seismo-volcanic crisis in afar: the 2005 boina rupture-eruption … · 2014-01-15 · 1 nerc...

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NERC GEOPHYSICAL EQUIPMENT FACILITY (Seis-UK) PROJECT 0803 SCIENTIFIC REPORT

Seismo-volcanic crisis in Afar: the 2005 Boina

rupture-eruption sequence

Cindy Ebinger*, Derek Keir#, James Hammond^, and Tim Wright#

*Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX; Now at: Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627 #Now at: School of Earth Sciences, University of Leeds, Leeds, LS2 9JT ^Department of Earth Sciences, University of Bristol, Bristol Abstract Continental rupture models emphasize the role of faults in extensional strain accommodation; extension by dike intrusion is commonly overlooked. A major rifting episode that began in September, 2005 in the Afar depression of Ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture. Earthquakes recorded on a temporary seismic array, direct observation of fault patterns, and geodetic data document ongoing strain and continued dike intrusion along the ~60 km-long Dabbahu-Wal’is rift segment. Epicentral locations lie along a ~3 km-wide, ~50 km-long swath that curves into the SE flank of Dabbahu volcano; a second strand continues to the north toward Gab’ho volcano. Two additional dike intrusions in June and July, 2006 indicate a central feeding source for basaltic magmas (Keir et al., 2009; Hamling et al., 2009). We interpret the depth distribution of microseismicity as the dike intrusion zone; the dikes rise from ~10 km to the near-surface along the ~60 km-long length of the tectono-magmatic segment. Focal mechanisms indicate slip along NNW-striking normal faults, perpendicular to the Arabia-Nubia plate opening vector. The seismicity, InSAR, continuous GPS, and structural patterns all suggest that magma injection from lower or sub-crustal magma reservoirs continued at least 3 months after the main episode. Cote et al (submitted) measure the low frequency content of volcano-tectonic earthquakes recorded on a temporary broadband seismic array following the September 2005 dike intrusion and volcanic eruption sequence in the Afar rift, Ethiopia. Stations at all 9 sites show signals with long low frequency codas. Strong variations in signal amplitude, frequency content and coda length are seen even between stations separated by only 2 km. The patterns of seismicity after the 2005 dike intrusion provide a 3D perspective of magma feeding systems maintaining the along-axis segmentation of this incipient seafloor spreading segment.

Background Diking events result in an instantaneous and localized extension marked by intense

swarms of shallow, commonly small magnitude, and sometimes low frequency earthquakes, with or without effusive volcanism (e.g., Einarsson and Brandsdottir, 1980; Rubin et al., 1998a, b). Migrating earthquake swarms and harmonic tremor recorded on seismic arrays, and satellite geodetic techniques allow us to map the vertical and lateral migration of magma during multi-episode, tectono-magmatic episodes, as well as the transient response of the plate to the stresses induced by the dike intrusion itself (e.g., Sigmundsson, 2006). Seismicity accompanies dike intrusion when ambient stress levels are near failure (Rubin et al., 1998). Dikes emanating from shallow reservoirs will solidify over the time periods of propagation (e.g., Fialko and Rubin, 1998), but dikes rising from greater depths, and, higher temperatures, may cool over periods of weeks to months, effectively maintaining an open conduit at depth (Buck et al., 2007).

In September 2005 a tectono-magmatic event of unprecedented scale and intensity occurred along a previously identified segment of the southern Red Sea rift in northern Ethiopia (Fig. 1). The brief event consisted of an ~60 km-long dike intrusion signaled by 163 5.5 > mb > 3.9 earthquakes between 4 September and 4 October (NEIC); models of InSAR data suggest 2.5 km3 of magma were emplaced during the month-long episode (Wright et al., 2006; Ayele et al., 2007) (Fig. 1). The 2005 Afar seismo-volcanic crisis

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provides an opportunity to study the response of a stretched continental plate to the rapid emplacement of a large magma volume (Yirgu et al., 2006; Ayele et al., 2007). Although the event occurred in a region with few permanent settlements and no two-story structure, the scale and duration of deformation are sobering, and require re-evaluation of rift-related hazards.

Wright et al. (2006) analyse the teleseisms and InSAR data from the 2005 activity. Ebinger et al. (2008) report seismicity patterns from 19 October 2005 to 1 April 2006, and a comparison with deformation patterns deduced from field observations (Rowland et al., 2007), InSAR, and GPS data over the same time period. Keir et al. (2009) focuses on the time-space relations of two subsequent dike intrusion episodes in June and July, 2006, and their implications for magma sources as well as dike emplacement processes. Cote et al. We use these results to evaluate models for dike intrusion and their contribution to the creation and maintenance of along-axis segmentation in rifts at the transition from continental to oceanic rifting. These results also form a baseline for ongoing hazard mitigation efforts in Afar.

