Auxiliary Material for Paper 2010GC003434

Seismicity during lateral dike propagation: Insights from new data in the recent Manda Hararo-Dabbahu rifting episode (Afar, Ethiopia)

R. Grandin
Ecole Normale Superieure, Paris, France
Institut de Physique du Globe de Paris, Paris, France

E. Jacques and A. Nercessian
Institut de Physique du Globe de Paris, Paris, France

A. Ayele
Institute of Geophysics, Space Science and Astronomy, Addis Ababa University, Addis Ababa, Ethiopia

C. Doubre
Institut de Physique du Globe de Strasbourg, Strasbourg, France

A. Socquet
Institut de Physique du Globe de Paris, Paris, France

D. Keir
National Oceanography Centre, University of Southampton, Southampton, UK

M. Kassim
Geophysical Observatory of Arta, Arta, Djibouti

A. Lemarchand and G. C. P. King
Institut de Physique du Globe de Paris, Paris, France


Grandin, R., E. Jacques, A. Nercessian, A. Ayele, C. Doubre, A. Socquet, D. Keir, M. Kassim, A. Lemarchand, and G. C. P. King (2011),
Seismicity during lateral dike propagation: Insights from new data in the recent Manda HararoDabbahu rifting episode (Afar, Ethiopia),
Geochem. Geophys. Geosyst., 12, Q0AB08, doi:10.1029/2010GC003434.

Introduction

Auxiliary material for this paper consists of three figures and one table providing details on the inversion of earthquake locations using S-P phase delays (Sections 2.2 and 2.3).
Figure S1 shows the gradient velocity model used for the probabilistic inversion. The layered velocity model used for Hypo71 inversions is also shown for comparison. Figure S2
shows the ellipses of uncertainty on earthquake locations deduced from the probabilistic inversion of S-P phase delays. Figure S3 shows the earthquake locations deduced from
the inversion of S-P phase delays, compared to the locations determined with Hypo71. Table S1 provides information about the geometry of the network, and the distances between
stations and dike-induced earthquake swarms.

1. 2010gc003434-fs01.eps
Figure S1. Velocity models used in this study. Model A was used for locations with Hypo71 [Lee and Lahr, 1975], and was derived from profile V by
Berckhemer et al. [1975]. Model B, a smoothed version of Model A, was used for locating earthquakes based solely on S-P phase delays at three stations
(SM1, SM2 and SM4) with the probabilistic approach of Tarantola and Valette [1982]. S-wave velocity is deduced using a Vp/Vs ratio of 1.77.


2. 2010gc003434-fs02.eps
Figure S2. Uncertainty on earthquake locations determined by inversion of S-P phase delays, assuming that earthquakes occur at a fixed depth of 5 km.
The ellipses demarcate the limit of the region where the earthquake is expected to have occurred, with the size of the region depending on the confidence
in the estimation of eathquake location (black: 50%; grey: 90%). In a, the geometry of the network is shown, together with the location of the three dikes
studied here (note that dike d11, which overlapped with dike d10, has been shifted toward the NE to enhance clarity). b, c and d show each dike in detail.
Thick color lines represent the traces of the dikes at the surface, deduced from InSAR [Grandin et al.,2010b]. Different earthquakes yield different sizes
of the ellipses because errors on phase pickings are not homogeneous within the catalogue. The shift of earthquake locations toward the SW at the southern
end of the July 2008 dike likely results from poorly resolved phase pickings at SM1, whereas the epicentral distance relative to less distant stations SM2
and SM4 is better constrained.


3. 2010gc003434-fs03.eps
Figure S3. Comparison of the epicentral locations determined with Hypo71 (grey x) versus locations determined solely with S-P phase delays (black +).


4. 2010gc003434-ts01.pdf
Table S1. Station-epicenter distances (Delta_SMx) and largest azimuthal gap between azimuthally adjacent stations (GAP) for the three dikes presented in this study.


