Analysis of GRACE Range-Rate Time Series during Strong Earthquakes: A Case Study in Iran

The applicability of the Gravity Recovery and Climate Experiment (GRACE) level ۱B range-rate data to detect gravity changes caused by significant earthquakes (M۶.۰-۶.۹) has been investigated. The most common product of the GRACE mission is the level ۲, science data, as the spherical harmonic Stokes’ coefficients of the geopotential. These coefficients have been generated from Level ۱B data, resulting in missing some information during the smoothing process. In this study, the GRACE level ۱B K-band range-rate measurements over three selected cells in Iran were analyzed, including two cells containing the epicenters of the Borujerd earthquake (۶.۱ Mw) and the Zarand earthquake (۶.۴ Mw), which occurred on March ۳۱, ۲۰۰۶, and February ۲۲, ۲۰۰۵, respectively, and one cell far enough from those two cells. Additionally, the range-rate time series attributed to Iran's main catchments containing the aforementioned zones have been extracted to distinguish between the impacts of earthquakes and hydrology on the range-rate time series. Besides, the impact of factors other than earthquakes, such as tides and non-gravitational accelerations acting on the GRACE satellites has been corrected. To better explore the extracted signals, their details have been derived using wavelet transforms, and the corresponding anomalies have been detected using the boxplot method. The considerable anomalies observed in areas within or near the  epicenters of earthquakes before and after the events indicate that the GRACE and GRACE Follow-On range-rate time series can be considered as potential precursors to a major earthquake.