[1] We examine the potential triggering relationship between large earthquakes and methane mud volcano eruptions. Our data set consists of a 191-year catalog of eruptions from 77 volcanoes in Azerbaijan, central Asia, supplemented with reports from mud volcano eruptions in Japan, Romania, Pakistan, and the Andaman Islands. We compare the occurrence of historical regional earthquakes (M > 5) with the occurrence of Azerbaijan mud volcano eruptions and find that the number of same-day earthquake/ eruption pairs is significantly higher than expected if the eruptions and earthquakes are independent Poisson processes. The temporal correlation between earthquakes and eruptions is most pronounced for nearby earthquakes (within $100 km) that produce seismic intensities of Mercalli 6 or greater at the location of the mud volcano. This assumed magnitude/distance relationship for triggering observed in the Azerbaijan data is consistent with documented earthquake-induced mud volcano eruptions elsewhere. We also find a weak correlation that heightened numbers of mud volcano eruptions occur within 1 year after large earthquakes. The distribution of yearly eruptions roughly approximates a Poisson process, although the repose times somewhat favor a nonhomogenous failure rate, which implies that the volcanoes require some time after eruption to recharge. The volcanic triggering likely results from some aspect of the seismic wave's passage, but the precise mechanism remains unclear.

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Seafloor and sub-seafloor interpretive maps derived from broad-band 3D Short-Offset seismic augmented with 2D High-Resolution seismic and seabed piston core data were utilized to assess potential hazards and constraints to mobile drilling operations over the Nakhchivan block, offshore Azerbaijan. The Nakhchivan block is located along the shelf break and upper slope environment of the Azerbaijan sector of the South Caspian Sea. Water depth ranges from 70 to 788 meters across a northwest trending ridge that is the seafloor expression of a subsurface anticline. A flat-topped mud volcano complex and associated mudflow deposits occur along the anticlinal crest within a large depression bound on three sides by steep high-relief slump scarps and rotated blocks. The depression opens to the east into a large sea valley bound on both sides by slump scarps. Surface mudflows trend seaward through this valley on the eastern flank of the anticline. Gravity cores acquired within the mud volcano complex and surface mudflows recovered liquid to very soft black to grey muds. Geochemical analyses revealed anomalous concentrations of methane in all these cores. A linear compressional anticline, trending from northwest-southwest to north-south, dominates the shallow structure. The axis of the structure, a complex fault graben, is truncated by a mud volcano. Immediately north of the mud volcano, a northwest to northeast trending radial fault pattern, aligned with the anticlinal axis, forms numerous narrow horst and graben structures. Four mapped horizons illustrate the subsurface structure and morphology. The latest Pleistocene Lower Khvalynian horizon is an unconformity that separates muds and minor sands above from interbedded clays, silty-clays, sands, and mudflow breccia below. The late Pleistocene intra-Lower Khazarian horizon is a transgressive surface that overlies a sequence of clays, thin sands, and mudflow breccia. The middle Pleistocene Apsheron horizon is an unconformity that overlies a sequence of thick clays, shales, and thin interbedded and basal channel-fill sands. The Akchagyl horizon, at the Plio-Pleistocene boundary, overlies thick clays, shales, interbedded and channel-fill sands, evaporites, and bioclastic carbonates. Sediments within the underlying upper Surakhany Series consist of interbedded shales, thin sands, and evaporites. Rapid structural growth, associated seismicity and mud volcanism that was initiated at the onset of Apsheron time continues to the present. During early Apsheron time mud volcanism exploited the faulted axial crest of the anticline to form a mud lake and associated mud cones at the seafloor. Subsequent periods of mud volcano eruption, fault movement, and seismicity precipitated the formation of large slump blocks and surface slides. These processes formed the seafloor caldera observed today. Seafloor mudflows, active slumps and slides represent potential foundation hazards. Structural and stratigraphic gas traps occur along the updip flanks of the anticline throughout the latest Tertiary and Quaternary section. Overpressured aquifers associated with interbedded sands and evaporites occur within the Akchagyl and Surakhany Series. Shallow overpressured water and gas sands represent a potential well-control hazard. Introduction The interpretive results of a mobile drilling rig site investigation on the SOCAR and ExxonMobil Nakhchivan exploration block are presented.

We investigated the mode of growth of flat-topped mud volcanoes, through the study of three active edifices onshore Azerbaijan: they are the Bozdag-Guzdek, the Ayazakhtarma and the Akhtarma-Pashaly mud volcanoes. The three edifices are up to 80 m high and 3 km wide, and the recurrence time between eruptions from a few months to a few years. Surface changes during and between eruptions were documented by a combination of mapping from satellite pictures, repeated direct observation over five years and structural analysis. In addition, resistivity profiles and microgravity measurements were used to decipher their subsurface geometries. We interpret the flat-topped character of the mud volcanoes as the result of rapid "isostatic" readjustment of a brittle surface crust, < 1 m to tens of meters thick, overlying a ductile layer. The surface structure typically shows a concentric transition from an extensional regime in the vicinity of the emission center, to strike slip movements in a median ring, to a compressional regime with thrusts, and pop-up blocks or folds in the outer part of the plateau. Both the flat surface of these mud volcanoes and observed radial displacements of the surface, combined with the very low Bouguer anomaly of the Ayazakhtarma mud volcano, give arguments to propose that km-diameter, flat-topped mud volcanoes likely reflect the presence at a shallow depth (a few tens of meters?) of a large volume of soft mud. Rapid compaction at the surface forms a crust that moves away from a central feeding area, thereby transferring mud added at the center into lateral spreading, building a Coulomb prism all around the mud volcano and strongly limiting vertical buildup. Highlights ► We propose a new model of growth for flat-topped mud volcanoes fed from the center. ► Flat-topped MVs grow by radial expansion, making a concentric structural pattern. ► Fractures evolve from extensional in center to compressional at periphery. ► Gravity inversion shows a ca 300 m thick lowdensity lens below the surface. ► Soft mud in the lens limits vertical growth and flattens the MV by isostasy.