A model with rigid rotations and slip deficits for the GPS-derived velocity field in Southwest Japan

We interpret the GPS-derived velocity field in southwest Japan by a superposition of the elastic deformation caused by fault interactions (slips or slip deficits) on the rigid motion of tectonic blocks (or plates). Based on the strain rate field and crustal seismicity, we apply a model with three blocks (Inner Arc, Outer Arc, and the northern Ryukyu block) and slip deficits along the block boundaries.Several characteristics of the synthesized contributions are found:

(1) Westward motion of the outer arc relative to the Amurian plate and the inner arc,

(2) southeastward motion of the northern Ryukyu block relative to the Amurian plate,

(3) 2−4 mm/yr deficits of left lateral slip rates along the boundary at 32°N in southern Kyushu,

(4) 0−8 mm/yr deficits of right lateral slip rates along the Median Tectonic Line and the Beppu-Shimabara Graben,

(5) slip deficit rates on the plate interface smaller than those in the case without any consideration for rigid block motions,

(6) clockwise deflection of slip deficit rate vector on the plate interface from that estimated when not taking rigid block motions into consideration.

Dipolarization fronts in the magnetotail plasma sheet

We present a THEMIS study of a dipolarization front associated with a bursty bulk flow (BBF) that was observed in the central plasma sheet sequentially at X=−20.1, −16.7, and −11.0R_E. Simultaneously, the THEMIS ground network observed the formation of a north-south auroral form and intensification of westward auroral zone currents. Timing of the signatures in space suggests earthward propagation of the front at a velocity of 300 km/s. Spatial profiles of current and electron density on the front reveal a spatial scale of 500 km, comparable to an ion inertial length and an ion thermal gyroradius. This kinetic-scale structure traveled a macroscale distance of 10R_E in about 4 min without loss of coherence. The dipolarization front, therefore, is an example of space plasma cross-scale coupling. THEMIS observations at different geocentric distances are similar to recent particle-in-cell simulations demonstrating the appearance of dipolarization fronts on the leading edge of plasma fast flows in the vicinity of a reconnection site. Dipolarization fronts, therefore, may be interpreted as remote signatures of transient reconnection.     

Anomalous change in atmospheric radon concentration sourced from broad crustal deformation: A case study of the 1995 Kobe earthquake

Before the Kobe earthquake, an anomalous increase in atmospheric Rn concentration was observed. By separating the measured concentration of atmospheric Rn into three components according to the distance from the monitoring station, the variation of Rn exhalation rate can be estimated for the respective area using the daily minimum and maximum concentrations. The mean rate of Rn exhalation gradually increased in an area of 20 km around the monitoring station, becoming five times higher than normal in the period between October 1994 and the date of the earthquake. This area had a large co-seismic displacement of up to 30 cm, which roughly corresponds to the crustal strain of 10⁻⁶-order, and it is considered the main source for the atmospheric Rn prior to the Kobe earthquake. Analyses revealed that the pre-seismic change in the atmospheric Rn concentration exhibited an anomalous pattern which would yield information on the spatial distribution of the mechanical response of the ground.     

Analytical methods for error propagation in planar space–time prisms

The space–time prism demarcates all locations in space–time that a mobile object or person can occupy during an episode of potential or unobserved movement. The prism is central to time geography as a measure of potential mobility and to mobile object databases as a measure of location possibilities given sampling error. This paper develops an analytical approach to assessing error propagation in space–time prisms and prism–prism intersections. We analyze the geometry of the prisms to derive a core set of geometric problems involving the intersection of circles and ellipses. Analytical error propagation techniques such as the Taylor linearization method based on the first-order partial derivatives are not available since explicit functions describing the intersections and their derivatives are unwieldy. However, since we have implicit functions describing prism geometry, we modify this approach using an implicit function theorem that provides the required first-order partials without the explicit expressions. We describe the general method as well as details for the two spatial dimensions case and provide example calculations.     

Crustal transect from the North Atlantic Knipovich Ridge to the Svalbard Margin west of Hornsund

The crustal structure along a 312 km transect, stretching from the axial mountains of the North Atlantic Knipovich Ridge to the continental shelf of Svalbard, has been obtained using seismic reflection data and wide angle OBS data. The resulting seismic V_p and V_s models are further constrained by a 2-D-gravity model. The principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow-spreading counterpart of the North Atlantic Ridge Systems. V_p is estimated to be 3.50–6.05 km/s for the upper oceanic crust (oceanic layer 2), with a marked increase away from the ridge. The measured V_p of 6.55–6.95 km/s for oceanic layer 3A and 7.10–7.25 km/s for layer 3B, both with a V_p/V_s ratio of 1.81, except for slightly higher values at the ridge axis, does not allow a clear distinction between gabbro and mantle-derived peridotite (10–40% serpentized). The thickness of the oceanic crust varies a lot along the transect from the minimum of 5.6 km to a maximum of 8.1 km. The mean thickness of 6.7 km for the oceanic crust is well above the average thickness for slow-spreading ridges (<10 mm/year half-spreading rate). The areas of increased thickness could be explained by large magma production-rates found in the zones of axial highs at the ridge axis, which also have generated the off-axial highs adjacent the ridge. We suggest that these axial and off-axial highs along the ridge control the lithological composition of the oceanic crust. This approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments (the axial and off-axial highs) and mantle-derived peridotite outside these zone.     

Seasonal variation in water column conditions in the upper Gulf of Thailand

Seasonal variation in water column conditions in the upper Gulf of Thailand (UGoT) was analyzed by considering four major factors including surface heat flux, freshwater discharge, tidal and wind stirrings. The coincidence of surface heat loss, low river discharge and strong wind resulted in vertical well-mixing in December. Strong stratification developed in September and October due to large river discharge and moderate heat flux. Strong surface heating in April and May has a potential to generate strong stratification, although not as large as that in September and October due to low river discharge. Although no factors are prominent during January and March, and June and August, weak to moderate stratification results, because the influences of river discharge and surface heating are still larger than those of tidal and wind stirrings. The results of water column analysis based on monthly average data agree well with analyses derived from cruise data in the same months. Most analytical results correspond to the distributions of temperature and salinity from field observations. Disagreement, however, was found in December 2003 (cruise CU-2) when stratification in some small regions occurs in the distribution of water properties, but the water column analysis suggests vertical well-mixing. This phenomenon is triggered by non-uniform distribution of freshwater over UGoT, which is related to river discharge, monsoonal wind and current. Compared to a previous study regarding surface chlorophyll dynamics, water column conditions may be used to explain the occurrence of phytoplankton bloom in this region.