High-resolution seismic stratigraphy of the Yamato Basin, Japan Sea and its geological application

Abstract High-resolution seismic stratigraphy of the Yamato Basin, Japan Sea, was successfully established using core-log-seismic data integration. The construction of synthetic seismograms by the combination of physical properties and well-log data from the Ocean Drilling Program (ODP) Site 797 was the key to accomplishing the high-resolution seismic stratigraphy. To achieve resolution comparable with well-log data and core lithology, single channel seismic reflection data taken from ODP underway geophysics were reprocessed, and then carefully compared with synthetic seismogram, core and well log profiles to identify seismic units. Ten seismic stratigraphic units were identified at the site, and seismic stratigraphic interpretation was successfully extended from the site to the nearby area along the Yamato Basin margin. The opal-A/opal-CT (biogenic silica/metastable diagenetic silica) boundary has different appearances at places from strong to weak, and mostly discontinuous. One of the significant results achieved from this study is clear distinction of the opal-A/CT boundary from a very strong reflector, which appears at 22 m below the opal-A/CT boundary. Through well-log and physical properties characterization of the different units, resistivity was found to be the best indicator of diatom content and with gamma-ray it also is an indicator of chert layers in the opal-CT zone. Velocity is not greatly effected by diatom ooze in the opal-A zone, however, it shows strong peaks and has an indirect relationship with gamma-ray in the opal-CT zone. Finally, successful correlation of Gamma-ray Attenuation Porosity Evaluator density and resistivity peaks with strong seismic reflectors from upper and lower stratified layers may provide new information on the late Neogene paleoceanography of the Japan Sea in high-resolution scale.     

Flow and transport in channels with submerged vegetation

This paper reviews recent work on flow and transport in channels with submerged vegetation, including discussions of turbulence structure, mean velocity profiles, and dispersion. For submerged canopies of sufficient density, the dominant characteristic of the flow is the generation of a shear-layer at the top of the canopy. The shear-layer generates coherent vortices by Kelvin-Helmholtz (KH) instability. These vortices control the vertical exchange of mass and momentum, influencing both the mean velocity profile, as well as the turbulent diffusivity. For flexible canopies, the passage of the KH vortices generates a progressive wave along the canopy interface, termed monami. The KH vortices formed at the top of the canopy penetrate a distance δ _e into the canopy. This penetration scale segregates the canopy into an upper layer of rapid transport and a lower layer of slow transport. Flushing of the upper canopy is enhanced by the energetic shear-scale vortices. In the lower layer turbulence is limited to length-scales set by the stem geometry, and the resulting transport is significantly slower than that of the upper layer.     

Effect of ice sheet growth and melting on the slip evolution of thrust faults

Field investigations suggest that postglacial unloading and rebound led to the formation or re-activation of reverse faults even in continental shields like Scandinavia. Here we use finite-element models including a thrust fault embedded in a rheologically layered lithosphere to investigate its slip evolution during glacial loading and subsequent postglacial unloading. The model results show that the rate of thrusting decreases during the presence of an ice sheet and strongly increases during deglaciation. The magnitude of the slip acceleration is primarily controlled by the thickness of the ice sheet, the viscosity of the lithospheric layers and the long-term shortening rate. In contrast, the width of the ice sheet, the rate of deglaciation or the fault dip have an only minor influence on the slip evolution. In all experiments, the slip rate variations are caused by changes in the differential stress. The modelled deglaciation-induced slip acceleration agrees well with the occurrence of large earthquakes soon after the melting of the Fennoscandian ice sheet, which led to the formation of spectacular fault scarps in particular in the Lapland Fault Province. Furthermore, our model results support the idea that the low level of seismicity in currently glaciated regions like Greenland and Antarctica is caused by the presence of the ice sheets. Based on our models we expect that the decay of the Greenland and Antarctica ice sheets in the course of global warming will ultimately lead to an increase in earthquake frequency in these regions.     

