relocation of the 31 march 2011 earthquake in the ...

Eritrean Journal of Science and Engineering Vol. 1, No.1, May 2014, pp.93-102

ISSN 2312-0118

RELOCATION OF THE 31 MARCH 2011 EARTHQUAKE IN THE NABRO VOLCANIC RANGE OF SOUTHERN ERITREA GHEBREBRHAN OGUBAZGHI & BERHE GOITOM Department of Earth Sciences Eritrea Institute of Technology, P.O. Box 12676, Asmara, Eritrea ALEM KIBREAB Departments of Mines, P.O. Box 272, Asmara, Eritrea JAMES G KING Department of Physics, University of Botswana, Private bag 00704, Gaborone, Botswana (Paper received: 25th September 2013; Paper accepted after revision: 20th March 2014)

ABSTRACT On 31 March 2011, a 4.5 magnitude earthquake hit the eastern part of the Afar Triangle, and was located by the United States Geological Survey (USGS) at 13.17º N, 41.83º E, some 25 km southeast of Nabro volcano in southern Eritrea. A field trip was carried out on 15 May 2011 in Nabro and its surrounding areas to investigate the event. This work revealed a maximum intensity of VII (in the modified Mercalli scale) inside the caldera of Nabro volcano where damages to housing and ground cracks were observed. No such features were observed in the USGS epicentral region. As local seismic data for the region are sparse (the nearest permanent seismic station is about 200 km away), and given the preliminary nature of the USGS epicentral location, intensity data were used to constrain the location of the event at the point where maximum intensity was observed, which is located at 13.33º N, 41.68º E. The significance of the relocation is reviewed in the context of the eruption of Nabro volcano on the night of 12 – 13 June 2011, suggesting that the earthquake might have initiated the eruption process.

Keywords: Nabro Volcanic Range, Earthquake intensity, epicentre relocation, macro-seismic data, Danakil Block.

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Corresponding author: Ghebrebrhan Ogubazghi ([email protected])

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Ghebrebrhan Ogubazghi, Berhe Goitom, Alem Kibreab & James G King 1. INTRODUCTION The Afar Triangle (AT), shown in Fig.1, is the area where the Red Sea, the Gulf of Aden, and the East African Rift System (EARS) meet Barberi et al [4]. This is a zone of lithospheric extension where the Arabian, Nubian, and Somali plates are moving away from each other (Chu and Gordon [6] and McClusky et al. [13]). The region shows abundant evidence of geologically recent seismic and volcanic activity. Prominent during the last half-century have been the 1969 earthquake sequence at Sardo in central Afar (Dakin et al. [7]), the 1989 earthquake sequence in the Dobi graben east of Sardo (Sigmundsson [15]), the 1993 Bada earthquake swarm of southern Eritrea (Ghebreab and Solomon [10] and Ogubazghi et al.[14]), the 2002 earthquake sequence of northern Afar (Ayele et al. [1]), the 2005 volcanic and seismic activities in the same region (Wright et al. [18] , Ayele et al. [2] , Ebinger et al. [8]), and some other events (Hagos et al. [11]). The Erta’Ale volcano in northern Afar is also continuously active with a permanent lava lake (Barberi and Varet [5]).

Fig. 1: Regional map of the AT and surrounding areas. The AT is bordered by the Nubian block, the Somali block, the Red Sea, and the Gulf of Aden. The area bounded by the rectangle in the DB encompasses the Nabro Volcanic Range and is detailed in Fig. 7. AD stands for Afar Depression.

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Relocation of The 31 March 2011 Earthquake In The Nabro Volcanic … On 31 March 2011, an earthquake of magnitude 4.5 hit the eastern boundary of the Afar depression within the Danakil Block, in southern Eritrea. Preliminary location parameters - origin time 18:33:38 UCT; epicentral coordinates 13.17º N, 41.83º E have been published in the Preliminary Determination of Epicentres (PDE) of the USGS. This epicentral location is about 25 km south east of Nabro volcano. The European-Mediterranean Seismological Centre (EMSC) has also provided coordinates of 13.11º N, 41.98º E, and an attributed magnitude of 4.6.

