Examining the Causes of Low-frequency Hybrid Earthquakes During Dike Intrusions in the Afar Rift, Ethiopia

Abstract:

Between 2005 and 2012, there were 14 large dike intrusions into the Dabbahu rift segment in the Afar rift, Ethiopia. Swarms of earthquakes with local magnitudes between 1.45<ML<4.7 occurred during the magma intrusion sequence, and hundreds of faults had visible surface displacements. Spectral analyses of the swarms accompanying the November 2007 and October 2008 diking events found 173 earthquakes with more low-frequency content and longer codas than expected for typical volcano-tectonic events. These unusual earthquakes were classified as low-frequency hybrid events based on peak frequency and percent of energy below 2 Hz.

Previous studies in other volcanic regions have found similar events, which has led to debate about what causes these events and how best to classify them. Explanations for hybrid events include both source and path effects, though the results from previous work and this study suggest that the Afar hybrids are largely a result of path effects based on high attenuation (Q ~ 200) and azimuthal dependence of spectral content. However, large (~3m) surface displacements on short faults indicate that unusual source processes, such as slow rupture times, may also be a factor in these hybrid events. 

The aims of this study are to distinguish between path and source effects, to characterize the source processes of these events, and to explore the relation between hybrid and normal tectonic events in the region – are the differences in the source or only in the path? For closely located earthquakes, an Empirical Green's Function approach is a great method to isolate the source-time function. Spectral analysis of the source-time function can be used to provide insights into the rupture time, stress drop, and scaling relations of the earthquakes. These results will be used to further refine earthquake classifications and determine if there are any defining characteristics of the classes that associate them with specific faulting processes, such as surface ruptures, that occur during diking events.