Development of A New Ocean Bottom Seismometer (Model IV of Kyoto University)

Seventy percent of the surface of the earth is covered by ocean. It is necessary to observe earthquakes in the sea for the study of solid earth. The group of ocean-bottom seismological observation of Kyoto University has developed ocean bottom seismometers (OBS) of three types over the last several years. In the present study, we present our newly developed OBS which deploys free-fall and pop-up methods. This OBS is equipped with a three-component geophone (Mark Product L-22D) and a digital recorder. The recorder digitizes seismic data by a 16 bit analog-to-digital converter and writes the data on a magneto-optical disk with a capacity of 326 megabytes (MB). The electronic circuit boards are all housed in a 17-inch-diameter glass sphere. Thus, we will be able to obtain seismic data of high quality reading the seismographs from this high dynamic range seismometer installed on the sea bottom. The present OBS has an electrical corrosion mechanism permitting the release of the main OBS unit from its anchor at the sea bottom when it is retrieved. Recording tests on land and popping-up tests in the sea were performed, confirming that the present system is reliable enough to record seismic data under long-time submarine deployment

Ocean Bottom Seismometer Handled by Submersible Vessel and Its Observation Prior to the 1993 Hokkaido Nansei-Oki Earthquake

Nowadays, most ocean bottom seismometers (OBS) are of the free-fall and pop-up type. For the observations, they are dropped from ships and self land on the flat sea floor where generally thick sediments cover basement rocks. However, soft sediment affects and distorts incoming seismic signals. If we could place ocean bottom seismometers on hard rock, it could produce high quality seismograms which contained more information on structure and earthquake sources. For this purpose, a submersible vehicle is necessary as it can carry and place an OBS properly on exposed sea floor hard rock. We designed a new type of OBS which can be handled by a submersible vehicle. This new OBS contains instruments within two aluminum cylinders with syntactic foam blocks outside the cylinders for buoyancy. The submersible vessel, Shinkai-6500, was used to set the OBS to observe seismicity in the Okushiri Ridge Area. It was placed on a relatively flat and hard basement outcrop at a depth of 3338m. The OBS was released to the sea surface through a self pop-up system after two days of observation. During this observation period, we noticed very high micro-earthquake activity around the Okushiri Ridge area where one year later the Hokkaido-Nansei-Oki earthquake of M7.8 took place. Such seismic activity could not have been detected by land-based seismic networks alone

長基線GPS歪計 : 高速サンプリング定常GPS観測網の提案

文部省科学研究費補助金・総合研究 (A) 「測地学におけるGPSの高度利用に関する総合的研究」, 研究代表者: 加藤照之, 課題番号: 04302019p.73と74の間に追加1頁(著者原稿)ありAn experiment for OPS strain seismometer, which possibly records largeamplitude near-field seismic motions with flat frequency responses from zero to several 0.1 Hz, has been conducted. Assigning displacement motions with the double amplitude of 15 cm and with periods of 25-300 seconds to a GPS rover antenna, we have made 3-hour GPS observations also at 2 other base sites 160 m and 160 km apart from the rover site, with a sampling interval of 1 second. Post-processing kinematic analyses determine the rover site motion every second from two base sites. Comparison of post-processing results determined from two base sites 60 m and 160 km apart from the rover site show that kinematic GPS can resolve horizontal displacement motions with an accuracy of 1-2 cm every second even from a site 160 km apart, though there exist drift components up to 10 cm during a period of 3 hours. These drift components are, however, mainly due to the use of broadcast ephemerides, and these components will be removed if using precise ephemerides. Thus, our experiment shows a possibility of GPS strain seismometer, which records large-amplitude nearfield ground displacement motion with flat frequency responses from zero to several 0.1 Hz

Seismometric Observations of Matsushiro Swarm Earthquakes

This paper is the report of seismometric observations of Matsushiro swarm earthquakes made by Abuyama Seismological Observatory and Disaster Prevention Research Institute, independently or co-operatively. Continuous observation at Shinko has been carried out to investigate the time variation of the seismic activity since the joint observation of ultramicro-earthquakes at the end of 1965. As the seismicity became active, continuous observation was started at Sanada, too, on June, 1966, with the main purpose of investigating the "Ishimoto-Iida's relation" and the time variation of seismic activity, but it was abandoned on September, 1966 because of overlapping the station with that of the Earthquake Research Institute. For studying the attenuation of seismic wave in short hypocentral distance and formulating the equation for calculating the magnitude of an earthquake, the Matsushiro swarm earthquakes are a very good source observations. On the basis of this idea, we picked out four observation points on the array line from Matsushiro to Komoro, and installed completely equipped low magnification seismographs of displacement type. The observations were begun on June, 1966. Further, on November the same year, the array line turned about 90 degrees for the first line which was extended from Matsushiro to Toyoshina through Akashina. The observations are still being continued at present. As had been planned in the earthquake prediction program in Japan, mobile observations were carried out three times from June to November, 1966, for the purpose of investigating the field problems of this observation system and the seismic activity outside the so-called seismic region of Matsushiro swarm earthquakes. Although this is a simple method by which only the frequency distribution of S-P duration times were observed, notwithstanding its simplicity the mobile observation system for the purpose of surveying the seismic activity was proved useful in the region where the dense observation network had not then been set up. Though the idea that the earthquakes are almost always generated in the vicinity of the fault is exceedingly Americanized, we carried out the observation of ultramicro-earthquakes by the seismometer-array, with an expectation that some relation between the cracks around Mt. Minakami and ultramicro-earthquakes may exist. However this expectation has been proved completely to the contrary

Precise Acoustic Ranging System Using Ultrasonic Waves

The Philippine Sea Plate and Pacific Plate are subducting under the Japanese Islands. Large earthquakes have occurred along these pla te boundaries. It is important to measure the amount of movement of the plate directly for the research of earthquake occurrences. Ultrasonic waves are used for precise positioning of a reference point on the seafloor, where electromagnetic waves and lights are not transmittable in the sea water. For seafloor positioning with a precision of several cm, we developed an acoustic ranging system using linear FM signals around 10 kHz. This report describes the measurement experiments using this system carried out in Osaka Bay. It is confirmed that precision of the acoustic ranging is 3 cm or less by the experiments under the condition of calm water. However, occasionally acoustic signals were disturbed by the multiplex resections at the sea surface and/or the seafloor. It is important that the transducer of the surface unit is installed at the good position apart longer than 1.3 m from reflectors such as the bottom of the ship.＜総説

Shallow Crustal Structure Beneath Taal Volcano, Philippines, Revealed by the 1993 Seismic Explosion Survey

We carried out an seismic explosion survey at Taal volcano in March, 1993. The explosions were done at the west of the volcanic island and digital event recorders were deployed along a fan-shooting survey line at the east of the volcanic island and also along a short straight survey line across the island. Small-aperture arrays were also operated at two sites west of the volcanic island. We found that P waves that traveled just beneath the main crater strongly attenuated and showed later arrivals. This result suggests the existence of a low-velocity and low-Q region around the main crater at a depth of about 1.5km, although the location and extent cannot be determined exactly. P wave velocity of this abnormal region may be eatimated to be lower than that of the surroundings by much greater than about 25%, if we assume the region is restricted just beneath the main crater. We applied the NMO correction to the later part of seismograms and found a reflector around the east of the main crater at a depth of about 6km, which may suggest the top surface of the magma reservoir