On 12/13 June 2011, about ten weeks after the March seismic event, the hitherto dormant Nabro volcano erupted at about midnight local time (UCT + 3 hours). Before the eruption, on 15 May 2011, a field trip had been made to the area to investigate the March earthquake. The macro-seismic evidence collected during this brief survey pointed to a revised epicentral location of the event, which further indicated that the earthquake was involved in the eruption initiation process. 2. NABRO AND ITS LOCATION Nabro volcano is located in the Danakil Block (DB), or as it is also called Danakil horst or Danakil microplate, along the eastern margin of the Afar Depression (Fig. 1). Among several publications dealing with the geology and kinematics of the DB are those of Chu and Gordon [6], Eagles et al. [9], Lahitte et al. [12], and McClusky et al. [13]. Compared with the floor of Afar Depression, the DB in general has low seismicity (Chu and Gordon [6], and Ayele et al. [1]). However, the Dubbi volcanic eruption of 1861 (Wiart and Oppenheimer [16]), which is the largest volcanic eruption in the African continent in recent times, occurred about 25 km northeast of Nabro volcano in the DB. Both eruptions have occurred on the Nabro Volcanic Range (Wiart and Oppenheimer [17]), which extends from eastern Afar north-northeast wards to the Red Sea coast (area enclosed inside the rectangle of Fig.1).

Unlike active volcanoes, which are mostly closely monitored using a variety of instruments, there were no ground based monitoring instruments on the vicinity of Nabro volcano when it erupted. The nearest permanent seismic stations are located in Yemen and the Republic of Djibouti, and they are more than 200 km away. To the west of Nabro volcano, the stations are very sparse and the nearest permanent station is more than 400 km away. After the eruption of 31 March 2011, 11 broadband 95

Ghebrebrhan Ogubazghi, Berhe Goitom, Alem Kibreab & James G King seismographs were installed in the area, 8 of which are in or in the immediate vicinity of Nabro, which were expected to give more information about the post eruption processes. An international team also surveyed the area during October 2011 and took samples. Given these circumstances, it has been necessary to use macro-seismic data to constrain the location of the epicentre of the 31 March 2011 event. The observations made during the field trip are elaborated below.

3. FIELD OBSERVATIONS The field observations for the 31st March 2011 earthquake were made about six weeks after the event. Unfortunately, rain during the intervening period had blurred or erased some of the ground damage made by the earthquake. However, the local community were able to show the field team where such blurring or erasure had taken place. Three ground cracks were still preserved at one location (see Figs. 2a, 2b). They were simple tensional openings with no vertical or horizontal strike-slip displacements. The stone walls of some houses had tumbled (Fig. 3). Away from the cracks, a hole some half a meter deep was observed (Fig. 4). From the top of a small hill above the village of Sireru, boulders had been dislodged by the tremor and rolled to the bottom of the hill (Fig. 5). The positions of these observations, all in the western side of the Nabro caldera, are shown in Fig. 6. The village of Sireru extends from point AD to DM.

Fig.2: (a) One of three cracks observed on the ground. Although rain has covered most of the segments, signs of deep cracking were visible. This crack was visible for more than 10 meters length, although the inhabitants claimed that before the rain its length was several tens of meters. (b) The lengths and orientations of the three observed cracks. 96

Relocation of The 31 March 2011 Earthquake In The Nabro Volcanic …

Fig.3 : Several poorly built walls of buildings of the village, like the one shown, have been damaged. Better built walls, like the one shown behind the damaged wall, only sustained cracking.

Fig.3

Fig.4

Fig.4: A hole that might have resulted from the collapse of some sort of cavity/vent below the sediment

Fig. 5 Fig.6 Fig. 5: Scene of dislodged stones. Note the big boulders towards the bottom of the hill and the traces they have left behind. Fig.6: Location of the field observations inside the Nabro caldera. Shown are the locations of the cracks (CR), the damaged houses (DM), the adopted epicentre (AEP), the observed hole (HO), the bottom of the hill where stones were dislodged (HL), and the office of the village administrator (AD). North is in the vertical direction. International Space Station photograph

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Ghebrebrhan Ogubazghi, Berhe Goitom, Alem Kibreab & James G King

All of the damaged houses were built on sedimentary infill on the caldera floor. Those built with non-mortared stone walls almost all suffered severe damage and collapse. Those with mortared walls suffered only varying degrees of cracking.

