Pit crater chains are common topographic features on Mars and several other planetary bodies, and a wide range of mechanisms has been proposed for their origin. Two rifting-related seismic events in 1975-1976 and 1978 along the Mid-Atlantic Ridge near the northern coast of Iceland, associated with the Krafl a volcanic eruptions to the south, produced an array of pit chains in unconsolidated sediments overlying Holocene basalt fl ows. Fault scarps and extension fractures in basaltic lava fl ows are traceable laterally into overlying unconsolidated fl uvial deposits, revealing contrasting deformation styles in the two mechanical layers. Map-scale structures in basalt with little or no sedimentary cover include (1) fault scarps, (2) extension fractures and fracture swarms, (3) faulted monoclines, (4) widened fractures with caverns, and (5) localized circular or elongate collapse pits. Where unconsolidated fl uvial sand and gravel deposits >3 m thick cover the basaltic lava fl ows, structural geomorphic features are dominated by (1) grabens bounded by normal faults with ~1 m displacement, (2) cone-to bowlshaped pit craters with depths up to 2.8 m, and (3) elongate troughs. Formation of these structures in fl uvial sediment was triggered by reactivation of faults and extension fractures in the underlying basalt. Pit craters are readily explained by downward "draining" of poorly consolidated material into subterranean cavities produced by fault and extension fracture dilation in underlying cohesive material (basalt). High-resolution imagery on Mars shows geomorphic patterns that are directly analogous to these Icelandic pit chains, suggesting similar processes have occurred on Mars.

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Seismicity in the Eagle Ford play grew to 33 times the background rate in 2018. We identified how hydraulic fracturing (HF) contributed to seismicity since 2014 by comparing times and locations of HF with a catalog of seismicity extended with template matching. We found 94 ML ≥ 2.0 earthquakes spatiotemporally correlated to 211 HF well laterals. Injected volume and number of laterals on a pad influence the probability of seismicity, but effective injection rate has the strongest effect. Simultaneous stimulation of multiple laterals tripled the probability of seismicity relative to a single, isolated lateral. The 1 May 2018 MW 4.0 earthquake may have been the largest HF‐induced earthquake in the United States. It occurred ~10 km from a MW 4.8 earthquake in 2011 and was thought to be induced by fluid extraction. Thus, faults in this area are capable of producing felt and potentially damaging earthquakes due to operational activities.

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