Long-range weather forecasting

This article describes long-range forecasting, examples in Japan being mainly quoted. The article stresses the approach with which the author is most familiar, namely that used in the Japan Meteorological Agency; it employs statistical, synoptic, and physical techniques which contain implicitly many of the considerations used in long-range forecasting in many countries. After World War II, with an increase in quantity of upper-air data over the Northern Hemisphere, many studies of long-range forecasting were made in relation to the general circulation of the atmosphere. For the past several years the studies of long-range forecasting in Japan have been especially concentrated on the development of the techniques of seasonal weather forecasting. The relevant approaches to long-range forecasting are now being developed from synoptic and dynamical viewpoints in many countries, by the use of data in both the troposphere and the stratosphere over the globe and electronic computers.     

K-Ar dating on volcanic rocks in the Bonin Islands and its tectonic implication

The K-Ar dating method was applied to the volcanic rocks, the boninite and related rocks, from the Bonin Islands. Four fresh samples gave K-Ar ages of about 40 Ma. These ages suggest that the main volcanism of the Bonin Islands occurred about 40 Ma ago, as a result of the initiation of subduction of the Pacific plate at its western margin. Results of the K-Ar dating on other rocks suggest that some volcanism or thermal disturbance may have occurred 5 ~ 10 Ma ago, at least, in Chichijima.     

Estimating rupture scenario likelihood based on dynamic rupture simulations: the example of the segmented Middle Durance fault, southeastern France

The Middle Durance fault system, southeastern France, is a slow active fault that produced moderate-size historical seismic events and shows evidence of at least one   M _w ≳ 6.5  event in the last 29 000 yr. Based on dynamic rupture simulation, we propose earthquake scenarios that are constrained by knowledge of both the tectonic stress field and of the 3-D geometry of the Durance fault system. We simulate dynamic rupture interaction among several fault segmentations of different strikes, dips and rakes, using a 3-D boundary integral equation method. 50 combinations of reasonable stress field orientations, stress field amplitudes and hypocentre locations are tested. The probability of different rupture evolutions is then computed. Each segment ruptures mainly as a single event (44 per cent of the 50 simulations test in this paper). However, the probability that an event triggers simultaneously along three segments is high (26 per cent), leading to a potential rupture length of 45 km. Finally, 2 per cent of the simulations occur along four adjacent segments, producing the greatest total rupture length of 55 km. The simulation results show that the southernmost segment is most easily ruptured (40 per cent), because of its favourable orientation with respect to the tectonic stress and of its favourable location for interaction with the other segments. South-bound unilateral propagation is slightly preferable (41 per cent), compared to north-bound unilateral and bilateral propagation modes. Although, these rupture scenarios cannot be directly translated into probabilities of occurrence, they do provide a better insight as to which rupture scenarios are more likely, an important element to better estimate near-field strong ground motion and seismic hazard.     

Bathymetry, biota and sediments on the Hirota Reef, Tane-ga-shima – the northernmost coral reef in the Ryukyu Islands

Abstract Investigations were conducted on bathymetry, reef biota and sediments on the Hirota Reef, Tane‐ga‐shima, North Ryukyus, near the northern limit for coral‐reef formation. A bathymetric profile from shore to the reef edge was depicted along an approximately 420‐m transect on the Hirota Coast of this island. A total of 20 quadrats (1 m × 1 m) were analyzed along the profile at 10‐ or 20‐m intervals to clarify distribution of macrobenthos inhabiting the reef. The Hirota Reef is divided into four geomorphologic zones according to their depth, gradient, surface roughness, substrate and characteristic macrobenthos. They are, from shore to offshore, shallow lagoon, seaward reef flat, reef edge and reef slope. The shallow lagoon comprises a shoreward depression (∼160 m wide on the transect) with a sand/gravel bottom that inclines gently toward offshore, and a seaward patch zone (∼70 m wide). The patches (<2 m high) are covered with fleshy algae, coralline algae and hermatypic corals. The seaward reef flat (∼190 m wide) is a flat plane that is constructed by biogenic carbonates and is covered with turf algae, with hermatypic corals scattered. Although the seaward reef flat of the Hirota Reef cannot be differentiated into different geomorphologic zones, similar seaward reef flat areas in the Central and South Ryukyus can be clearly subdivided into inner reef flat, reef crest and outer reef flat. This difference may be attributed to a lower reef growth rate and/or the later reef formation of the Hirota Reef in Holocene time than the southern examples. The coral fauna on the Hirota Reef is delineated by low diversity and characterized by taxa typical of high‐latitude, non‐reefal communities. The algal flora consists of tropical to subtropical species associated with warm‐temperate species. These faunal and floral characteristics may be related largely to lower water temperature in Tane‐ga‐shima than those in typical coral‐reef regions.     

Temporal Variations of Seismic Wave Velocity Associated with 1998 M6.1 Shizukuishi Earthquake

—?We attempt to detect temporal variations of seismic wave velocity before and after 1998 M6.1 Shizukuishi, northeastern Japan, earthquake by using waveform data from explosions and earthquake doublets spanning the period immediately before and after the earthquake. Direct P waves of the second explosion are delayed by ～20 ms at observation stations with epicentral distances less than 15 km. This tendency does not change if the analysis frequency band is changed. Our result suggests that the P-wave velocity decreased by at least 1% in the extremely shallow region of the hanging wall of the M6.1 thrust event after its occurrence. On the other hand, there was the frequency dependence of the coda wave delays for both artificial sources and for natural events. At 5–10 Hz, immediate sharp increases by more than 20 ms in time delays and lower coherency were observed at several stations. We estimated the region in which P-wave velocity might have decreased after the M6.1 earthquake. Maximum depth of the region is 13 km. The region includes the aftershock area of the M6.1 earthquake, but is eccentric to the earthquake in the west. Considering the frequency band analyzed (5–10 Hz), the scale of the spatial inhomogeneity which led to the coda wave delay is several hundreds meters. We investigated candidates for the cause of the direct P-wave and coda wave delay. Observed direct P-wave delay can be partly explained by the stress changes caused by coseismic fault slip. However, the coda wave delay cannot be explained by the stress changes that are limited to the superficial area. Crustal heterogeneity should have changed at coseismic time in the deeper area where aftershocks of the M6.1 earthquake occurred.     

Groundwater microtemperature in earthquake regions

Microtemperature measurements of groundwater with a relative precision better than 1/1000°C have been made in several seismically active areas in Japan. The measured temperatures show clear coseismic signals as well as a correlation with atmospheric pressure. Simultaneous observations at various depths have shown that these temperature changes were not induced by simple groundwater level changes. Also, distinctive signals occurred before several earthquakes and seem to be caused by a different mechanism than the coseismic signals. The microtemperature at some observation sites shows excellent correlation with records of nearby sensitive borehole strainmeters. Simultaneous recording of microtemperature and strain has been initiated in some boreholes.     

Interferometer observations of the solar brightness distribution at 8.6 mm wavelength

The brightness distribution of the quiet Sun at 8.6 mm wavelength is synthesized from off-meridian observations using an eight element east-west interferometer with a maximum base line of 16.38 m (1913). The observed brightness distribution is practically flat from the disk center to the optical limb. The effective radius of the nearly uniform component is 1.01 R . If the limb brightening is present, the brightening located between 0.95 R and 1.01 R , and the total flux density of the limb brightening is less than 1% of the total flux density of the Sun. In addition to the nearly uniform component there exists a coronal component just outside the optical limb.