Gravity Changes Due to Large Earthquakes Detected in GRACE Satellite Data via Empirical Orthogonal Function Analysis
We report on the seven largest earthquakes for which we can detect unequivocal coseismic signals in the time-variable gravity (TVG) data of the GRACE (Gravity Recovery and Climate Experiment) satellite mission during the GRACE era of 2002-2017. After reducing the land-hydrological effect according to model Global Land Data Assimilation System, we employ the empirical orthogonal function (EOF) analysis to solve for the spatial pattern and time history of the coherent (standing-oscillation) modes, which provide in an objective manner an overall view of the spatiotemporal scenario. The solved EOF mode for an earthquake-induced TVG typically features a spatial pattern (along with polarity) for the coseismic jump, in conjunction with the time series showing the jump (plus possible preseismic and postseismic variability). In all cases the earthquake EOF mode solutions match well with the least squares coseismic jump in both spatial pattern and amplitude and agree generally with previous reports in the literature. We conclude according to the examined cases that the lowest earthquake magnitude threshold that can be detected by GRACE is the Mw 8.3 of the 2013 deep-focus Okhotsk event, which in fact provides a testable case where the TVG we observe versus the seismologically derived source models may reflect different aspects of the source mechanism under different timescales.