Modelling and Observing the Mw 8.8 Chile 2010 and Mw 9.0 Japan 2011 Earthquakes Using GOCE

Abstract

Earthquakes change the gravity field of the area affected by the earthquake due to mass redistribution in the upper layers of the Earth. In addition, for sub-oceanic earthquakes deformation of the ocean floor causes relative sea-level changes and mass redistribution of water that has again a significant effect on the gravity field. Two such recent, large sub-oceanic earthquakes are the 27 February 2010 Chile Maule earthquake with a magnitude of Mw 8.8 and the 11 March 2011 Japan Tohoku earthquake with a magnitude of Mw 9.0. The goal of ESA's satellite GOCE-launched in March 2009-is to map the Earth's gravity field with unprecedented accuracy and resolution. To this end, GOCE carries a gravity gradiometer. Although the mean gravity field is to be mapped, the sheer size of both earthquakes and associated mass redistribution make them both potential candidates for detecting the co-seismic gravity changes in the GOCE gradiometer data. We assess the detectability of gravity field changes in the GOCE gravity gradients by modelling these earthquakes using a forward model. Furthermore, we analyse the GOCE data before and after the respective earthquakes and assess their quality. Based on these analyses we conclude that despite its small signal size at GOCE altitude the Japan earthquake may be visible in the gravity gradients when more post-earthquake data become available. Because of the short data period before the Chile earthquake this signal will probably not be visible.

Observing coseismic gravity change from the Japan Tohoku‐Oki 2011 earthquake with GOCE gravity gradiometry

“…The predicted coseismic gravity change, expressed in geoid heights, results in 0.8 to À1.2 cm for the Wei slip model developed to spherical harmonic degree and order 250 which corresponds to 80 km spatial resolution (see Figure 3). The coseismic gravity change is about ±300 μGal [Bouman et al, 2014]. Our modeled results are lower compared with Zhou et al [2012] who found up to 2.5 cm geoid change and a coseismic gravity change of À1000-600 μGal.…”

“…The obtained geoid series expansions have been used to compute gravity gradient observations along the GOCE orbit for the period 11 March 2011 to 31 March 2012. The gradient signal developed from the forward computed model up to degree and order 450 (see Figure ) is at GOCE orbit height in the order of 0.6 mE [ Bouman et al ., ]. The forward computed gradients undergo the same processing procedure as explained in section 2.…”

Distributed fault slip model for the 2011 Tohoku‐Oki earthquake from GNSS and GRACE/GOCE satellite gravimetry

“…The GRF measured gravity gradient of V xx is pointing approximately in the satellite's flight direction, V yy in cross-track direction, and V zz in radial direction (toward the Earth's center). The combined gravity gradients are a representation of the radial gravity gradient V rr in a local Earth-centered Earth-fixed system [Bouman et al, 2013].…”

“…Using the combined gravity gradient data set (section 2.3), we combine the four accurately measured GOCE and GRACE gravity gradient components in a regional least squares estimation. To that aim spherical prisms [Grombein et al, 2011] located at Earth's surface are used to interpolate the measured gravity gradient signals at satellite's mean orbit height in a least squares sense [Bouman et al, 2013]. To compute the gravity gradient grids in an area of interest of 40 ∘ by 40 ∘ , centered at 37 ∘ north and 141 ∘ east, we set up a surface grid with nodes of 1 ∘ by 1 ∘ .…”