Seismicity and subsidence following the 2011 Nabro eruption, Eritrea: Insights into the plumbing system of an off‐rift volcano

Abstract

Nabro volcano, situated to the east of the Afar Rift Zone, erupted on 12 June 2011. Eruptions at such off‐rift volcanoes are infrequent, and consequently, the plumbing systems are poorly understood. We present posteruption Synthetic Aperture Radar (SAR) images from the TerraSAR‐X satellite and posteruption continuous seismic activity from a local seismic array. Interferometric analysis of SAR data, reveals a circular, 12 km wide, signal subsiding at ∼200 mm/yr. We inverted for the best fit Mogi source finding a 4 ± 1 × 107 m3/yr volume decrease at 7 ± 1 km depth. Between 31 August and 7 October 2011, we located 658 and relocated 456 earthquakes with local magnitudes between −0.4 and 4.5. Seismicity beneath the SE edge of Nabro at 11 km depth is likely associated with high strain rates from deep magma flow into the modeled reservoir. This suggests that magma is supplied through a narrow conduit and then stored at ∼7 km depth. We interpret seismicity at 4–6 km depth as brittle fracturing above the inferred magma reservoir. Focal mechanisms delineate a thrust fault striking NE‐SW and dipping 45° to the SE across the caldera floor. We propose that the crustal response is to slip on this fault which crosscuts the caldera rather than to deform on ring faults. The NE‐SW fault plane is not associated with measurable surface deformation, indicating that it does not contribute much to the caldera deformation. We show that subsidence of the caldera is controlled by magma chamber processes rather than fault slip.

The 2011 eruption of Nabro volcano, Eritrea: perspectives on magmatic processes from melt inclusions

“…Using a crustal density of 2780 kg m −3 (Makris and Ginzburg 1987), we calculate that most melt inclusions were trapped at 5-10 km depth (consistent with seismic data for the depth of a magma body; Hamlyn et al 2014), but some may have been trapped close to the base of the crust, around 20 km depth . The magma storage region is most likely composed of a series of sills and old eruptive products, evidenced by the presence of xenocrystic material in the magmas.…”

“…The tephra field was welldefined to the south-west of the edifice (towards and blanketing Mallahle volcano). During the eruption, seismicity was also detected under Mallahle, raising the possibility of an underground connection between the two volcanoes [e.g., Hamlyn et al (2014)]. The 2011 eruption represents an unprecedented opportunity to study an eruption at one of the large caldera volcanoes on the margins of the Afar Depression.…”

What causes subsidence following the 2011 eruption at Nabro (Eritrea)?

“…Pinatubo emitted approximately 20 Tg of SO 2 to the atmosphere during the 1991 eruption (Bluth et al 1992). Hamlyn et al (2014) previously found that the magma chamber which fed the eruption is centred under the SW caldera rim and can be adequately modelled by a Mogi solution at a depth of ∼ 7 km. There is no indication that a magmatic link exists between Nabro and neighbouring Mallahle volcano.…”

“…There is no indication that a magmatic link exists between Nabro and neighbouring Mallahle volcano. However, changes to the local stress regime following the eruption induced seismicity beneath Mallahle (Hamlyn et al 2014).…”

When do fractured media become seismically anisotropic? Some implications on quantifying fracture properties

“…For instance, fractures have a significant influence on the integrity of boreholes and the sealing capacity of the reservoir overburden and their ability for maintaining barriers between potable water and hydrocarbon, CO 2 or radioactive waste. For non-geo-industrial applications, such as monitoring volcanoes, landslides and earthquakes, fractures have a significant influence on the stability of the rock mass and so have important implications on geo-hazard assessment (e.g., Hamlyn et al, 2014).…”