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The Main Ethiopian Rift (MER) represents an area where volcanism and tectonics interact to create closely linked volcano-tectonic features. This linkage is paramount in the axial portion of the rift, where magmatic segments localize several large peralkaline eruptive centres. Many of them evolved into caldera collapse (the best preserved of which are younger than ${

On 24 August 2016 a seismic event (Mw 6.0) was the first of the long Central Italy sequence (ongoing at the end of 2017) of medium-to-high magnitude earthquakes, with nine Mw ≥5 up to October 2017, and with about 74.000 seismic events registered after 1 year. The largest was the Mw 6.5 30 October 2016 event near Norcia. After the major seismic events, 17 mud volcanoes erupted around Monteleone di Fermo village (Marche region). Mud volcano eruptions generally occurred a few hours to a few days after the main earthquakes, suggesting a seismic triggering. We analyzed the peak ground velocities and dynamic stresses during the three largest earthquakes. We also evaluated the static stress changes in order to assess the potential influence of normal stress changes on the feeder system of the activated mud volcanoes. We find a correlation with dynamic stresses, whereas static stress changes are negligible or negative (with values reaching −0.44 bar, clamping feeder dykes). We conclude that seismic shaking (up to ~3.9 bar during Norcia earthquake) is the dominant driver for these eruptions. Finally, we evaluated the response ratio as a function of the dynamic stress. It increases exponentially with peak dynamic stress varying from 0.3 bar to >50% for peak dynamic stress >2 bar, indicating a link between earthquake shaking and mud volcano activity.

Earthquakes can trigger the eruption of mud. We use eruptions in Azerbaijan, Italy, Romania, Japan, Andaman Islands, Pakistan, Taiwan, Indonesia, and California to probe the nature of stress changes that induce new eruptions and modulate ongoing eruptions. Dynamic stresses produced by earthquakes are usually inferred to be the dominant triggering mechanism; however static stress changes acting on the feeder systems of mud volcanoes may also play a role. In Azerbaijan, eruptions within 2-10 fault lengths from the epicenter are favored in the year following earthquakes where the static stress changes cause compression of the mud source and unclamp feeder dikes. In Romania, Taiwan, and some Italian sites, increased activity is also favored where the static stress changes act to unclamp feeder dikes, but responses occur within days. The eruption in the Andaman Islands, and those of the Niikappu mud volcanoes, Japan are better correlated with amplitude of dynamic stresses produced by seismic waves. Similarly, a new island that emerged off the coast of Pakistan in 2013 was likely triggered by dynamic stresses, enhanced by directivity. At the southern end of the Salton Sea, California earthquakes increase the gas flux at small mud volcanoes. Responses are best correlated with dynamic stresses. The comparison of responses in these nine settings indicates that dynamic stresses are most often correlated with triggering, although permanent stress changes as small as, and possibly smaller than, 0.1 bar may be sufficient to also influence eruptions. Unclamping stresses with magnitude similar to Earth tides (0.01 bar) persist over time and may play a role in triggering delayed responses. Unclamping stresses may be important contributors to short-term triggering only if they exceed 0.1-1 bar.

The relations between mud volcanism and active tectonic strain were investigated at the Nirano mud volcano field (NMVF), near the active Pede-Apennine thrust front of the Northern Apennines thrust-and-fold belt (Italy). Active fluid release occurs through numerous vents within an elliptical depression developed over the crest of a thrust-related anticline. This depression is interpreted as a caldera collapse-like structure that may have developed in response to the deflation of a shallow mud chamber triggered by eruption and sediment fluid evacuation events. Like many volcanic calderas, the NMVF caldera is elongated nearly parallel to the direction of least horizontal compressive stress [S.sub.h], and is thus deduced to reflect the regional tectonic stress axes. It is concluded that mud volcano calderas exhibit mechanical similarities to the igneous analogs, and that the methods used in this first test could be applied to other calderas imaged by seismic data and in the field. Keywords: mud volcanism, elliptical caldera depression, active thrusting. Northern Apennines.

The Main Ethiopian Rift (MER) has a complex structural pattern composed of southern, central, and northern segments. Ages of onset of faulting and volcanism apparently indicate a heterogeneous time-space evolution of the segments, generally referred to as a northward progression of the rifting process. New structural, petrological, and geochronological data have been used to attempt reconciling the evolution of the distinct MER segments into a volcanotectonic scenario accounting for the propagation of the Afar and the Kenya Rifts. In this evolutionary model, extension affected the Southern MER in the early Miocene (20-21 Ma) due to the northward propagation of the Kenya Rift-related deformation. This event lasted until 11 Ma, then deformation decreased radically and was resumed in Quaternary times. In the late Miocene (11 Ma), deformation focused in the Northern MER forming a proto-rift that we consider as the southernmost propagation of Afar. No major extensional deformation affected the Central MER in this period, as testified by the emplacement at 12-8 Ma of extensive plateau basalts currently outcropping on both rift margins. Significant rift opening occurred in the Central MER during the Pliocene (~5-3 Ma) with the eruption of voluminous ignimbritic covers (Nazret sequence) exposed both on the rift shoulders and on the rift floor. The apparent discrepancy between the heterogeneous propagation of the three MER segments could be reconciled by considering the opening of Central MER and the later reactivation of the Southern MER as due to a southward propagation of rifting triggered by counterclockwise rotation of the Somalian plate starting around 10 Ma.