Ultraslow spreading mid-ocean ridges with full spreading rates up to 20 mm/yr are described as the melt poor endmember of the entire mid-ocean ridge system. The melt supply along ultraslow spreading ridges is uneven resulting in the formation of volcanic centres and amagmatic segments. Amagmatic segments show thicker brittle lithosphere of up to 30 km, whereas magmatic segments have much thinner lithosphere of less than 15 km. It is supposed that melt travels along the lithosphere - asthenosphere boundary from amagmatic segments to magmatic events, where it can reach the seafloor and erupt. These spreading events are rare at ultraslow spreading ridges compared to faster spreading ridges and in situ observations hardly exist. During an ocean bottom seismometer (OBS) experiment at the eastern Southwest Indian Ridge two earthquake swarms were accidentally recorded. The swarms occurred in January and April 2013 and both lasted for a few days. In this thesis, the events of the earthquake swarms were relatively located with HypoDD for better spatial resolution. I created earthquake catalogues to study the characteristics of the earthquake swarms. The results allowed for studying active spreading processes at an ultraslow spreading ridge. The earthquakes occurred in depths, where the magma chamber of the nearby Segment 8 volcano is located. This magma chamber potentially fed a sill intrusion, which was recorded as earthquake swarms. During the first hours of the earlier earthquake swarm a migration pattern was identified. The hypocentres migrated away from the Segment 8 volcanic centre and slightly downwards. Later events occurred more randomly in the active area. Horizontal intrusion can be favoured due to rotation of stresses in extensional settings and rigidity contrast between peridotite and serpentinised peridotite. Simultaneously with the swarms seismic tremor was recorded at the station closest to the swarm locations. The tremor lasted longer for the shorter earthquake swarm in April. Amplitude and frequency of the tremor signal are modulated with a 12 hour period. I therefore speculate that the heat of the intrusion drove hydrothermal activity and fluid flow modulated by the tides produced the tremor signal.