The 31 March 2011, earthquake struck at 21:34 (local time) when the villagers were asleep. According to the villagers, the shaking was so strong that everybody woke up and ran out of their houses for safety. They said that during and after the earthquake there was a roaring sound coming from the direction of the location indicated as RN in Fig. 6. In these remote regions, with no electricity, people sleep early, and as a result the March 31 event was felt only by people in the Sireru area itself, where the ground and house damages also were restricted.

The Nabro volcano area is rugged and with very limited access by road. It is also located in a very sensitive border area where free movement of the field team was not possible. As a result, ground surveys and interviews with inhabitants of this sparsely populated area were restricted. Easier access was available to pastoralists moving more freely through the wider region. No reports were obtained of any earthquake shakings or damage outside of Sireru and the Nabro volcano.

We obtained an estimated maximum intensity value, on the modified Mercalli scale, of VII in the Sireru area. The intensity values in the surrounding villages, including Maebele and Wade (Fig. 7, MA and WA), are estimated not to have exceeded IV as people there were not awakened from sleep.

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Fig. 7: The Nabro Volcanic Range and its surroundings. The EMSC, the USGS, and the adopted epicentres (AD) are shown using red circles. The villages of Maebele (MA) and Wade (WA) are indicated with red Xs. The red cross indicates the Nabro reference point. The volcanoes shown are: Dubbi (1), Mabda (2), Nabro (3), Mallahle (4), Bara’Ale (5), and Sork’Ale (6).

4. DISCUSSION AND CONCLUSIONS The epicentres derived by USGS and by EMSC are closer to the villages of Wade and Maebele (Fig. 7) than to Sireru (the distances involved are given in Table 1). However, our field data yield a maximum seismic intensity for Sireru, and not Wade or Maebele. This relocates the epicentre to Nabro caldera itself, the site of the major volcanic eruption which followed two-and-a-half months later. Table 1: Coordinates of locations and places mentioned in the text and their aerial

distances from Nabro volcano. Place Nabro Reference point USGS epicenter EMSC epicenter Wade village Maebele village Adopted epicenter

Latitude Longitude (degrees) (degrees) 13.38 41.71 13.17 13.11 13.18 13.25 13.33

41.83 41.98 41.90 41.75 41.68 99

Arial distance from Nabro (km) 0 26 41 30 15 6

Ghebrebrhan Ogubazghi, Berhe Goitom, Alem Kibreab & James G King

Intensity data have their limitations, especially for sparsely inhabited areas like Nabro. Consideration must also be given to the varying characteristics of the propagation path through which seismic waves travel from the source to the final receivers. All three contribute (possibly in varying manners) to the intensity values at various locations. Bakun and Wentworth [3] state that the intensity value at a particular location is a function of the earthquake source (magnitude, epicentre location,

source directivity,

...),

propagation path (attenuation and

scattering,

reflections from the Moho and other layers, …), site amplification (due to material properties of site geology, topography, and soil types), and the response of the receiver (various sensitivities of human beings, and various types of structures). Following the above reasoning, and due to the preliminary nature of the USGS and EMSC locations, it is reasonable to relocate the epicentre of the 31 March 2011 event in the location of highest intensity. This is inside the Nabro caldera in the environs of Sireru village. Using simple averaging, we adopt the midpoint between the locations where the cracks and the hole appeared, at about 13.33º N, and 41.68º E. This value may be updated if and when better instrumental or other data are eventually obtained.

The adopted epicentre lies close to the area where the villagers reported hearing the roaring sound, suggesting that the 31 March earthquake may be part of the eruption initiation process, although the details are not clear at present. The data from the survey made by the international team and from the newly installed seismic instruments, together with those from the surrounding seismological and other instruments, may provide a better picture of the eruption.

ACKNOWLEDGEMENTS

We acknowledge the kind cooperation we received from the members of the Southern Red Sea Region Administration and to the Sireru village administration during the field trip. Special thanks go to Kibrom Nerayo, Andemichael Solomon, Zerai Berhe, and Ahmed Mohamed. We are most grateful to Paul Mohr and an anonymous reviewer for reviewing the manuscript and giving valuable suggestions. We also thank Raeesi and J. Hammond for providing the base maps for Figures 1and 7, respectively.

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