Measurement of directional spectrum of wind waves using an array of wave detectors

A method is proposed to estimate the true directional spectrum of wind waves by making use of more than four wave detectors. The true spectrum of wind waves whose wave lengths lie between about 1.1 and 2.6 times the largest span between the wave gauges can be recovered with an error less than 0.5 %. An additional wave gauge with fixed maximum span extends its effectivity to shorter wave lengths, but does not effect the upper limit of wave length.The method is based on the fact that the spectrum estimated byBarber (1961) is connected with true directional spectrum by the Fredholm integral equation of the first kind if the wave component satisfies the dispersion relation. Solving the equation by using the Fourier series method, we can get true spectrum.     

A widespread simultaneous increase of the microearthquake seismicity and minor crustal movements in and around the assumed rupture zone of the Tokai earthquake

Data from an extensive routine network and a high-density temporary seismic observation using the ocean bottom seismometer in and around Suruga Bay, Japan, showed that a simultaneous increase of microearthquake seismicity occurred from mid-July 1984 (Ukawa et al., 1988). The area extended over a region of about 60 km (NS) × 80 km (EW), and is nearly included in the assumed fault zone of the presumed Tokai earthquake. The analyses of the bore-hole ground tilt and volumetric strain data revealed that an anomalous small ground tilt and volumetric strain change occurred at the time of the seismic activity with a duration of about a month. Data of geodetic measurements, groundwater, and radon content were examined with the result that many observational items showed small anomalies during that period. The observed crustal movement was compared with that of slip models of the Philippine Sea plate around Suruga Bay, suggesting that some type of episodic aseismic subducting motion occurred and, thus, caused a crustal movement in the overlying continental plate in the very region of the future fault zone.     

Generation of block and ash flows during the 1990–1995 eruption of Unzen Volcano, Japan

Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990–1995 eruption of Unzen volcano, Japan. Two different types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991–1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows.     

日本地震预警系统在2011年3月11日地震中的效能、不足与改进

自2007年10月以来,成熟的公共紧急地震速报(earthquake early warning,EEW)系统(国内称为地震预警系统)在日本得到应用。2011年日本东北近海(Tohokuoki)地震和海啸(也被称为3·11)发生时,震中附近的几百万人在最严重的振动发生前约15~20s,收到了地震预警(EEW)信息,同时更多周边地区的人在不太严重的振动开始前,有更多的预警时间。这些人中大约90%能够预先采取行动拯救自己及其家庭成员的生命,或根据事先计划采取其他措施,有些行为措施是基于对警报的直观反应。如此高的效率,取决于学校及社会中关于EEW系统知识的教育。尽管EEW系统存在着一些不足,但其有效性促使日本加密现有的地震与海啸监测台网。该台网在日本东部近海岛弧有约150个台站,并提供一个特殊的终端,使EEW系统更进一步应用在超过53 000名学生的学校中。当前对其分析和发布方案的改进也正在进行中。     

A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone,central Japan

We installed a real-time operating regional observation network of Ocean-Bottom-Seismometers, connected to an electro-optical fiber communication cable, at the Sagami trough subduction zone, just south of the Tokyo metropolitan area, central Japan. The network, called ETMC, has six seismic observation sites at approximately 20 km spacing. In addition, there are three tsunami observation sites along the ETMC network to monitor the propagation process of tsunamis around the Sagami trough region.The on-line data from the ETMC has been improving the detection capability of smaller-magnitude earthquakes even at areas close to the margin of the trough. The ETMC data analyzing system, which has a function of real-time digital filtering for each seismic channel, can read the arrival times of P- and S-waves precisely, constraining well the automatic on-line hypocenter locations. The network has been providing useful information regarding the bending and downgoing process of the Philippine sea plate at the Sagami trough subduction zone.The pressure sensors of the installed network have a detection capability of tsunami wave trains with an amplitude of less than 1 cm. For example, the sensors recorded the full time history of tsunami wave trains, with mm order resolution, originating from a tsunami earthquake with 5.7 M_W and the tsunami magnitude of 7.5 occurred near Tori Shima (Tori Is.) of the Izu-Bonin Is. arc on September 4, 1996. The maximum amplitude of the tsunami signals on the trough-floor was approximately 1 cm (P-P), in contrast with approximately 20 cm (0-P) at a coastal site on Izu-Oshima, near the trough. Also, the pressure sensors observed tsunamis due to a large tsunami earthquake (7.1 M_W) at the northern New Guinea, on July 17, 1998.     

Characterization of low-temperature pyroclastic surges that occurred in the northeastern Japan arc during the late 19th century

Three eruption events occurring in the central part of the northeastern Japan arc were investigated and compared: Adatara AD1900, Zao AD1895, and Bandai AD1888. Producing low-temperature (LT) pyroclastic surges, these events are characterized by steam eruptions ejecting no juvenile material. These eruptions' well-preserved eruptive deposits and facies facilitated granulometric analyses of the beds, which revealed the transport and deposition mechanisms of LT surges. Combining these results with those of investigations of documents reporting the events, we correlated each eruption to the relevant individual bed and reconstructed the LT surge development sequence. Important findings related to the transport and deposition modes are the following. (1) Bed sets consisting of thin, laminated ash and its overlying thick massive tuff were recognized in the Adatara 1900 proximal deposits. The bed set was probably produced by a strong wind that discharged and propagated quickly from the vent (leading wind) and a gravitationally segregated, highly concentrated flow originated from the eruption column, within a discrete eruption episode. A similar combination might have occurred during the first surge of the Bandai 1888 event. (2) Comparison of the proximal and distal facies for the largest eruption of Adatara 1900 event indicates that the initial turbulence of the eruption cloud decreased rapidly, transforming into a density-stratified surge with a highly concentrated part near the base. Similar surges occurred in the climatic stage of Zao 1895. (3) Bandai 1888 ejecta indicate massive beds deposited preferentially at topographic lows. Co-occurring planar beds showed no topographic affection, as indicated by the topographic blocking of a stratified surge. The observed facies–massive tuffs, crudely stratified tuffs, and thin bedded tuffs–are compatible with those for high-temperature surges. At Bandai, absence of dune bedded tuffs and commonly poorer sorting in the LT surge deposits might be attributable to poor thermally induced turbulence of eruption columns. Condensation of vapor in the surges might have contributed to the poor sorting. The estimated explosion energies were 6 × 10¹³ J for Adatara AD1900, 6.5 × 10¹⁰ J for Zao AD1895, and 6.5 × 10¹⁵ J for Bandai AD1888, implying that the three events were hydrothermal eruptions with distinctive eruptive mechanisms. Regarding eruption sources, the Adatara 1900 event was caused solely by thermal energy of the hydrothermal fluid, although magma intrusion likely triggered evolution of hydrothermal systems at Zao in 1895. Steam eruptions in the Bandai 1888 event occurred simultaneously with sudden exposure of the hydrothermal system, whose triggers require no internal energy.     

Microearthquakes along the back-arc spreading system in the eastern Bismarck Sea

OBS's were deployed for 26 to 29 days in the eastern Bismarck Sea to investigate the back-arc spreading. Hypocenters of 186 shallow earthquakes were determined using P- and S-waves from at least five stations. In the western survey area, a transform fault zone is marked by a linear micro-earthquake activity striking N65°W and less than 5 km wide. The predominant type of their focal mechanisms is strike-slip. In the eastern area, several intermittent zones of micro-earthquakes and their strike-slip type focal mechanisms suggest the location of short-length transform faults separating en-echelon spreading ridges.