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Local Earthquake Magnitude Scale and Seismicity Rate for the Ethiopian Rift

¹ Department of Geology
Royal Holloway University of London
Egham, Surrey, TW20 0EX, United Kingdom
d.keir{at}gl.rhul.ac.uk
 (D.K.)
² School of Earth and Environment
University of Leeds
Leeds, LS2 9 JT, United Kingdom
graham{at}earth.leeds.ac.uk
 (G.W.S., A.J.)
³ Geophysical Observatory
Addis Ababa University
Addis Ababa, P.O.B. 1176, Ethiopia
atalay{at}geobs.aau.edu.et
 (A.A.)

^* Present address: Institut fur Geophysik, ETHZ Honggerberg, CH-8093 Zurich, Switzerland: andrew.jackson{at}erdw.ethz.ch.

A calibrated local earthquake magnitude scale is essential for quantitative analyses of seismicity. In Ethiopia, effective monitoring of earthquakes and resulting assessment of seismic hazard are especially important as regions with seismic and volcanic activity coincide with regions of economic significance and population growth. We have developed a local magnitude (M_L) scale for the northern Main Ethiopian rift (MER) using earthquake data collected during 2001–2003 on 122 three-component broadband seismic stations. Waveform data from 2139 local earthquakes were corrected for instrument response and convolved with the nominal Wood–Anderson torsion seismograph response appropriate for the original definition of local magnitude. The hypocentral distances considered are 5 to 800 km, with the best represented range from 5 to 150 km. A total of 30,908 maximum zero-to-peak amplitudes (A_WA) were incorporated into a direct linear inversion for individual earthquake local magnitudes (M_L), 244 station factors (C), and 2 linear distance-dependent factors (n, K) in the distance correction term, log (A_o), of the equation for local magnitude: M_L = log(A_WA) – log(A_o) + C. The resulting distance correction is given by –log(A_o) = 1.196997log(r/17) + 0.001066(r – 17) + 2, which implies that ground-motion attenuation is relatively high, consistent with ongoing magma intrusion and the presence of shallow magma reservoirs beneath the MER. Station corrections significantly reduce M_L residuals and range between ±0.42 M_L units. The catalog of earthquakes is complete above M_L 2.1 and the annual cumulative seismicity rate follows the relation log N = 4.5 – 1.13 M_L. Our results are critical for accurate routine quantitative analysis of past, current and, future seismicity in Ethiopia.