Anomalous earthquakes associated with Nyiragongo Volcano: Observations and potential mechanisms

A series of five unusual earthquakes (4.6 ≤ M ≤ 5.3) has been recently located near Nyiragongo volcano (D. R. Congo) in the Western Rift Valley of the East African Rift. e their moderate size, these earthquakes do not appear in global seismicity catalogs, but were located using long-period surface waves primarily recorded on the Global Seismographic Network. Three of the events occurred in the week following the January 2002 eruption of Nyiragongo, while the final two occurred in 2003 and 2005 respectively, and are not linked to a major eruption at Nyiragongo or its neighboring volcano, Nyamuragira. Several common techniques were used to investigate the characteristics of these seismic sources in the context of the volcanic activity of the region. The frequency content of the five anomalous earthquakes was compared to that from local events found in global catalogs, and the newly detected earthquakes were shown to be slow events, being greatly depleted in frequencies above 0.1 Hz. Each of the newly detected earthquakes was modeled by a series of forces and by a centroid-moment tensor. A deviatoric moment tensor was shown to provide a better fit to the data. The newly detected earthquakes are highly non-double-couple in nature, each having a large compensated-linear-vector-dipole component of the moment tensor. Drawing on models based on similar observations from other active volcanoes, we propose that the earthquakes are caused by slip on non-planar faults beneath the volcano. We suggest a mechanism in which these newly detected earthquakes are generated by the collapse of the roof of a shallow magma chamber along an inward-dipping cone-shaped ring fault. Diking events, which result in magma evacuation from shallow magma chambers, could trigger such earthquakes. Our results provide new constraints on the dynamics of the poorly understood magma system beneath Nyiragongo, an active volcano that is a significant threat to life and property.