Distance attenuation and local magnitude scale based on constant geometrical spreading in Northern Punjab, Pakistan

Development and calibration of distance attenuation curve for local seismic data are requisite for better seismo-tectonic modeling and seismic hazard estimation. Local magnitude scale developed for Northern Punjab (Potwar Plateau and Salt Range) has been tested against different geometrical spreading values. Trade-off or linear inter dependence has been observed between local magnitude scale parameters. To address this issue, either nonparametric approach with proper smoothing factor or parametric approach with single parameter ( a ) as constant may be used. In this study, we fixed geometrical spreading parameter to eliminate parameters dependency and hence reduction in error was achieved. Geometrical spreading was observed to be distance-dependent factor rather than a constant parameter. A variance reduction of 11%, with zero error in anelsatic attenuation parameter was achieved by fixing geometrical parameter and performing parametric inversion. The new magnitude scale with distance correction factor is given by the expression; - log A o = log ( R ) + 0.0006 ( R ) - 1.7419 , where R is hypocentral distance, valid for R < 130 km and depth < 30 km. Lower attenuation (i.e., b = 0.0006 ) was observed in new magnitude scale as compared to previously developed magnitude scale (i.e., b = 0.00115 ) for the same area. The pronounced lower attenuation of seismic waves beneath the intraplate region of Northern Punjab is apparently owes to regional tectonics. Northern Punjab is relatively stable part of the Indian shield with Precambrian basement rock overlaid by varying thickness of salt diapirs and thick sedimentary cover.