Data

Between October 19 and 21, 2005 we deployed 6 Güralp 6TD broadband seismometers in or near villages to the east and west of the Dabbahu -Hararo rift segments, and one in the regional capital, Semera (Fig. 1). Three seismometers were installed near the Da’Ure eruption site to monitor what we originally thought was a flank eruption of Dabbahu volcano. An unguarded site at ~12o 18’N was vandalized, limiting the resolution of the array in the southern part of the segment. Instruments recorded at a sampling rate of 50 Hz. High frequency (> 1 Hz) cultural noise was very low at BOOE, BOSE, and BOVE near the vent site; the other sites in villages were noisy during early evening when generators were used (Fig. 3). BOOE, BOVE, and BOSE show up to 20 shallow volcano-tectonic earthquakes and tornillos per hour until January, 2006 when activity became more episodic. The harsh field conditions and rapid deployment led to instrument failures, and a decrease in operational seismic stations over time. Additional data over parts of this time period were acquired from the permanent station DESE maintained by the Geophysical Observatory of Addis Ababa University. The temporary array allows accurate location of low magnitude seismicity not possible from permanent

Figure 1: Earthquakes during April - August 2006 recorded on temporary broadband seismic stations (triangles) and permanent short period stations (squares). Seismic stations are located in named towns. Three additional stations were located between Dabbahu (D) and Gab’ho (G) volcanoes; data from these 3 sites were used to analyze low-frequency EQs (Cote et al., submitted). Stars denote continuous GPS stations. Major volcanic centres are labelled; A= Ado’Ale silicic complex; D = Dabbahu, E = Erta’Ale, F = Fielle, G = Gab’ho. Top right inset shows location of Afar with respect to East Africa and Arabia. From Keir et al. (2009).


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regional stations (ATD, FURI).

Methods and Analyses Arrival times of P and S phases were measured manually on traces filtered using a

Butterworth bandpass filter (1 –15 Hz). Arrival times of P phases were assigned a quality factor of 0, 1, 2 or 3 according to estimated measurement errors of 0.05 s, 0.1 s, 0.15 s and 0.2 s, respectively. S wave quality factors of 0, 1, 2 and 3 were assigned to arrivals with estimated measurement errors of 0.1 s, 0.175 s, 0.25 s and 0.3 s, respectively. A total of 1939 earthquakes were recorded on 4 or more instruments during the period 19 October, 2005 to 1 April, 2006; Keir et al. (2009) analyzed a further 200 events during the period April-August, 2006. Event location, magnitude determination, focal mechanisms, and spectral analyses methods are described in Ebinger et al. (2008), Keir et al., (2009), and Cote et al. (submitted).

Results Persistent seismic swarms at 2 sites on Dabbahu volcano coincide with areas of

deformation identified in the InSAR data: (1) a northwestward-dipping zone of seismicity and subsidence interpreted as a collapse structure above a mid-crustal magma reservoir, and (2) a more diffuse, 8 km-radius zone of shallow seismicity (<2 km), and a largely aseismic zone between 2.5 and 6 km. A second, ~2 km diameter chamber at 4 km subsurface may underlie the Da’Ure vent, the site of a silicic eruption on 26 September, 2005. InSAR and continuous GPS data show uplift above a shallow source in zone (2). The patterns of seismicity provide a working model of magmatic systems maintaining the along-axis segmentation of this incipient seafloor spreading segment. By analogy to the Krafla rifting episode and the rock record in Afar, we have predicted, and seen continued activity in 2006/7, with a fissural eruption at the southern tip of the segment in 2007 (Yirgu et al., 2007).

40! 24" 40! 36" 40! 48"

12! 12"

12! 24"

12! 36"

12! 48"

B

B’

C’

C

D

E

E’

D’

A

A’

Ado ‘Ale

Gab’ho

Dabbahu

Da’Ure

Fig. 4, Ebinger et al.

SF

Figure 2. Earthquake epicenters relocated using the double difference method of Waldhauser and Ellsworth (2000). A-A’, B-B’, C-C’, D-D’ E-E’ mark the locations of topography and seismicity profiles shown in Figures 7-8 of Ebinger et al. (2008). Colours signify temporal variations: red, 19-31 October 2005; green, 1-30 November 2005; blue, 1-31 December 2005; violet, 1-31 January 2006; black, 1 February – 30 March, 2006. Ado ‘Ale, Dabbahu, Gab’ho are silicic volcanoes.


