Low-frequency earthquakes beneath Tullu Moye volcano, Ethiopia, reveal fluid pulses from shallow magma chamber

Abstract

The active magmatic processes beneath volcanoes in continental rifts is poorly understood. For example, until recently in the East African rift (EAR), the majority of the young volcanoes were thought to be inactive. More recent studies have shown that numerous volcanoes in the EAR are seismically active and deforming rapidly. However, an unambiguous sign of actively degassing magma hosted in shallow magma bodies has eluded most investigators. Here we present detailed analysis of the first low-frequency (LF) earthquake swarms to be observed in the Main Ethiopian Rift. The earthquakes locate to beneath Tullu Moye volcano and are directly related to the presence of a shallow magma body with a high fluid content. Using spectral modelling we show that the LF earthquakes appear to have low stress-drops (1-50 kPa) which we interpret in terms of low rupture velocities and high pore-fluid pressure. Careful relocation of the LF earthquakes place them approximately 4 km below the surface within one of two possible clusters. However, analysis of the correlation between earthquake waveforms show that each swarm contains a range of earthquake families and as such a diversity of earthquake source mechanisms. To explain these observations, we propose the seismicity is induced by H 2 O/CO 2 fluid pulses from the shallow magma body into a highly fractured region. Fluid pulses cause high pore fluid pressures, which also cause the low rupture velocities.

Magmatic Processes in the East African Rift System: Insights From a 2015–2020 Sentinel‐1 InSAR Survey

“…The temporal evolution, U ( t ) can be modeled by an exponential decay function, , characterizing a hydraulic connection between two magma bodies (Lengliné et al., 2008; Rivalta, 2010) with an asymptotic displacement, U ∞ = 14 cm and characteristic time τ = 1.25 years (Figure 2e—pink solid line). These observations confirm the episodic behavior of this magmatic system and the exponential decay suggests a migration of fluids between two reservoirs, in agreement with seismic and MT studies (Greenfield et al., 2019a, 2019b; Samrock et al., 2018).…”

“…The spatial extent of ground unrest correlates with the location of seismic swarms recorded during a campaign in 2016-2017 (Greenfield et al, 2019b). Although the majority of events are volcano-tectonic, two clusters located below Tullu Moje contained low-frequency waveforms that are likely to be triggered by pulses of hydrothermal fluids (H 2 O/CO 2 ) into a fractured region from a shallow magma body located 4-6.5 km below the surface (Greenfield et al, 2019a(Greenfield et al, , 2019b The Sentinel-1 survey detected a new pulse of ground deformation between the three volcanic centers-Tullu Moje, Bora, and Bericha-elongated in an NW-SE direction (Figure 2b). The deformation started in early 2016 at a rate of 5.8 ± 0.6 cm/yr (LOS descending), decreasing rapidly with time to an average rate of 1.9 ± 0.2 cm/ yr for the period 2017-2020 ( Figure 2e-red solid line).…”

“…The spatial extent of ground unrest correlates with the location of seismic swarms recorded during a campaign in 2016–2017 (Greenfield et al., 2019b). Although the majority of events are volcano‐tectonic, two clusters located below Tullu Moje contained low‐frequency waveforms that are likely to be triggered by pulses of hydrothermal fluids (H 2 O/CO 2 ) into a fractured region from a shallow magma body located 4–6.5 km below the surface (Greenfield et al., 2019a, 2019b).…”

Not all Icequakes are Created Equal: Basal Icequakes Suggest Diverse Bed Deformation Mechanisms at Rutford Ice Stream, West Antarctica

“…Instead, we propose that a weakening of the till resulting from the presence of fluid penetrating the till layer (Rathbun et al, 2008) or fluctuations in the hydraulic pressure caused by fluids (Röösli et al, 2016) may eventually result in a series of stick-slip events adjacent to regions of free water at the ice-bed interface. The role of fluids in promoting icequakes would also explain the temporal and spatial event clustering we observe (Fischer, Horálek, et al, 2014;Greenfield et al, 2019) and the large b-values (El-Isa & Eaton, 2014;Schlaphorst et al, 2017;Wilks et al, 2017). Large b-values are indicative for swarm-like earthquake behavior, which, in turn can be triggered from fluid induced pressure variations.…”

The Coupled Magmatic and Hydrothermal Systems of the Restless Aluto Caldera, Ethiopia

“…Over the past decade, a number of studies have sought to better understand the dynamics and eruptive histories of these volcanoes. Experiments have revealed a rich complexity of seismicity beneath Corbetti (Lloyd, et al, 2018a;Lavayssière et al, 2019), Bora-Tulu Moye (Greenfield et al, 2019a;Greenfield et al, 2019b) and Aluto (Wilks et al, 2017). Satellite imaging suggests frequently recurring and shallow magmatic activity (Biggs et al, 2011;Hutchison et al, 2016b;Lloyd et al, 2018a), and a role for pre-existing structures in the development of volcanism (Lloyd et al, 2018b).…”