Insights Into Fault-Magma Interactions in an Early-Stage Continental Rift From Source Mechanisms and Correlated Volcano-Tectonic Earthquakes

Strain in magmatic rifts is accommodated by both faulting and dike intrusion, but little is known of the frequency of dike intrusions in early-stage rifts. We use a new earthquake data set from a dense temporary seismic array (2013-2014) in the 7-Myr-old Magadi-Natron-Manyara section of the East African Rift, which includes the carbonatitic Oldoinyo Lengai volcano that erupted explosively in 2007-2008. Full moment tensor analyses were performed on M > 3.4 earthquakes (0.03- to 0.10-Hz band) that occurred during the intereruptive cycle. We find two opening crack-type and various non-double-couple earthquake source mechanisms and interpret these as fluid-involved fault rupture. From waveform analysis on the nearest permanent seismic station, we conclude that similar rupture processes probably occur over eruptive and intereruptive cycles. The repeated and dynamically similar fluid-involved seismicity, along with intrabasinal localization of active deformation, suggests that significant and persistent strain is accommodated by magmatic processes, modulated by tectonic cycles.