Gas-surface interactions and satellite drag coefficients

Information on gas-surface interactions in orbit has accumulated during the past 35 years. The important role played by atomic oxygen adsorbed on satellite surfaces has been revealed by the analysis of data from orbiting mass spectrometers and pressure gauges. Data from satellites of special design have yielded information on the energy accommodation and angular distributions of molecules reemitted from satellite surfaces. Consequently, it is now possible to calculate satellite drag coefficients from basic physical principles, utilizing parameters of gas-surface interactions measured in orbit. The results of such calculations are given. They show the drag coefficients of four satellites of different compact shapes in low-earth orbit with perigee altitudes in the range from about 150 to 300 km, where energy accommodation coefficients and diffuse angular distributions have been measured. The calculations are based on Sentman's analysis of drag forces in free-molecular flow. His model incorporates the random thermal motion of the incident molecules, and assumes that all molecules are diffusely reemitted The uncertainty caused by the assumption of diffuse reemission is estimated by using Schamberg's model of gas-surface interaction, which can take into account a quasi-specular component of the reemission. Such a quasi-specular component is likely to become more important at higher altitudes as the amount of adsorbed atomic oxygen decreases. A method of deducing accommodation coefficients and angular distributions at higher altitudes by comparing the simultaneous orbital decay of satellites of different shapes at a number of altitudes is suggested. The purpose is to improve thermospheric measurements and models, which are significantly affected by the choice of drag coefficients.     

Seismic activity related to the 2000 eruption of the Hekla volcano, Iceland

The 2000 Hekla eruption took place from February 26 to March 8. Its seismic expressions were a swarm of numerous small earthquakes related to its onset, and low-frequency volcanic tremor that continued throughout the eruption. A swarm of small earthquakes was observed some 80 min before the onset of the eruption, and the size of the events increased with time. Low-frequency volcanic tremor, with a characteristic frequency band of 0.5–1.5 Hz and dominant spectral peak(s) at 0.7–0.9 Hz, became visible at 18:19 GMT on February 26, marking the onset of the eruption. The tremor amplitude rose quickly and was very high in the beginning of the eruption. However, it soon began to decrease after about an hour. In general, the seismic activity related to the 2000 Hekla eruption was very similar to what was observed in the previous eruption in 1991. Based on knowledge gained from seismicity and strain observations from 1991, this was the first time that a Hekla eruption was predicted.Editorial responsibility: J Stix     

Land use change effects on catchment streamflow response in a humid tropical montane cloud forest region,central Veracruz,Mexico

Tropical montane cloud forests (TMCF) are recognized for their capacity to maintain high dry-season baseflow and a host of other ecosystem services. Substantial areas of TMCF have been converted to pasture and crops such as coffee, while in other areas TCMF are recovering. However, little is known about the effects of this complex dynamic on catchment hydrology. We investigated the effect of land use on rainfall-runoff response in five neighbouring headwater micro-catchments in central Veracruz, Mexico, dominated by either mature TMCF (MF), young (20 year-old) and intermediate (40 year-old) naturally regenerating TMCF (YF and IF, respectively), shaded coffee (SC), and an intensively grazed pasture (IP). We used a 4-year record of high-resolution rainfall and streamflow (10 min) data collected from 2015 to 2019. These data were analysed via comparison of hydrologic metrics that summarize streamflow responses at various time scales and magnitudes. Results showed no statistical difference in the regulation capacity of high flows in the micro-catchment with 20 years of natural regeneration, compared to the MF. In terms of baseflow sustenance, our results support the hypothesis that MF and IF better promote this hydrologic service than the other land uses. SC exhibited a high capacity to modulate peak flows comparable to that of MF, and an intermediate capacity to sustain baseflow, suggesting that the integrated functioning of this micro-catchment was largely preserved. Finally, 40 years of intense pasture management was found to have degraded the soil hydraulic properties of IP; mainly, reducing its infiltration capacity, causing a fivefold greater peak flow response and a lower baseflow compared to MF.     

Women in the Field: Critical Feminist Methodologies and Theoretical Perspectives*

Feminist scholars have traditionally emphasized the importance of incorporating “the everyday” worlds of women into the historically masculinist theoretical and empirical foundations of the social sciences. Such emphases have commonly resulted in smaller-scale research projects and more interactive kinds of research methods and methodologies. Feminist geographers have uniquely contributed to the body of feminist scholarship through drawing out the importance of place in everyday constructions of gender and, more recently, sexuality. Critical field-based research has therefore from the beginning been the mainstay of subdisciplinary research. Like the discipline as a whole, however, little explicit attention has been given in publications or pedagogically to the politics of fieldwork (including how a “field” is defined and the politics involved in choosing and working in a particular “field”) or the politics of representation (which includes considerations of the partiality of knowledge and how and to whom we represent our work, ourselves and others in various kinds of texts). These “Opening Remarks” show how these issues are addressed in the papers that follow and how feminist geography has much to contribute to critical analyses of global and multinational processes, including patriarchy, capitalism, and racism.