Sensitivity analysis of elastic models of coseismic gravity changes on the surface of Earth's Crust to input parameters

Earthquake process involves different variables some of which are determined more accurately than the others with non-modeling approaches. The scope of this research is to investigate the effect of individual geometrical and physical input parameters in coseismic gravity change models on the Earth surface. Among different physical and geometrical parameters, performing sensitivity analysis on less accurately determined parameters by field work is recommended. Among these parameters we can refer to fault dip and upper locking depth of the fault. Nevertheless in this research the role of all faulting parameters on gravity obtained data have been surveyed. To do this analysis the elastic model of Okubo (1992) was used. In this research, surface gravity change was modeled in three strike-slip, dip-slip and tensile slip reference faults, in a medium composed of an elastic half-space and a sensitivity analysis was performed on all geometrical and physical parameters. From the variability analysis, the location of the most appropriate gravity data was determined to obtain values for the studied parameters. To do sensitivity analysis, we considered areas with maximum and minimum gravity changes. These areas were located on one end of the surface projection of the reference fault plane in a strike slip case and middle of the surface projection of fault plane in dip slip and tensile slip cases. In all cases the characteristic horizontal length scales were fault dimensions. Maximum and minimum gravity changes were principally by the magnitude of slip or dislocation. On the other hand, fault size has a much smaller effect upon them. According to results obtained from the analysis, coseismic gravity changes showed a high dependency to fault slip above rupture surface of the fault; however it showed the least sensitivity to the fault length as well. Therefore, this model was not an appropriate tool to determine the fault length. Analyzing the coseismic gravity changes revealed a strong dependency on the dip angle of the fault plane. Observation points with large gravity changes also showed a large variability as the dip angle of fault varied. The area over the rupture plane was the one where the largest gravity changes occured. Therefore, surface measurements in this area were the most suitable to ascertain the most likely value for the dip angle. In the analysis of the coseismic gravity changes it was found that, on average, deviations from a reference model were large above the rupture plane when varying the upper locking depth of the fault. on the other hand, varying the elastic half-space density led to small differences, in general. It means that coseismic gravity change analysis shows a smallsensitivity to the elastic half-space density. This, in turn, indicates that coseismic gravity measurements are not recommendable for trying to ascertain an accurate value for this parameter. This model does not show any sensitivity to Lame coefficients for Poisson Solid. Earthquake parameter determination specially dip angle and upper locking depth using Multi-purpose Physical Geodesy and Geodynamics Network of Iran (MPGGVI) was an important applicable result of this research. Densifying of this network in seismic zones of Iran is recommended for better inverse problem solution using these network observations.Sensitivity analysis of Soldati (1998) model for viscoelastic half-space is recommended. The results of this analysis could be used for fault parameters determination by gravity network set up in postseismic mode.