Approximate coseismic displacements and near-field fling pulse from the Mw 6.5 30 October 2016 Central Italy earthquake

A study of the coseismic displacement and fling pulse recorded during the Mw 6.5 30 October 2016 Central Italy earthquake is presented. The near-field has been well documented, owing to the deployment of additional strong-motion stations following the earlier events of the 2016 Central Italy seismic sequence. As a result, there are numerous stations with evidence of coseismic displacement and fling pulse. In this study, 25 records with strike distance of less than 25 km and rupture distance under 28 km are considered. Approximate coseismic displacements have been recovered by a bilinear model to remove the low frequency noise in the records. The bilinear noise model uses two linear regression segments on the velocity trace to remove baseline offsets. After obtaining the coseismic displacement time series, the fling pulse period is examined. Existing methods of obtaining the fling pulse period are reviewed and a proposed algorithm is considered for automatic fling pulse detection. Both horizontal and vertical fling periods are obtained, unlike many studies which neglect the vertical fling. It is shown that the fling pulse period is highly variable (~?2–16 s) in the near-field region but exhibits some trends with various site-to-source distances.     

Chemistry of hot springs along the Eastern Lau Spreading Center

The Eastern Lau Spreading Center (ELSC) is the southernmost part of the back-arc spreading axis in the Lau Basin, west of the Tonga trench and the active Tofua volcanic arc. Over its 397-km length it exhibits large and systematic changes in spreading rate, magmatic/tectonic processes, and proximity to the volcanic arc. In 2005, we collected 81 samples of vent water from six hydrothermal fields along the ELSC. The chemistry of these waters varies both within and between vent fields, in response to changes in substrate composition, temperature and pressure, pH, water/rock ratio, and input from magmatic gases and subducted sediment. Hot-spring temperatures range from 229° to 363 °C at the five northernmost fields, with a general decrease to the south that is reversed at the Mariner field. The southernmost field, Vai Lili, emitted water at up to 334 °C in 1989 but had a maximum venting temperature of only 121 °C in 2005, due to waning activity and admixture of bottom seawater into the subseafloor plumbing system. Chloride varies both within fields and from one field to another, from a low of 528 mmol/kg to a high of 656 mmol/kg, and may be enriched by phase separation and/or leaching of Cl from the rock. Concentrations of the soluble elements K, Rb, Cs, and B likewise increase southward as the volcanic substrate becomes more silica-rich, especially on the Valu Fa Ridge. Iodine and δ⁷Li increase southward, and δ¹¹B decreases as B increases, apparently in response to increased input from subducted sediment as the arc is approached. Species that decrease southward as temperature falls are Si, H₂S, Li, Na/Cl, Fe, Mn, and ⁸⁷Sr/⁸⁶Sr, whereas pH, alkalinity, Ca, and Sr increase. Oxygen isotopes indicate a higher water/rock ratio in the three systems on Valu Fa Ridge, consistent with higher porosity in more felsic volcanic rocks. Vent waters at the Mariner vent field on the Valu Fa Ridge are significantly hotter, more acid and metal-rich, less saline, and richer in dissolved gases and other volatiles, including H₂S, CO₂, and F, than the other vent fields, consistent with input of magmatic gases. The large variations in geologic and geophysical parameters produced by back-arc spreading along the ELSC, which exceed those along mid-ocean ridge spreading axes, produce similar large variations in the composition of vent waters, and thus provide new insights into the processes that control the chemistry of submarine hot springs.     

Tight Reassembly of Gondwana Exposes Phanerozoic Shears in Africa as Global Tectonic Players

