Variations in teleseismic body-wave attenuation along the East African Rift

The East African Rift stretches from the Tanzanian craton to the Gulf of Aden and Red Sea. Low seismic wave speeds are observed beneath the Main Ethiopian Rift (MER); such observations are often attributed to elevated mantle temperatures. However, petrological constraints from MER lavas indicate a thermal anomaly of ~140 C, accounting for ~2% of the total ~6% decrease in Vp. In addition to temperature, seismic wave speeds can be impacted by the presence of melt, and variations in mantle composition, water contents, grain size, and oxygen fugacity. Seismic wave speeds alone cannot discriminate between these many factors, however seismic attenuation can provide additional constraints. Here, we produce a regional model of attenuation for the East African Rift using a waveform-matching approach on teleseismic P-wave data to assess the contributions from the presence of melt to the observed geophysical anomaly. Our attenuation measurements use data collected from more than 250 previously deployed stations, both temporary broadband seismic arrays and permanent broadband stations, utilizing P-wave arrivals from deep focus (>200 km depth) teleseismic events from 30 to 90 degrees epicentral distance and magnitudes >5.5 Mw. To compensate for limited temporal overlap among the various array deployments, long-running stations are used to calculate event statics. We calculate t*, the relative changes to the integrated effect of attenuation over the raypath, using a time-domain method that is more robust to confounding factors such as energy scattering compared to the traditional spectral-slope approach, and apply a linear inversion to each measurement to produce a smoothed 2D attenuation model for the study area. Initial results show a maximum t* of 0.1 - 0.14 seconds under the Afar Depression and t* of 0.01 - 0.04 seconds under the southern portion of the Kenyan Rift. The t* is lower under the Western Rift Valley (-0.05 to -0.08 seconds). The range of t* observed here (0.19-0.22 with a peak in the Afar Depression) is similar to t* under the Salton Trough (California, USA), where it is thought that the presence of melt attenuates seismic waves. The similarity in t* between the Salton Trough and the Afar Depression may indicate a role for the presence of melt in producing the prominent geophysical anomaly under the MER.