Investigation of macroseismic intensity of the Mw5.9 September 10, 2016 Kagera earthquake: Implications for site effect amplification

Abstract

The Kagera earthquake of magnitude Mw5.9 occurred on September 10, 2016 at 12:27 GMT, 15:27 local time, with an epicenter located northwest of Bukoba town, in Minziro village close to the border of Tanzania and Uganda. The earthquake resulted in the widespread damage within the Kagera region with an officially announced death toll of 17, many residential structures destroyed, and others damaged beyond repair. Following the earthquake, a macroseismic survey was conducted by interviewing various affected people using macroseismc questionnaires to delineate the distribution of seismic energy and establish the effects of ground shaking. The results of the macroseismic survey indicate variations of intensity values from IV-VIII on the MSK-64 scale with more frequently assigned values of intensity range from VI-VII. The orientation of the intensity pattern is elongated towards the southeast in accordance with the rupture directivity towards Bukoba town, which is in agreement with the orientation of the fault rupture from the focal mechanism and the distribution of aftershocks. The duration of shaking in many localities was observed to be over 20 s, resulting in resonance effects. Both findings of the observed variations of the intensity values and the duration of shaking indicate that site effects, which result in the amplification of the seismic signals, likely played a great role in the damage distribution. The maximum intensity of VIII, which corresponds to the instrumental magnitude of 6.2, differs from the instrumental result of magnitude 5.9 by a factor of 0.3; the difference is attributed to the amplification by site effects within the region. The observed site effects therefore emphasize the importance of reinforcing the buildings, and, in the future, adherence to seismic building codes and specifications in all regions within the East African Rift system (EARS). It is also hoped that the database of the intensity data points from macroseismic survey will be useful in the development of the seismic intensity prediction equation for the earthquakes within the region and for the assessment of the impact on regional seismic hazard and the entire EARS.