ScSp observed on North Island, New Zealand: implications for subducting plate structure

Seismic phase conversions provide important constraints on the layered nature of subduction zone structures. Recordings from digital stations in North Island, New Zealand, have been examined for converted ScS ‐to‐ p ( ScSp ) arrivals from deep (>150 km) Tonga–Kermadec earthquakes to image layering in the underlying Hikurangi subduction zone. Consistent P ‐wave energy prior to ScS has been identified from stations in eastern and southern North Island, where the subducted plate interface is at a depth of between 15 and 30 km. Two ScS precursors are observed. Ray tracing indicates that the initial precursor ( ScSp ₁) corresponds to conversion from the base of an 11–14 km thick subducting Pacific crust. The second precursor is interpreted as a conversion from the top of the subducting plate. The amplitude ratio, ScSp ₁: ScS , increases from 0.10 to 0.19 from northern to southern North Island. This is within the range expected from a simple first‐order velocity discontinuity at an oceanic Moho. A 1–2 km thick layer of low‐velocity sediment at the top of the subducting plate is required to explain the remaining ScSp waveform. Our results imply that the abnormally thick Hikurangi–Chatham Plateau has been subducting beneath New Zealand for at least 2.9 Myr, thus explaining the high uplift rates observed across eastern North Island.     

Water isotope characteristics of a flood: Brisbane River,Australia

Flooding associated with tropical storms can cause extreme perturbations in riverine and coastal ecosystems. Measuring isotope variability of tropical storm events can help investigate the impacts of flooding. We measured the water isotope composition (δD and δ¹⁸O) of rain and associated floodwater collected during two storms and subsequent major and minor flooding events in the subtropical coast of eastern Australia. Compared with baseline regional rainfall isotope values of ?15.0 ± 1.9‰ for δD and ?3.3 ± 0.2‰ for δ¹⁸O, floodwater had lower values with ?33.8 ± 2.5‰ δD and ?5.1 ± 0.4‰ δ¹⁸O for the major flood and ?29.4 ± 1.0‰ δD and ?4.6 ± 0.1‰ δ¹⁸O for the minor flood. The low isotope composition of the floodwater was associated with the transport of large quantities of suspended sediments, with sediment loads 30 to 70 times larger than during base flow conditions. Floods carried up to 35% of the annual phosphorus and up to 208% of the currently calculated average annual nitrogen load of the Brisbane River. The dramatic changes caused by a rapid increase in discharge from 2 to 2015 m³ s^?1 over 2 days in the major flood would have major consequences in riverine and coastal ecosystems of the region. These changes could potentially be traced using the isotope composition of the floodwaters. Copyright © 2015 John Wiley & Sons, Ltd.     

Origin of Lower Cretaceous quartzose arenites in northern India and the Indus Basins of Pakistan—The result of provenance composition,weathering or diagenesis?

Lower Cretaceous (Aptian-Albian) sandstones of the Ghaggar-Hakra Formation in the Barmer Basin of northwest Rajasthan, India, have a complex depositional history which is confusing given they are quartzose arenites. The heavy mineral grains are very well-rounded, and the assemblage is dominated by zircon and rutile grains suggesting that the sediments have been recycled multiple times, whilst the presence of staurolite indicates a metapelite provenance component. Petrographical analysis suggests that extreme diagenesis cannot account for the quartzose arenite composition, despite Early Cretaceous soil formation and at least two periods of subsequent telogenetic modification. An alternative explanation to extreme chemical weathering in the provenance area is that the Ghaggar-Hakra sandstones are multi-cycle sediments derived, at least in part, from the quartzose arenites of the Cambrian Jodhpur Group. This analysis suggests that variations in detrital mineralogy across the Western India Rift System and Indus Basins are the result of transcontinental fluvial transport systems sourcing sediment from specific basement highs (Nagar Parker High, Devikot High, Deodar Ridge and Aravalli Mountain Range) mixed with varying proportions of sediment derived from sandstones of the Jodhpur Group. Consequently, we suggest that Cretaceous fluvial systems were controlled by the local palaeogeographies within the failed rifts of the Barmer and Cambay Basins and that both basins formed barriers to sediment transport from the Aravalli Mountain Range across the northwest Indian plate and into surrounding basins.     

