We have derived a
local magnitude ML 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 ML 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.