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Figure 3: a) E-W motion recorded at cGPS station DA25. b) Difference in P-wave arrival time of earthquakes located along the axis of he Dabbahu rift at stations Semera and Abala against time Earthquakes are scaled and coloured by magnitude and error bars are estimated errors in arrival time measurement. c) Along-axis topographic profile. d) & e) axes as in b) but for a one day time interval. Histograms show estimated seismic moment release binned at hour e. f) Along-axis topography in the vicinity the Ado’Ale silicic centre. From Keir et al. (2009).

Multiple measurements from the BOOE site show that anisotropy is distributed fairly evenly in the brittle upper crust and anisotropy is generally parallel to tectonic / volcanic structures (Keir et al., in prep.). Receiver functions suggest that the crust thins towards Afdera from a thickness of ~24km beneath Silsa and Teru to ~16km beneath Afdera (Hammond et al., in prep.).

Da’UreSept 05 eruption

Dabbahuvolcano

Gab’hoinflation Oct-Mar

60 km

North

10?

Dabbahu magmatic system:Oct 2005-March 2006

??

?

>1 m-wide basaltic dikes

basaltic dikes ?

?

base of crust

Addo-Alevolcanic chain

Ebinger et al, GJI, 08

BOOEBOVE

Working model of magma migration through the crust beneath the Dabbahu segment, based on seismicity, InSAR, GPS, and structural obser-vations. Topo-graphic relief has been removed for simplification; volcanoes are indicated by black ellipses. Seismo-genic zones interpreted as red dikes; the main dike zone curves into the Dabbahu edifice, and a second dike to the east. Pink ellipses are magma chambers. The location of lower crustal/upper mantle source zones feeding the dikes and magma chambers is loosely constrained by the InSAR data, and subsequent deformation in 2006.


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Conclusions

Dense swarms of seismicity along the 60 km-length of the Dabbahu rift segment characterize the first 7 months of activity recorded on a temporary network after the start of the 2005 Dabbahu seismo-volcanic crisis. Along the length of the segment, focal depths determined using the double difference method lie between 10 km and the surface. Considering the large volume (~2.5 km3) of magma intruded during the September 2005 crisis, we interpret the zone of seismicity as failure on the sides and above the dike injection zone. Focal mechanisms show rift-normal opening. The persistent seismicity, InSAR, continuous GPS, and structural patterns all suggest that magma injection continued at least 3 months after the main episode. Unlike earlier dike intrusions in Krafla and Asal-Ghoubbet (Afar), there is no shallow magma reservoir feeding the dikes; InSAR patterns and the shallowing of seismogenic layer suggest the dike was fed from the base of or beneath the crust, near the centre of the segment at 12o 18N. The deep roots of the dike within the aseismic lower crust may have remained open from September to December, 2005.

Magma is sourced directly to the center of the rift segment from reservoirs in the upper mantle and/or lower crust and delivered into the upper 10-km inducing VT seismicity and associated fault growth. The current locus of diking in the Dabbahu rift correlates with surface volcanic features stable over ~2 My time scales showing the mid-segment magma source zone is spatially and temporally discrete from adjacent rift segments. Observations show the segmented magma supply responsible for the 2nd-order along-axis segmentation of oceanic rifts develops prior to continental breakup, and it is maintained by episodes of basaltic dike intrusion.

Low frequency earthquakes recorded on temporary stations near Dabbahu volcano show large variations in the frequency content as a function of propagation distance, suggesting large inverse scattering and absorption Q coefficients. If the unusual power at low frequencies of hybrid events from the Dabbahu volcano and recorded at nearby stations is due to high scattering and attenuation coefficients, then the prevalence of persistent swarms of low frequency hybrids in volcanic regions is not surprisingly associated with the local volcanic activity Scattering is likely to be stronger when there are strong local impedance contrasts, such as fluid filled cracks, Future work will explore the possibility that a matrix of fluid-filled cracks can account for the large scattering and attenuation in rays passing through basaltic dikes, and so account for the co-existence of low frequency hybrids and long period events with harmonic spectra associated with the magma emplacement process itself.

