Atsushi Urabe

Breakout flood from an ignimbrite-dammed valley after the 5 ka Numazawako eruption, northeast Japan

Numazawa volcano in northeast Japan erupted most recently at ca. 5 ka, forming a 2-km-diameter caldera and emplacing at least 4 km 3 of valley-confined ignimbrite. The ignimbrite dammed the Tadami River to a depth of >100 m, temporarily impounding >1.6 km 3 of water. Overtopping of the barrier triggered breaching and catastrophic release of the dam lake. Pyroclastic material redeposited by the ensuing flood is widely distributed along the Tadami and Agano Rivers in deposits tens of meters thick as far as the coastal Niigata Plain >150 km downstream of the volcano. Evidence for damming and flooding found along the rivers includes (1) fine-grained, thinly laminated lacustrine deposits upstream of the pyroclastic blockage; (2) 30-m-thick, pumiceous debris-flow and hyperconcentrated-flow deposits that show continuous sedimentation with no major hiatus downstream of the volcano; (3) fine-grained slackwater deposits at tributary river mouths; and (4) large flood boulders that form lags on stripped ignimbrite surfaces and younger terrace surfaces, or cropping out within hyperconcentrated-flow deposits. Paleohydraulic estimation techniques indicate a peak discharge of 30,000–50,000 m 3 /s at the breach point. Burial of Neolithic Jomon settlements by flood and volcaniclastic aggradational deposits at distal locations indicates that such indirect volcanogenic hazards are significant, even where direct primary consequences of volcanic eruptions are minor or absent.

Identification and characterization of a subsided barrier island in the Holocene alluvial plain, Niigata, central Japan

Abstract The Echigo Plain in the Niigata area of central Japan is a coastal alluvial plain facing the Sea of Japan. The Plain contains several rows of coastal sand dunes. During the Holocene maximum transgression, a barrier island sand body was formed on the plain. However, the distribution of this barrier island is partially intermittent. Based on facies analysis of cores, the barrier island succession was found to be buried by alluvium in the central part of the Echigo Plain. The barrier sand bodies do occur in the central part of the Echigo Plain, but they are deeply subsided and tilted by an active fault in the western margin of the plain. The alluvium in this area records a history of subsidence and changes in the depositional system due to the movements on the active fault.

A New Miocene Three-Spined Stickleback (Pisces: Gasterosteidae) from Central Japan

Abstract. Gasterosteus kamoensis sp. nov. is described from the Upper Miocene Minamiimogawa Formation in Kamo City, Niigata, Japan. The new species represents the oceanic complete morph. It is characterized by: short and high ectocoracoid; presence of posttemporal bone; fully developed pelvic girdle with convex anterior margin; wide and not tapering posterior process of pelvic bone; four dorsal spines; comparatively long spines in dorsal, pelvic and anal fins; and large body size, apparently exceeding 130 mm in standard length. Gasterosteus kamoensis sp. nov. is the third nominal fossil species of this genus described from East Asia, along with freshwater G. orientalis Sytchevskaya and the marine G. abnormis Gretchina. Together, these discoveries demonstrate the wide distribution and morphological and ecological diversity of this genus during the Miocene in the northwestern Pacific.

Simple assessment of shallow velocity structures with small-scale microtremor arrays: interval-averaged S-wave velocities

This article describes a method for processing microtremor records from a small-scale seismic array that allows interval-averaged S-wave velocities to be estimated for 10-m depth ranges down to a depth of 30 m. The method was applied to microtremor data obtained in the town of Mashiki, Kumamoto Prefecture, Japan, and the analysis results were evaluated through a comparison with available PS logs and sections obtained by surface-wave methods. It turned out that the interval-averaged S-wave velocity estimates may be subject to errors of up to 20–30% in absolute values, but it was shown that the method can help evaluate relative spatial variations in those S-wave velocities. In view of the simplicity of analysis, the analyser-independent nature of the results and the limitations of analysis accuracy, the interval-averaged S-wave velocity estimation method presented here could be used as an effective tool for the preliminary analysis of microtremor data from small-scale seismic arrays. This article describes a simple method for estimating interval-averaged S-wave velocities for 10-m depth ranges down to a depth of 30 m. Possibilities and limitations of this method are examined by using the analysis results of the microtremor data obtained in the town of Mashiki, Kumamoto Prefecture, Japan.