Session: B.2 - Solid Earth Sciences
(Convener: )

Title: What can we do with GRACE and GRACE-FO for the study of deep-focus earthquakes?
Presenter: Ivins, Erik
Co-Authors: G.A. Lyzenga, S. Adhikari, D.N. Wiese, M.M. Watkins

Abstract: The origin of deep focus earthquakes remains somewhat of an enigma in Earth sciences since the mechanisms of fracture initiation at depths below 420 km and above 660 km in the mantle require more exotic instabilities than those occurring at shallow depths. The fact that GRACE can now observe the co-seismic signal from deep focus earthquakes nucleating deeper than 600 km opens new possibilities for space geodetic research. Here we discuss a new observation and some of the issues behind the possible nucleation mechanisms and post-seismic relaxation. Since 2004, interest in short-term rheology has been heightened by the observations of post-seismic flow using GRACE and GPS data following three great subduction-zone earthquakes. A fourth subduction-zone earthquake is captured in the GRACE monthly solutions: the May 2013 Mw 8.3 Sea of Okhotsk Earthquake with rupture nucleation at 608 km depth, sitting just 52 km from the top of the lower mantle. The top of the lower mantle is a region that has recently been hypothesized to have a potent biviscous behavior anywhere in the mantle. Our strategy, therefore, is to examine the feasibility of using GRACE, or future gravity missions, as a test of the various hypotheses for the rheology of this zone where slabs flatten and stagnate.

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Postseismic gravity changes caused by persistent viscoelastic relaxation after a series of great earthquakes since 2004
Presenter: Han, Shin-Chan
Co-Authors: J. Sauber; F. Pollitz

Abstract: GRACE data has detected regional-scale coseismic and postseismic gravity changes after recent great earthquakes, including 2004 Mw9.2 Sumatra-Andaman, 2005 Mw8.5 Nias, 2007 Mw8.5 Bengkulu, 2010 Mw8.8 Maule, 2011 Mw9.0 Tohoku-Oki, 2012 Mw8.6 Wharton Basin (Indian Ocean), and 2013 Mw8.3 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 by not only megathrust ruptures (as large as Mw9.2), but also earthquakes (as small as Mw8.1) 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 gravity changes after these earthquakes may continue for several years to decades. 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 Mw8.3 Kuril thrust and Mw8.1 normal faulting events (doublet) was small but it produced substantial postseismic gravity change, indicating the prominent influence of viscous asthenosphere underlying the thin elastic lithosphere in the Kuril trench. The GRACE data could constrain the moment release ratio between the 2006 thrust and 2007 normal fault ruptures, the thickness of the lithosphere, and the viscosity of the underlying asthenosphere.

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. 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 correction” 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.

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