DETERMINATION OF LOCAL MAGNITUDE SCALE FOR UGANDA

We have derived a local magnitude M_L scale for Uganda using waveform data recorded by a temporary broadband seismic network deployed in Uganda and a permanent broadband station. We used 54 earthquakes recorded between July 2007 and November 2008. First, we determined hypocenters of these earthquakes using P and S phase arrivals. Most of their locations are associated with the western rift of the East African Rift System. We compared the hypocenters of seven earthquakes determined by this study to those reported by NEIC’s PDE catalog and IDC bulletins. They do not differ much, and they are roughly consistent with each other. To develop the M_L scale, we removed instrument responses in the waveforms and then applied the frequency response of the standard Wood-Anderson torsion seismograph for amplitude measurements. We obtained 529 amplitude data from horizontal components of 52 earthquakes whose focal depths are up to 34 km. We performed simultaneous linear inversion to determine the coefficients of distance correction function and local magnitudes to obtain the formula, where A is the maximum peak amplitude (mm) observed on the horizontal component seismogram, and r is the hypocentral distance (km). The coefficients of the above formula are smaller than those obtained for Southern California, and closer to those obtained for Tanzania. Uganda, through the Department of Geological Survey and Mines (DGSM), is in the process of upgrading its seismological monitoring network with modern digital monitoring and data acquisition systems. Therefore, the result of this study and its application to data from the upcoming new seismic network will be useful for improving earthquake monitoring and seismicity study in Uganda.