Earthquake Nucleation on Faults with a Revised Rate- and State-Dependent Friction Law

Recently Nagata et al. (J Geophys Res 117:B02314, 2012) have proposed a new version of rate- and state-dependent friction law (RSF) that seems to have eventually resolved all the previously known discrepancies in the existing RSFs from laboratory observations. The values of a and b, empirical RSF parameters determined by fitting the same laboratory experiments, have been revised to be five times greater and a newly noticed weakening effect by shear stress with a coefficient c has been introduced. By using this revised RSF, we reinvestigated a problem of 2D quasi-static nucleation on faults. A crack-like nucleation-zone expansion known for the ‘aging’ version of RSF is not sustainable with the ‘Nagata’ law, which is understandable as the Nagata law does not produce a slip-weakening distance proportional to the involved strength reduction, an aging law’s feature that contradicts laboratory observations. The later stage of Nagata-law nucleation shows localization of quasi-static slip within a limited spatial extent, but the localization is much milder than that predicted by the ‘slip’ version of RSF. With an appropriate c parameter of the Nagata law, the nucleation size seems to be reduced only by a factor from that of the aging law.     

Ice core record of Be over the past millennium from Dome Fuji, Antarctica: A new proxy record of past solar activity and a powerful tool for stratigraphic dating

The cosmogenic nuclide ¹⁰Be was analyzed by using accelerator mass spectrometry on an ice core drilled at the Dome Fuji station, inland Antarctica, for 700–1900 yr CE. The measured concentration of ¹⁰Be in the Dome Fuji ice core and the derived ¹⁰Be flux show similar fluctuations, with both increasing at known solar-activity minima over the last millennium in agreement with earlier observations of ¹⁰Be and ¹⁴C. Based on the similar nature of the ¹⁰Be flux to the reconstructed ¹⁴C production rate patterns, a ¹⁰Be–¹⁴C correlation age model for the Dome Fuji ice core was successfully constructed. This age model agrees well with the initial version of the tephrochronology of the core. The ¹⁰Be-flux record contains information on variability in the amount of cosmic radiation incident on the atmosphere, which is mainly attributable to high-frequency change in solar activity and low-frequency background intensity adjustment of the geomagnetic field. High-resolution ¹⁰Be analyses of the Dome Fuji ice cores promise to provide potentially important information on the history of cosmic radiation intensity over the past several hundred thousand years.     

A numerical study of the effect of frozen soil on dust emission during an East Asian dust event in December 2009

There are few dust simulation studies for East Asian dust events that took place in the wintertime, when the surface conditions of the dust source region differ from those of the springtime. The soil water turns into ice when the temperature falls below freezing, and the ice might prohibit wind erosion by increasing the binding strength between soil particles. However, the contribution of frozen soil to reducing dust outbreaks remains unclear. This study investigates the effect of frozen soil on dust emission through a case study of a severe wintertime East Asian dust event that originated on 23 and 24 December 2009 in Southern Mongolia and Inner Mongolia and reached Korea on 25 and 26 December 2009 using WRF/Chem with a new dust emission scheme. Model simulations with and without the effect of frozen soil were conducted. A temperature below 0°C and relative soil saturation exceeding 40% were used for frozen soil criteria, and the frozen soil was prohibited from emitting dust. The dust concentrations derived from the simulation without the effect of frozen soil were about three times higher than the observed PM₁₀ concentrations, while the results from the simulation with the frozen-soil effect were quite similar to those of the observation data. The simulation of the wintertime East Asian dust event with the frozen-soil effect improved the model representation. The sensitivity tests for frozen soil indicate that the criteria of frozen soil used in this study are appropriate for this case study.     

