of viscous asthenosphere underlying the thin elastic lithosphere in the Kuril trench. The similar results were also found after the 2009 Samoa-Tonga earthquake doublet. We have an opportunity to examine the Earth's deformations from various types of earthquakes at time-scales from months to decades comprehensively using nearly 15 years of continuous gravity measurements from GRACE and to be extended for another decade or longer by GRACE-FO. In this presentation, we review the GRACE observations of gravity changes from those earthquakes and provide the numerical modelling results of gravity change predominantly by viscoelastic relaxation. We will show the regional variability in the rheological structures along different plate boundaries.