Macroseismic analysis and the determination of a focal mechanism of the 31 October 2019, KwaZulu-Natal earthquake in South Africa

Abstract

An earthquake (magnitude, ML = 3.8) occurred on 31 October 2019 at 11:19 hours Greenwich Mean Time (GMT) in the Sunduza area of the KwaZulu-Natal (KZN) Province in South Africa. The earthquake was located near the broad fracture zone of the Ntlakwe-Bongwan fault and was felt along the eastern coast of South Africa. The effects of the event were assessed by conducting macroseismic investigations through interviewing members of the public and completing questionnaires. Analysis of all the collected macroseismic data showed that maximum shaking with Modified Mercalli Intensity of IV-V and V were experienced near the epicentral area as well as along the coast. Similar intensity values were also experienced in some parts of Durban about 100 km northeast of the epicentre. Shaking at such long distances was attributed to ground motion amplification due to site effects. A fault plane solution of the event obtained using both the first motion polarities and amplitude ratios showed normal faulting along a fault oriented in a strike of 185.0°, dipping at 68.0° with a rake of -52.0°. The strike of the focal mechanism solution aligns with the strike of the Ntlakwe-Bongwan fault zone and nearby faults suggesting that the event might have ruptured along one of the fracture segments of this fault mapped to the north. The obtained dip is also in line with a high angle fault comparable to that observed along the Ntlakwe-Bongwan fault. The epicentral region is highly faulted although the activity is not known.