Application research and product evaluation of GRACE in monitoring co-seismic deformation of large subduction earthquakes

The GRACE mission, which continuously collects large-scale gravity change data over long periods, provides a new tool for monitoring co-seismic deformation. GRACE′s time-variable gravity field products are primarily derived through spectral or spatial domain modeling. Spherical harmonic products, the most classical and widely used, are based on spectral domain modeling. The most popular spatial domain products are Mascon (Mass concentration) products. Currently, 22 spherical harmonic products and 4 Mascon products are available worldwide. Among them, spherical harmonic products employ various post-processing methods, including filtering. However, selecting the appropriate GRACE product for earthquake co-seismic deformation studies remains a question that needs to be resolved. This study uses a unified (stacking) method to extract co-seismic gravity changes from three M9 subduction earthquakes (the 2004 Sumatra, 2011 Tohoku, and 2010 Chile earthquakes). GRACE monitored these earthquakes with spherical harmonic and Mascon products from major international agencies. Evaluations based on spherical dislocation theory show that any spherical harmonic product can be chosen for co-seismic deformation studies combined with P3M6 decorrelation and Gaussian filtering. The Gaussian filter radius should be adjusted based on the earthquake signal strength: for more minor earthquakes with significant noise, a larger radius Gaussian filter is needed. Evaluation based on dislocation theory shows that any spherical harmonic product can be used to study co-seismic deformation. Pair it with P3M6 decorrelation and Gaussian filtering for post-processing. The Gaussian filter radius should be adjusted based on the signal strength. For more minor earthquakes with significant interference, use a larger radius. The GSFC-M Mascon product, combined with a 300 km Gaussian filter, is the best choice for studying co-seismic deformation in the three earthquakes. The unfiltered GSFC-M product demonstrates the most significant enhancement of co-seismic gravity signals among the three Mascon products, with the extracted signal intensity reaching 82% of the original theoretical model. This study compares various GRACE spherical harmonic and Mascon products and offers more precise guidance for selecting the best product.