Aeromagnetic surveys help reveal the geometry of Precambrian terranes through extending the mapping of structures and lithologies from well-exposed areas into areas of younger cover. Continent-wide aeromagnetic compilations therefore help extend geological mapping beyond the scale of a single country and, in turn, help link regional geology with processes of global tectonics. In Africa, India and related smaller fragments of Gondwana, the margins of Precambrian crustal blocks that have escaped (or successfully resisted) fracture or extension in Phanerozoic time can often be identified from their aeromagnetic expression. We differentiate between these rigid pieces of Precambrian crust and the intervening lithosphere that has been subjected to deformation (usually a combination of extension and strike-slip) in one or more of three rifting episodes affecting Africa during the Phanerozoic: Karoo, Early Cretaceous and (post-) Miocene. Modest relative movements between adjacent fragments in the African mosaic, commensurate with the observed rifting and transcurrent faulting, lead to small adjustments in the position of sub-Saharan Africa with respect to North Africa and Arabia. The tight reassembly of Precambrian sub-Saharan Africa with Madagascar, India, Sri Lanka and Antarctica (see animation in http://kartoweb.itc.nl/gondwana) can then be extended north between NW India and Somalia once the Early Cretaceous movements in North Africa have been undone. The Seychelles and smaller continental fragments that stayed with India may be accommodated north of Madagascar. The reassembly includes an attempt to undo strike-slip on the Southern Trans-Africa Shear System. This cryptic tectonic transcontinental corridor, which first formed as a Pan-African shear belt 700–500 Ma, also displays demonstrable dextral and sinistral movement between 300 and 200 Ma, not only evident in the alignment of the unsuccessful Karoo rifts now mapped from Tanzania to Namibia but also having an effect on many of the eventually successful rifts between Africa-Arabia and East Gondwana. We postulate its continuation into the Tethys Ocean as a major transform or megashear, allowing minor independence of movements between West Gondwana (partnered across the Tethys Ocean with Europe) and East Gondwana (partnered with Asia), Europe and Asia being independent before the 250 Ma consolidation of the Urals suture. The relative importance of primary driving forces, such as subduction ‘pull’, and ‘jostling’ forces experienced between adjacent rigid fragments could be related to plate size, the larger plates being relatively closely-coupled to the convecting mantle in the global scheme while the smaller ones may experience a preponderance of ‘jostling’ forces from their rigid neighbours.     

Micromorphological evidence for subglacial multiphase sedimentation and deformation during overpressurized fluid flow associated with hydrofracturing

Hydrofracture systems are being increasingly recognized within subglacial to ice‐marginal settings and represent a visible expression of the passage of pressurized meltwater through these glacial environments. Such structures provide a clear record of the fluctuating hydrostatic pressure and of the resulting brittle fracturing of the host sediment/bedrock and the pene‐contemporaneous liquefaction and introduction of sediment‐fill. A detailed macro‐ and microstructural study of a hydrofracture system cutting Devonian sandstone bedrock exposed at the Meads of St John, near Inverness (NE Scotland), has revealed that this complex multiphase system was active over a prolonged period and accommodated several phases of fluid flow. The main conduits that fed the hydrofracture system are located along bedding within the sandstone, with the site of the wider, steeply inclined to subvertical, transgressive linking sections being controlled by the contemporaneous development of high‐angle fractures and normal faults, the latter occurring in response to localized extension within the bedrock. A comparison with published engineering hydraulic fracturing data indicates that the various stages of sediment‐fill deposited during a flow event can be directly related to the fluctuation in overpressure during hydrofracturing. A model is proposed linking the evolution of this hydrofracture system to the retreat of the overlying Findhorn glacier. The results of this study also indicate that the development and repeated reactivation of subglacial hydrofracture systems can have a dramatic effect on the permeability of the bed, influencing the potential for overpressure build‐up within the subglacial hydrogeological system, and facilitating the migration of meltwater beneath glaciers and ice sheets.     

An XMM-Newton observation of the extreme narrow-line Seyfert 1 galaxy Mrk 359

We present XMM-Newton observations of Mrk 359, the first narrow-line Seyfert 1 galaxy (NLS1) discovered. Even among NLS1s, Mrk 359 is an extreme case with extraordinarily narrow optical emission lines. The XMM-Newton data show that Mrk 359 has a significant soft X-ray excess which displays only weak absorption and emission features. The     continuum, including reflection, is flatter than that of the typical NLS1, with     . A strong emission line of equivalent width ≈200 eV is also observed, centred near 6.4 keV. We fit this emission with two line components of approximately equal strength: a broad iron line from an accretion disc and a narrow, unresolved core. The unresolved line core has an equivalent width of ≈120 eV and is consistent with fluorescence from neutral iron in distant reprocessing gas, possibly in the form of a 'molecular torus'. Comparison of the narrow-line strengths in Mrk 359 and other low–moderate luminosity Seyfert 1 galaxies with those in QSOs suggests that the solid angle subtended by the distant reprocessing gas decreases with increasing active galactic nucleus luminosity.