Beginning on 30 June 2021, hundreds of earthquakes were detected beneath Semangko Bay in southernmost Sumatra, which is located adjacent to the Sunda Strait, a narrow sea passage that separates the islands of Java and Sumatra. A number of these earthquakes were large enough to be felt by people living in the city of Lampung, some 100 km to the east. In terms of magnitude and temporal distribution, the earthquakes did not follow a typical mainshock–aftershock sequence because the onset was marked by a cluster of five earthquakes with local magnitudes that ranged between 4.2 and 4.6, followed by a rapid decay in the number of detected events. We have relocated 254 of the 258 earthquakes that were recorded between 30 June and 14 July 2021, with a local magnitude range between ML 0.9 and 4.6, using the double-difference relocation method (hypoDD); focal mechanisms were also determined for a subset of events with a magnitude >4. Our results show that the seismicity pattern and focal mechanism solutions are more consistent with a multiple event episode caused by the rupture of several antithetic faults that have a similar strike to the west Semangko fault in southernmost Sumatra rather than a single fault plane. These faults appear to be part of a small graben system located beneath Semangko Bay, which was likely activated by ongoing extension in the Sunda Strait.