Improved Source Parameter Constraints for Recent Large Undersea Earthquakes from High-degree GRACE Gravity and Gravity Gradient Change Measurements

Abstract:

The north component of gravity and gravity gradient changes from the high degree (up to degree 96) data products of the Gravity Recovery And Climate Experiment (GRACE) are used to study the coseismic gravity change for five earthquakes over the last decade including the 2004 Sumatra-Andaman and 2005 Nias earthquakes, the 2010 Maule, Chile earthquake, the 2011 Tohoku earthquake, the 2012 Indian Ocean earthquakes, and the 2007 Bengkulu earthquake. We demonstrate the advantage of these components to reduce north-south stripes and preserve higher spatial resolution signal in GRACE Level 2 (L2) data, revealing gravity change up to –33 μGal for the 2004 Sumatra-Andaman and 2005 Nias earthquakes, which is by far the highest coseismic signal achievable by innovative GRACE data processing, and revealing the detectability of earthquakes as small as Mw 8.6 (e.g., the 2012 Indian Ocean earthquakes). The localized spectral analysis is applied as an efficient method to determine the practical spherical harmonic truncation degree leading to acceptable signal-to-noise ratio, and to evaluate the noise level for each component of gravity and gravity gradient change of the seismic deformations. For each of the earthquakes, the centroid moment tensor parameters, in particular the centroid location, depth, and slip rake angle are estimated via forward modeling inversions using the improved GRACE gravity and gravity gradient data.