GRACE and follow-on (FO) gravity missions: Contributions to understanding the earthquake seismic cycle and prospects for the future
Jeanne Sauber (Code 61A, NASA GSFC)
Shin-Chan Han (U of Newcastle, Australia)
Fred Pollitz (USGS)
Since launch in 2002, data from the Gravity Recovery and Climate Experiment (GRACE) have provided a synoptic view of mass change associated seismic cycle and other processes [Figure 1}. As part of the GRACE and now the GRACE-FO science teams, we have studied the long-wavelength gravity signal associated with varied earthquakes, including the great subduction zone earthquakes (e.g. Sumatra, 2004; Maule, Chile, 2010, and Tohuku, Japan, 2011), as well as strike-slip and normal faulting events that also produce discernible gravity changes (e.g. 2012 Indian Ocean earthquakes, Han et al., 2015).
Although many GRACE studies use monthly spherical harmonic coefficients provided by the GRACE project, we have also used individual arcs of GRACE inter-satellite tracking data to invert for earthquake source parameters (scalar seismic moment, Mo, dip, rake, and strike; [Han et al., 2013]). Also, we have tested early, varied distributions of coseismic finite-fault slip derived from seismic data against the GRACE range-rate and range-acceleration data [Han et al., 2010, 2011] for earthquakes with Mw ≥ 8.4.
Low-latency inter-satellite range data (now available following earthquakes from GRACE, and available in the future for the GRACE-Follow-on satellite [GRACE-FO; launch late 2017]) , could provide timely, relevantcoseismic source information reflective of the long-wavelength mass change on time scales of days to weeks. Based on the performance simulations of the Laser Ranging Instrument (LRI) on GRACE-FO and earthquake simulations that we have completed, we anticipate the improvements in inter-satellite range data will enable researchers to estimate coseismic source parameters for Mw ≥ 8.0 earthquakes particularly with the GRACE-FO test Laser Ranging Instrument (GRACE-FO-LRI). Longer-term prospects for discerning gravity change associated with the seismic cycle from satellite data have been explored in performance scenarios for future gravity missions beyond GRACE-FO. For example at the 2016 GRACE Science Team meeting we reported on the predicted mass change due to coseismic slip in earthquakes, for a Mw 7.8 Threshold Scenario (minimum mission requirement) and Mw 7.0 for the Target Scenario (desired mission requirement) [Sauber et al., 2016].
Probably the most unique contribution to seismic cycle studies is using the long-wavelength viscoelastic response to great earthquakes, as estimated by GRACE, to further constrain Earth’s rheological structure [Han et al., 2014]. Even the 2006 Kuril megathrust earthquake (Mw 8.3, 3.5 x 1021 N m) and the 2007 Kuril normal faulting earthquake (Mw 8.1, 1.8 x 1021 N m) of the central Kuril Islands resulted in significant long-wavelength postseismic gravity change (~4 mgal between 2007-2015) [Han et al., 2016, Figure 1]. The GRACE data were best fit witha model of 25-35 km for the elastic thickness and 1018 Pa s for the Maxwell viscosity of the asthenosphere. Following larger events such as the 2004 Sumatra-Andaman earthquake, the GRACE data continues to show ongoing gravity change and may suggest multiple relaxation times including deeper relaxation processes (Figure 1). While we have, so far, attributed the short-term postseismic gravity change to the asthenosphere (depth < 220 km), the deeper upper mantle is expected to undergo a substantial coseismic stress perterbation and hence relax following such a long rupture. The time series of GRACE, and anticipated GRACE-FO, data wll provide complementary information to the individual land-based GPS and InSAR data.
For more information, please contact Jeanne Sauber <Jeanne.M.Sauber-Rosenberg@nasa.gov>
References in reverse chronological order:
Sauber, J., S. Han, and F. Pollitz, 2016, GRACE, GRACE-FO and NGGM: Contributions and Future Prospects for Advancing Seismic Cycle Science, Grace Science Team meeting, Potsdam, October.
Han, S., J. Sauber and F. Pollitz, 2016, Postseismic gravity change after the 2006-2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands, Geophys. Res. Lett., 42, doi: 10.1002/2016GL068167.
Han, S, J. Sauber and F Pollitz, 2015, Coseismic compression/dilatation and viscoelastic uplift/subsidence following the 2012 Indian Ocean earthquakes quantified from satellite gravity observations, Geophy.Res.Lett.,42(10),3764-3772, doi: 10.1002/2015GL063819.
Han, S, J. Sauber, and F Pollitz, 2014, Broad-scale postseismic gravity change following the 2011 Tohoku-Oki earthquake and implications for deformation by viscoelastic relaxation and afterslip, Geophy. Res .Lett., 41(16), 5797-5805, doi:10.1002/2014GL060905.
Han, S., R. Riva, J. Sauber, and E. Okal, 2013, Source parameter inversion for recent great earthquakes from a decade-long observation of global gravity fields", J. Geophys. Res. Solid Earth, 118, 1240-1267, doi:10.1002/jgrb.50116
Han, S., J. Sauber and R. Riva, 2011, Contribution of satellite gravimetry to understanding seismic source processes of the 2011 Tohoku-Oki earthquake, Geophys. Res. Lett., 38, L24312,, doi:10.1029/2011GL049975
Han, S., J. Sauber, and S. Luthcke, 2010, Regional gravity decrease after the 2010 Maule (Chile) earthquake indicates large-scale mass redistribution, Geophys. Res. Lett., 37, L23307, doi:10.1029/2010GL045449.