Postseismic gravity changes caused by persistent viscoelastic relaxation after a series of great earthquakes since 2004

Abstract:

GRACE data has detected regional-scale coseismic and postseismic gravity changes after recent great earthquakes, including 2004 M_w9.2 Sumatra-Andaman, 2005 M_w8.5 Nias, 2007 M_w8.5 Bengkulu, 2010 M_w8.8 Maule, 2011 M_w9.0 Tohoku-Oki, 2012 M_w8.6 Wharton Basin (Indian Ocean), and 2013 M_w8.3 Okhotsk earthquakes. Those earthquakes caused abrupt gravity field changes and triggered gradual postseismic adjustment that are expected to continue for years to decades due to viscoelastic relaxation. Significant postseismic gravity changes were recorded in GRACE not only for megathrust ruptures (as large as M_w9.2), but also for non-megathrust earthquakes (as small as M_w8.1) with very different mechanisms and properties, such as strike-slip earthquakes (not causing large vertical motion, e.g., 2012 Wharton Basin earthquake) and normal faulting events (e.g., 2007 Kuril earthquake). The cumulative postseismic gravity change can be even larger than the coseismic change depending on the rupture mechanism and the Earth’s rheological structure around the region. The results from the newest GRACE Release-05 (RL05) Level-2 (L2) solutions found that the combined coseismic gravimetric signal from M_w8.3 Kuril thrust and M_w8.1 normal faulting events (doublet) was small but it produced substantial postseismic gravity change. Our preliminary results are consistent with the prominent influence of a biviscous asthenosphere underlying a thin elastic lithosphere in the Kuril trench.

Now we have a unique opportunity to examine various types of earthquakes at time-scales from months to decades comprehensively using nearly 14 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 postseismic gravity changes from those earthquakes and provide the numerical modelling results of gravity change anticipated from viscoelastic relaxation. By optimizing the model parameters, we will show the regional variability in the rheological structures along the plate boundaries.

Our study will be indispensable not only for advancing earthquake physics but also for enabling climate investigations relying on GRACE data free of tectonic signals. We will discuss our progress to provide the optimized physical models of co- and post-seismic gravity changes to the GRACE Project and the broader science community for “earthquake corrections” to L1B and L2 data (just like “Atmosphere and Ocean De-aliasing”, AOD products) to improve quantification of secular trends of ocean, cryosphere, and hydrological mass transport from GRACE measurements.