Development of local magnitude scale for the Northern Punjab, Pakistan

We developed a local magnitude scale for the Northern Punjab (Potwar Plateau and Salt Range), using seismic data of CES (Centre for Earthquake Studies). The region is tectonically stable, but surrounded by active geological structures (i.e., Main Boundary Thrust, Sulaiman Range, and Jhelum Fault). The imparted stresses adjust both seismically (moderate to small magnitude earthquakes) and aseismically in decollement. Previously, Southern California magnitude scale was being used to determine local magnitude for compilation of seismic catalog. Difference in geology and tectonics between Southern California and Northern Punjab may lead to systematic errors in hazard assessment based on this catalog. To address this problem, we selected 231 seismic events for a period of 07 years with M-L 2.0 and above, that correspond to 2800 records on 21 seismic stations. We chose data of seismic events with hypocentral distance less than 600 km and recorded on five stations at least. We then inverted the synthetic Wood-Anderson amplitudes of selected data for station and distance corrections. Accordingly, the new magnitude scale for Northern Punjab is given by the following: M-L = logA +0.869log(r) +0.00115(r) -1.53, where A is the amplitude in nanometers on synthetic Wood-Anderson seismograph and r is the hypocentral distance in kilometers. We observed lower attenuation of seismic waves in our study area as compared to Southern California. Lower standard deviation (i.e., reduction in variance to 45.4% ) in magnitude residuals shows that there is less deviation for newly developed scale as compared to that of Southern California scale. Values of station correction factors for different stations of local network vary between -0.5 to +0.5, which suggest variation in station site effects.