Characteristics of seismicity in Eritrea (2011-2012): Implications for rifting dynamics

Friday, 18 December 2015
Poster Hall (Moscone South)
Berhe Goitom, University of Bristol, Bristol, United Kingdom, James O S Hammond, Imperial College London, London, United Kingdom, Michael J- Kendall, University of Bristol, School of Earth Sciences, Bristol, United Kingdom, Ghebrebrhan Ogubazghi, Eritrea Institute of Technology, Asmara, Eritrea, Derek Keir, University of Southampton, Southampton, United Kingdom, Atalay Ayele, Addis Ababa University, Institute of Geophysics, Space Science, and Astronomy, Addis Ababa, Ethiopia and Finnigan Illsley-Kemp, University of Southampton, Southampton, SO14, United Kingdom
Eritrea hosts the final stages of on-land East-African rifting, yet questions remain about how rifting transits from the Afar Depression to the Red Sea. In this study, we use data from recent deployments of 6 broadband seismometers in Eritrea together with deployments in Ethiopia to locate seismicity and determine the current focus of strain. Over 1000 events have been located with local magnitudes 0.7-5.0. A significant period of seismicity was observed on 1 July 2012 around Nabro volcano and is associated with the biggest event of mL 5 preceded by 33 events in the previous two days. It may be related to magma movement below Nabro. Other significant seismicity was observed on 25 December 2011 and is correlated with an earthquake of mL 4.2 and associated with 13 other events on the same day. This event is located around Hayli Gubbi and Ale Bagu volcanoes and could be related to the activities around these volcanoes. We use double difference relocations to improve accuracy and show two main clusters of seismicity, one oriented NW-SE in the Bada-Alid axis along the north-western boundary of the Danakil microplate and the other NE-SW, following the trend of the Biddu-Nabro volcanic complex. Our new estimates of seismicity demarcate the boundary between the Nubian, Somalian, and Danakil Microplate and suggest that the Danakil microplate may be broken in two along the Biddu-Nabro Volcanic complex. We estimate b-values for the different clusters of events and show that close to the major border faults near Massawa, average b-values are lower (0.65) than that found near the volcanic centres (1.2 – Bada-Alid, 0.81 – Biddu-Nabro). This may indicate that the stress is less in the volcanic regions and the seismicity is due to movement of magma fluids and strain is accommodated by the injection of magma. In contrast the earthquakes around Massawa occur in relatively stronger rocks suggesting strain may be accommodated by movement on larger faults.