Source Parameter Estimation of Indian Ocean Earthquake from observation of GRACE Gravity Gradient Tensor

Armin Rahimi,Mehdi Naeeni

doi:10.4401/ag-7394

Abstract

In this study, the focal mechanism of Indian Ocean earthquake is determined by combination of GRACE gravity gradient change and analytical model of Okubo (1992). To do so, the co-seismic gravity gradient change resulting from Indian ocean earthquake is derived using monthly solution of GRACE level 02 data after removing the contaminations from hydrological effects and post-seismic creep. Since, De-striping methods lead to reduction and distortion of a co-seismic gravity change signal, it is suggested to compute a set of gravity gradient tensor components e.g. ΔVxx and ΔVxz, because they are more sensitive to small-scale signals and they reduce the striping errors without need to any de-striping filtering. Then the computed gravity gradient components are compared with the gravity gradient components from Okubo model (1992), which accounts the focal mechanism of the earthquake. By the way, the nonlinear inversion algorithm is constructed and solved by Genetic algorithm to find the best value of fault parameters for Indian Ocean earthquake. For better constraining the fault parameters in the inversion process, a sensitivity analysis is also conducted which reveals that the selected model is highly sensitive to a strike angle, dip angle, length, width and average slip, although it is less sensitive to a depth of a fault. The ultimate optimal estimate of the fault parameters shows a good correspondence compared with some existing slip models obtained by various constraints or via inversion of seismic observations.

Highlights

Earthquake affects lives of many people every year and poses inevitable threats to everyone who lives in seismically active regions
Since the fault parameters are determined by the inversion of gravity gradient components using Okubo [1992] model, it is necessary that the sensitivity of this model to fault parameters is carefully investigated to show which fault parameters can be better constrained in the inversion process. This analysis reveals that the model may be transparent with a respect to some fault parameters and their values should be determined from other observations
In this study, using the observations of Level 02, monthly gravity field solution of Gravity Recovery And Climate Experiment (GRACE) satellites, the changes in the gravity gradient tensor resulting from Indian Ocean earthquake have been derived

Summary

Introduction

Earthquake affects lives of many people every year and poses inevitable threats to everyone who lives in seismically active regions. Han et al [2013] demonstrated the fault parameters inversion of large earthquakes, including that of Indian Ocean, based on multiple centroid moment tensors and normal mode formulation They inverted four double-couple parameters (i.e. strike, dip, rake and scalar seismic moment) simultaneously from the gravity changes [Han et al 2013]. Similar techniques were applied to invert the four double-couple parameters (i.e. strike, dip, rake and scalar seismic moment) for Indian Ocean earthquake from north component of GRACE gravity and gravity gradient change measurements [Die et al 2016]. Model II, by Shao et al [2012], is calculated using 28 teleseismic broadband P waveforms, 27 broadband SH waveforms, and 45 long period surface waves The fault that they proposed has the same direction (strike=20) as the Wei [2012] while it has distinctly different fault parameters and slip history. This analysis reveals that the model may be transparent with a respect to some fault parameters and their values should be determined from other observations

Coseismic gravity and gravity gradient change using GRACE observation

Findings

Conclusion