Using Low-Frequency Volcano-Tectonic Earthquakes to Probe the Distribution of Melt During Large-Scale Dike Intrusion Events in Afar

New broadband seismic data from the 2005-present Dabbahu, Afar rifting event offer unprecedented insights into the influence of magmatic fluids in the production of low frequency volcano-tectonic earthquakes. We analyze persistent low-frequency earthquakes recorded on a temporary array deployed three weeks after the intrusion of a 60 km-long, 8 m-wide basaltic dike. The dike intruded and reheated existing silicic magma chambers beneath and surrounding Dabbahu volcano and Gab'ho volcanoes. The multiple magma chambers and the large volume basaltic intrusion provide a diverse data set of volcano-tectonic events, and the complementary seismic, geodetic, and structural data sets provide constraints on the source and path effects. We analyze the spectral characteristics of near source low-frequency earthquakes, and their spatial and temporal distribution to differentiate between competing models for their generation. Earthquakes from the three months following the intrusion are classified using their peak intensities in the frequency domain and by percentage of power below 2Hz. We notice non-uniform increases in relative low frequency power based on distance from station and wave propagation direction. Stations north of Dabbahu volcano detect earthquakes with extreme attenuation at frequencies above 10Hz over a distance less than 20km, in many cases less than 5km, whereas the eastern instruments see a lesser change in distribution of power over a distance of over 70km. A small number of the earthquakes require further calculation as they are dominated by low frequency power at all sites and are possibly due to a different type of mechanism entirely. Our data implies bodies that strongly attenuate high frequencies near and around Dabbahu volcano. The large impedance contrasts to explain our observations suggest the presence of melt in the crust from the initial dike intrusion. This study provides another approach to modeling the complexities in the Afar region and offers a unique chance to characterize the nature of low frequency events in a zone of sea floor spreading.