The Carbon Cycle of Sandy Lands in China and its GlobalSignificance

The quantitative C dynamics of desertifiedlands in Northern China were predicted for the years2000 and 2030, based on the areas and conversion rates(positive and negative) of desertified lands in thepast forty years and organic carbon contents of soils.The top 1.0 m soil layer of natural desertified landsin China contained some 7,841 Tg of organic carbon asof 1992. In China, over the last 40 years, a total of2,812 Tg of organic carbon was released from desertlands and, in the reverse process about 644 Tg oforganic-C were fixed into lands undergoingdesertification. Thus, China's desert lands have showna net release of 2,168 Tg of organic-C over the past40 years, equivalent to 7,949 Tg of CO₂. By theyear 2000, the area of desertified lands in China hadincreased 40,300 km² and released 368 Tg oforganic-C into the atmosphere. By 2030 this area willincrease to 249,700 km² and release about 1,996 Tg of organic-Cinto the atmosphere. Net releases of151 Tg and 1,243 Tg of organic-C can be expectedby the year 2000 and 2030, respectively. This wouldbe equivalent to a net release of 553 Tg of CO₂by 2000 and 4,558 Tg by 2030. Thus, the organiccarbon released through land desertification in Chinacould be an important factor affecting changes inconcentrations of greenhouse gases worldwide.     

Wind-Tunnel and Numerical Simulations of the Coastal Thermal Internal Boundary Layer

Wind-tunnel experiments in a thermally stratified wind tunnel and direct numerical simulations were performed to simulate the thermal internal boundary layer (TIBL) that developed over a coastal area in a sea-breeze flow. The results of the simulations were analyzed to investigate turbulence structure in the TIBL. To study the effects of the atmospheric stability over the sea on the TIBL, two vertical profiles of temperature were created in the upstream portion of the wind-tunnel experiment and the direct numerical simulation. Turbulence statistics of the TIBL changed significantly according to the temperature profile over the sea, indicating that the stability of the flow over the sea has a significant effect on the structure and turbulence characteristics of the TIBL. Furthermore, the TIBL heights were estimated from the vertical profiles of the local Richardson number. The estimated TIBL heights agreed with those predicted by a pre-existing relation, suggesting that both the wind-tunnel experiment and the direct numerical simulation accurately reproduced the growth of the TIBL.     

Thickening model of the continental lithosphere

Summary. The thickening plate theory proposed by Yoshii and Parker & Oldenburg for the oceanic lithosphere is extended to include the continental lithosphere. The theory is based on the assumption that the lithosphere—asthenosphere boundary is a solidus and that as a result solidification of the top of the asthenosphere is occurring. Observational data imply that the relationship between the plate thickness and basement age for the North American continent is y = 1.7 √ t + (50 ± 10), where y (km) is the plate thickness and t (Myr) is the basement age.
The theory is tested against changes with basement age of the observed surface heat-flow and seismic estimate of plate thickness. The following conclusions are inferred:
(1) The changes both of the observed heat flow and plate thickness with basement age are explained by this theory.
(2) The surface erosion and vertical distribution of radiogenic heat sources are important factors in controlling the thickening process of the continental lithosphere.
(3) The equality of the average surface heat-flow over the oceans and over the continents is a consequence of a faster release of latent heat at the lithosphere—asthenosphere boundary under the oceans, instead of a higher heat production in the continental crust.     

A magnetodynamic mechanism for loop flares

Loop flares are given a new magnetodynamic interpretation. In this model, the top of the magnetic loop is heated up by a collision of magnetic twist-wave packets (non-linear torsional Alfven wave) which are produced in the process of the loop emergence, and stored and released from the footpoints of the loop with some retardation. The appearance of the blueshifted component in CaXIX and FeXXV lines a minute or so before the impulsive phase, and the so-called instantaneous acceleration of ions deduced from the nearly simultaneous (with a delay of seconds) occurrence of -ray line emission with the impulsive hard X rays, are very naturally explained in the present model which originally aims at providing an explanation of the source of energy, a blackbox located at the top of the loop in the loop flare theories discussed thus far.     

The force-free magnetosphere around an oblique rotator

The force-free magnetosphere around an obliquely rotating pulsar is studied. The basic equations reduce to two equations for two Euler potentials. One of the Euler potentials is regarded as a generalization of the stream function of the poloidal magnetic field lines in an axisymmetric rotator. Two divergence-free vectors become tangential to the surface on which this Euler potential is constant.