Data obtained by GRACE (Gravity Recovery and Climate Experiment) have been used to invert for the seismic source parameters of megathrust earthquakes under the assumption of either uniform slip over an entire fault or a point-like seismic source. Herein, we further extend the inversion of GRACE long-wavelength gravity changes to heterogeneous slip distributions during the 2011 Tohoku earthquake using three fault models: (I) a constant-strike and constant-dip fault, (II) a variable dip fault, and (III) a realistically varying strike fault. By removing the post-seismic signal from the time series, and taking the effect of ocean water redistribution into account, we invert for slip models I, II, and III using co-seismic gravity changes measured by GRACE, de-striped by DDK3 decorrelation filter. The total seismic moments of our slip models, with respective values of 4.9×1022 Nm, 5.1×1022 Nm, and 5.0×1022 Nm, are smaller than those obtained by other studies relying on GRACE data. The resulting centroids are also located at greater depths (20 km, 19.8 km, and 17.4 km, respectively). By combining onshore GPS, GPS-Acoustic, and GRACE data, we obtain a jointly inverted slip model with a seismic moment of 4.8×1022 Nm, which is larger than the seismic moment obtained using only the GPS displacements. We show that the slip inverted from low degree space-borne gravimetric data, which contains information at the ocean region, is affected by the strike of the arcuate trench. The space-borne gravimetric data help us constrain the source parameters of a megathrust earthquake within the frame of heterogeneous slip models.
The 13 November 2016 Kaikoura earthquake occurred in the northeastern coastal region of the South Island, New Zealand. The Mw 7.8 mainshock generated a complex pattern of surface ruptures, and was followed within about 12 hours by three moderate shocks of Mw ≥ 6.0. Here we use teleseismic waveforms to invert for the source rupture of the Kaikoura earthquake. The resulting slip-distribution model exhibits insignificant slip near the hypocenter and three pockets of major slip zones with distinct senses of motion. The mainshock started from a rupture near the hypocenter, grew into thrust on shallow crustal faults ~50 km northeast of the hypocenter, and then developed into two slip zones: a deeper one with oblique thrust and a shallower one with almost purely right-lateral strike-slip. Locations of the thrust and strike-slip motions in the slip-distribution model agree well with reported coastal uplifts and horizontal offsets. The overall slip pattern is dominated by horizontal motion, especially at shallow depth, due to the partitioning of thrust and strike-slip motions above the subduction zone megathrust. Aftershock distribution suggests that most aftershocks tend to occur near the edges of the major slip zones of the mainshock. This observation on aftershock locations may provide useful information for seismic hazard assessments after large earthquakes.