Combined inequality in wealth and risk leads to disaster in the climate change game

It is generally agreed that the risk of catastrophic climate change can only be reduced if agents cooperate to reduce greenhouse gas emissions over the course of the 21st Century. Previous economic experiments have suggested that sufficient cooperation can often be achieved providing individuals are adequately and convincingly informed of the consequences of their actions and the stakes involved. However, this previous work, has not allowed for the fact that in the real world agents vary in both: (1) their resources to mitigate climate change, and (2) the consequences that they face from climate change. We develop and expand the protocol of previous economic experiments to investigate the introduction of such combined asymmetries. We find that when inequality in resources is combined with a greater relative risk for poorer members, cooperation collapses, with tragic consequences. This is because the rich invest proportionally less into preventing climate change when they are less at risk. We also find, through the use of a post-game questionnaire, that those individuals who were more skeptical about climate change in the real world cooperated less in our games. Insofar as such experiments can be trusted as a guide to either people’s everyday behaviour or the interactions of nation states, these results suggest that voluntary cooperation to avoid climate catastrophe in the real world is likely to be hard to achieve.     

The Turkish Dilatancy Project (TDP3): multidisciplinary studies of a potential earthquake source region

Summary. The section of the North Anatolian Fault lying near the city of Izmit, at the east of the Marmara Sea, has been identified as a seismic gap and the possible site of a future major earthquake. Previously published studies of records from an earthquake swarm within the gap (TDP1 and TDP2) provided the first evidence that shear-wave splitting occurs in earthquake source regions, a conclusion since verified by many studies at other locations. A third field study (TDP3) was mounted in the Izmit region during the summer of 1984. Observations were made over an eight-month period and included geomagnetic and geoelectric measurements in addition to a series of observations utilising dense arrays of three-component seismometers. Earthquake activity in the principal study area was monitored over a period of eight months. Records showed features similar to those observed in the earlier studies. In particular: (1) almost all shear waves emerging within the shear-wave window displayed shear-wave splitting; and (2) the polarizations of the first arriving (faster) split shear-waves showed sub-parallel alignments, characteristic of propagation through a distribution of parallel vertical cracks striking perpendicular to the minimum compressional stress.
These and other observations support the conclusion of earlier studies – that the upper crust is pervaded by distributions of micro-cracks aligned by stress, known as extensive-dilatancy anisotropy. A search for time dependence in shear-wave phenomena has revealed temporal variations in the delays between the split shear-waves throughout the course of the TDP3 study, but as yet this has not been correlated with specific earthquake activity.     

Mean Motion Resonances, Gas Drag, and Supersonic Planetesimals in the Solar Nebula

We examine the orbital evolution of planetesimals under the influence of Jupiter's perturbations and nebular gas drag, under the assumption that gas persisted in the asteroid region for some time after Jupiter attained its final mass. Two distinct mechanisms, associated with the 2 : 1 and 3 : 2 mean motion resonances, can excite eccentricities to high values, despite the damping effect of drag. If Jupiter's eccentricity was comparable to its present value, planetesimals can be temporarily trapped in the 2 : 1 resonance. Bodies crossing the 3 : 2 resonance can enter a region of phase space with overlapping high-order resonances. Both mechanisms can produce eccentricities greater than 0.5 for asteroid-sized planetesimals. The combination of resonant perturbations and drag causes secular decay of semimajor axes, resulting in migration of bodies from the outer to inner belt. Inclinations remain low, implying significant collisional evolution during this migration. Velocities of resonant bodies relative to the gas are highly supersonic; these would have been a source of shock waves in the solar nebula.This revised version was published online in October 2005 with corrections to the Cover Date.     

The 2dF Galaxy Redshift Survey: galaxy luminosity functions per spectral type

We calculate the optical b _J luminosity function (LF) of the 2dF Galaxy Redshift Survey (2dFGRS) for different subsets defined by their spectral properties. These spectrally selected subsets are defined using a new parameter, η , which is a linear combination of the first two projections derived from a Principal Component Analysis. This parameter η identifies the average emission- and absorption-line strength in the galaxy rest frame spectrum, and hence is a useful indicator of the present star formation. We use a total of 75 000 galaxies in our calculations, chosen from a sample of high signal-to-noise ratio, low-redshift galaxies observed before 2001 January. We find that there is a systematic steepening of the faint-end slope ( α ) as one moves from passive  ( α =-0.54)  to active  ( α =-1.50)  star-forming galaxies, and that there is also a corresponding faintening of the rest frame characteristic magnitude   M *-5 log₁₀( h )  (from −19.6 to −19.2). We also show that the Schechter function provides a poor fit to the quiescent (Type 1) LF for very faint galaxies  [ M _{b J}-5 log₁₀( h )  fainter than −16.0], perhaps suggesting the presence of a significant dwarf population. The LFs presented here give a precise confirmation of the trends seen previously in a much smaller preliminary 2dFGRS sample, and in other surveys. We also present a new procedure for determining self-consistent k -corrections, and investigate possible fibre-aperture biases.