Earthquakes involve mass redistribution within the solid Earth and the ocean, and as a result, perturb the Earth's gravitational field. For most of the shallow (<60 km) earthquakes with M_w > 8.0, the GRACE satellite gravity measurements suggest considerable volumetric disturbance of rocks. At a spatial scale of hundreds of km, the effect of volumetric change exceeds gravity change by vertical deformation; for example, negative gravity anomalies associated with volumetric expansion are characteristic patterns after shallow thrust events. In this study, however, we report contrasting observations of gravity change from two intermediate-depth (100–150 km) earthquakes of 2016 & 2017 Mw 8.0 (two combined) Papua New Guinea thrust faulting events and 2019 Mw 8.0 Peru normal faulting and highlight the importance of compressibility in earthquake deformation. The combined 2016/17 thrust events resulted in a positive gravity anomaly of 5–6 microGal around the epicenter, while the 2019 normal faulting produced a negative gravity anomaly of 3–4 microGal. Our modeling found that these gravity changes are manifestation of vertical deformation with limited volumetric change, distinct from gravity changes after the shallow earthquakes. The stronger resistance of rocks to volume change at intermediate-depth results in largely incompressible deformation and thus in a gravity change dominated by vertical deformation. In addition, malleable rocks under high pressure and temperature at depth facilitated substantial afterslip and/or fast viscoelastic relaxation causing additional vertical deformation and gravity change equivalent to the coseismic change. For the Papua New Guinea events, this means that postseismic relaxation enhanced coseismic uplift and relative sea level decrease.

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The GRACE and GRACE Follow-On era is a rare opportunity from which we can study the Earth’s response to great earthquakes across diverse tectonic settings at time scales from days to decades. Earthquakes cause abrupt gravity field changes and triggered postseismic adjustment expected to continue for years to decades. There were a total ~20 great earthquakes (Mw≥8.0)during the GRACE era (2002-2017). GRACE data has detected regional-scale coseismic and postseismic gravity changes, including 2004 Mw9.2 Sumatra-Andaman, 2005 Mw8.5 Nias, 2006/2007 Mw8.3 Kuril, 2007 Mw8.5 Bengkulu, 2009 Mw8.1 Samoa-Tonga, 2010 Mw8.8 Maule, 2011 Mw9.0 Tohoku-Oki, 2012 Mw8.6 Indian Ocean (Wharton Basin), and 2013 Mw8.3 Okhotsk earthquakes. These events include shallow megathrust, strike-slip, normal faults ruptures as well as a deep (~600 km) source earthquake. The GRACE gravity data demonstrated the importance of compressibility in governing large-scale coseismic gravimetric change. For the largest of these events (Mw≥8.6), we were able to constrain the seismic source depth and long-wavelength slip distribution, bulk modulus, elastic lithosphere thickness, sea water redistribution, and the Earth’s rheological structure. Most of the 20 events with Mw≥8.0, during the GRACE era, exhibited a postseismic response best approximated by bi-viscous relaxation with range of asthenospheric viscosities including a lower range of 5x10^17–10^18 Pa s. In this presentation, we review these events using monthly time series of the entire GRACE geopotential change datasets from 2002 to 2017. Using the elastic and viscoelastic normal mode computations, we analyze the GRACE data to optimally constrain the earthquake source models and the regional rheology models using a self-consistent approach. We determine the coseismic and postseismic geopotential change models by inverting the GRACE L2 data for those events. Additionally, our modeling results will be provided to the GRACE Project and science community so that they can be used to remove the earthquake-related GRACE response to enable climate investigations that rely on GRACE data free of tectonic signals. Our comprehensive products can be used to improve quantification of secular trends of ocean, cryosphere, and hydrological mass transport from GRACE and GRACE Follow-On measurements. @en