References

Ayele, A., E. Jacques, M. Kassim, T. Kidane, A. Omar, S. Tait, A. Nercessian, J-B de Chabalier, G. King, 2007. The volcano-seismic crisis in Afar, Ethiopia, starting September, 2005, Earth Planet. Sci. Letts., 255, 187-197.

Buck, R., P. Einarsson, & B. Brandsdottir, 2007. Tectonic stress and magma chamber size as controls on dike propagation: Constraints from the 1975-1984 Krafla rifting episode, J. Geophys. Res., 111, doi:10.1029/2005JB003879.

Doubre, C., I. Manighetti, L. Dorbath, C. Dorbath, E. Jacques, J-C. Delmond, 2007a. Crustal structure and magmato-tectonic processes in an active rift (Asal-Ghoubbet, Afar, East Africa); 1. Insights from a 5-month seismological experiment, J. Geophys.


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Res., doi: 10.1029/2005JB003940. Einarsson, P., 1991. The Krafla rifting episode 1975-1989, in Náttúra Myvatns, (The

Nature of lake Myvatn), edited by A. Gardarsson, and Á. Einarsson, pp. 97-139, Icelandic Nature Science Society, Reykjavíc.

Rubin, A., D. Gillard, J-L. Got, 1998a. Re-interpretation of seismicity associated with the January 1983 dike intrusion at Kilauea volcano, Hawaii, J. Geophys. Res., 103, 10003-10015.

Rubin, A., & D. Gillard, 1998b. Dike-induced earthquakes: Theoretical considerations, J. Geophys. Res., 103, 10017-10030.

Ruegg, J.-C., 1975. Structure profonde de la croute et du manteau superieur du Sud-Est de l’Afar d’après les données sismiques, Ann. Geophys. 31, 329– 360.

Sigmundsson, F., 2006. Iceland Geodynamics: Crustal Deformation and Divergent Plate Tectonics, Springer-Praxis, 228p.

Yirgu, G., 2007. The August 2007 fissure eruption on the Dabbahu rift segment, Active Volcanism and Continental Rifting Conference, 26th ECGS Workshop, Luxembourg, November 2007.

References stemming from this research: Coté, D., A. Quillen, C. Ebinger, D. Keir, M. Belachew, Low frequency hybrid

earthquakes near a magma chamber in Afar: Quantifying path effects, to be submitted to Geophys. J. Int., February, 2009.

Ebinger, C., Keir, D., Ayele, A., Belachew, M., Calais, E., Wright, T., Campbell, E., Buck, R., Magma intrusion and faulting processes in a zone of continental rupture: Seismicity of the Dabbahu (Afar) rift, Geophys. J. Int., doi: 10.1111/j.1365-246X.2008.03877.x, 2008.

Ebinger, C., Yirgu, G., Wright, T., Calais, E., Lewi, E., Ocean birth through rifting and rupture, McGraw-Hill Yearbook of Science and Technology, 161-165, 2007.

Hammond, J., J-M. Kendall, D. Keir, A. Ayele, C. J. Ebinger, Crustal structure beneath the Afar depression derived from receiver function analyses, in prep.

Hamling, I., A. Ayele, E. Calais, C. Ebinger, D. Keir, E. Lewi, T. Wright, G. Yirgu, Geodetic observations of new dyke intrusions in the Dabbahu rift segment, Afar, Ethiopia, Geophys. J. Int., in press, 2009.

Keir, D., A. Ayele, E. Calais, I. Hamling, C. Ebinger, T. J. Wright, E. Jacques, M. Kassim, J.O.S. Hammond, Manahloh Belachew, E. Baker, J. Rowland, Repeated dike injection sourced beneath the center of the Dabbahu segment in the Afar rift, Geology, 37, 59-62, doi:10.1130/G25147A.1, 2009.

Keir, D., Ebinger, C., Belachew, M., Hammond, J., Variations in crustal anisotropy at a rift-rift-rift triple junction, to be submitted to GRL, in prep.

Rowland, J., E. Baker, C. Ebinger, D. Keir, T. Kidane, J. Biggs, N. Hayward, Fault growth at a nascent slow-spreading ridge: 2005 Dabbahu rifting episode, Afar, Geophys. J. Int., doi: 10.1111/j.1365-246X.2007.03584.x, 2007.

Wright, T., C. Ebinger, J. Biggs, A. Ayele, G. Yirgu, D. Keir, A. Stork, Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode, Nature, 442, doi:10.1038/nature04978, 2006.


seismo-volcanic crisis in afar: the 2005 boina rupture-eruption … · 2014-01-15 · 1 nerc...

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