Ocean contribution to seismic gravity changes: the sea level equation for seismic perturbations revisited

Abstract

S U M M A R YDuring megathrust earthquakes, great ruptures are accompanied by large scale mass redistribution inside the solid Earth and by ocean mass redistribution due to bathymetry changes. These large scale mass displacements can be detected using the monthly gravity maps of the GRACE satellite mission. In recent years it has become increasingly common to use the long wavelength changes in the Earth's gravity field observed by GRACE to infer seismic source properties for large megathrust earthquakes. An important advantage of space gravimetry is that it is independent from the availability of land for its measurements. This is relevant for observation of megathrust earthquakes, which occur mostly offshore, such as the M w ∼ 9 2004 Sumatra-Andaman, 2010 Maule (Chile) and 2011 Tohoku-Oki (Japan) events. In Broerse et al., we examined the effect of the presence of an ocean above the rupture on long wavelength gravity changes and showed it to be of the first order.Here we revisit the implementation of an ocean layer through the sea level equation and compare the results with approximated methods that have been used in the literature. One of the simplifications usually lies in the assumption of a globally uniform ocean layer. We show that especially in the case of the 2010 Maule earthquake, due to the closeness of the South American continent, the uniform ocean assumption is not valid and causes errors up to 57 per cent for modelled peak geoid height changes (expressed at a spherical harmonic truncation degree of 40). In addition, we show that when a large amount of slip occurs close to the trench, horizontal motions of the ocean floor play a mayor role in the ocean contribution to gravity changes. Using a slip model of the 2011 Tohoku-Oki earthquake that places the majority of slip close to the surface, the peak value in geoid height change increases by 50 per cent due to horizontal ocean floor motion. Furthermore, we test the influence of the maximum spherical harmonic degree at which the sea level equation is performed for sea level changes occurring along coastlines, which shows to be important for relative sea level changes occurring along the shore. Finally, we demonstrate that ocean floor loading, self-gravitation of water and conservation of water mass are of second order importance for coseismic gravity changes.When GRACE observations are used to determine earthquake parameters such as seismic moment or source depth, the uniform ocean layer method introduces large biases, depending on the location of the rupture with respect to the continent. The same holds for interpreting shallow slip when horizontal motions are not properly accounted for in the ocean contribution. In both cases the depth at which slip occurs will be underestimated.

Topographic effects on coseismic gravity change for the 2011 Tohoku‐Oki earthquake and comparison with GRACE

“…We first calculate changes for a uniform land model and then compute the seawater effect for the oceanic areas. The seawater correction is significant [e.g., Heki and Matsuo , ; Broerse et al ., ]. As suggested by Broerse et al .…”

“…Horizontal deformation will lead to equivalent vertical displacement in an area with inclined topography [e.g., Tanioka and Satake , ; Hooper et al ., ; Broerse et al ., ], resulting in mass redistribution and gravity change. Figure is a schematic depiction of equivalent vertical displacement and mass redistribution within a uniform‐density ramp‐body model.…”

“…An exception is Broerse et al . [] who estimated mass redistribution due to seawater column changes associated with horizontal movement of sloped seafloor and corresponding gravity effects of the 2011 Tohoku‐Oki earthquake.…”

“…Broerse et al . [] found that the seawater‐column‐change effect due to topography should be considered when using GRACE observations to estimate earthquake source parameters, either separately or in conjunction with seismic and other geodetic data. In addition to the seawater contribution, solid Earth horizontal deformation in regions of inclined topography will also contribute.…”

Postseismic gravity change after the 2006–2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands

“…Broerse et al . [] found that the horizontal motion of the bathymetry is important to understand the coseismic gravity change. However, the horizontal postseismic motions during the observation period are ~2 orders of magnitude less than the coseismic horizontal motions, and hence, they contribute a much smaller fraction of the net gravity change.…”