Hb M Akita disease is a cyanotic hemoglobinopathy found in Akita Prefecture, Japan. The abnormal hemoglobin was found to be the same as Hb M Hyde Park (beta92 His replaced by Tyr) by chemical analysis in 1967. In this disease signs of accelerated hemolysis (serum bilirubin, 2.4 mg/dl; splenomegaly, 2 finger breadths; Hb, 10.7 g/dl; reticulocyte index, 2.7) were noted, but the causes of its slight anemia were revealed to be fairly complex by ferrokinetic study, RBC life-span measurement, and 99mTc myeloscintigram. The anemia in this disease is caused not only by shortened erythrocyte survival (T 1/2 = 11.5 days by 51Cr-tagging method) and sequestration of red cells in the spleen (Spleen: liver ratio = 2.5 approximately 3.0 by 51Cr-surface counting), but also by slow supply of erythrocytes to the peripheral blood from the bone marrow, presumably, related to the existence of unstable Hb M Akita and its derivative (Hb Akita) in the erythroid cells. Both Carrell's isopropanol test and Heinz body formation test were positive. In spite of maximally increased total erythropoiesis (8 times as high as the normal level; M:E ratio = 0.22:1.0), supply of red cells from the bone marrow to the peripheral blood was significantly decreased. The distribution of hematopoietic sites throughout the body was reasonably uniform.

Hypocenters and focal mechanisms of microearthquakes have been investigated at the Rodriguez Triple Junction in the Indian Ocean. Little was known on microearthquake activity in this region. We deployed 18 ocean bottom seismographs during the KH93–3 cruise of the R/V Hakuho‐Maru (Ocean Research Institute, University of Tokyo) from July 30 to August 20, 1993. We obtained 579 well‐constrained hypocenters and 13 focal mechanisms. Microearthquakes were found to be active along all of the three ridges: the Central Indian Ridge, the Southeastern Indian Ridge, and the Southwestern Indian Ridge. Especially at the triple junction there was an earthquake swarm within narrow area of approximately 15×5 km2. All of the 13 focal mechanisms showed normal or strike‐slip faultings, which means that the extensional stress field characterizes this region.

The eastern margin of the Japan Sea is a nascent convergent plate boundary. Previous studies proposed the existence of a seismic gap along this boundary between 39°N and 40°N. The trend of this gap is reported by Ohtake (Island Arc 4, 156–165, 1995) to be north-northwest to south-southeast, but by Ishikawa (Gekkan Kaiyo, Suppl. 7, 102–107, 1994) and Matsuzawa (Prog. Abstr., Seismol. Soc. Jpn. 2, B92, 1995) to be north-northeast to south-southwest. During one month ocean bottom seismic observations were conducted using nine ocean bottom seismometers to investigate seismicity in and around the seismic gap area in detail. The observations revealed that the earthquake epicentral distribution had an echelon shape and could be divided into three groups. These groups have a north-northeast to south-southwest trend. This trend is consistent with the fault system in this area, which was formed by the back-arc spreading in the Early to Middle Miocene. This suggests that previously formed tectonic structures affect the present seismo-tectonics and that this area has weak planes with a north-northeast to south-southwest trend.

We conducted two seismic surveys in the Yap region to investigate tectonic activity in this area. One was conducted in the northern Yap Trench for ten days using ocean bottom seismometers, the other was conducted on the Yap Islands for eight months using a small array. From these observations we,detected many earthquakes beneath the Yap region and found a characteristic pattern of seismicity. Many earthquakes occurred in the inner trench slope, no seismicity was observed in the axial region of the trench, and a few earthquakes occurred in the outer trench slope. This pattern is similar to the typical pattern in active subduction zones. This result indicates that the Yap Trench is still actively subducting. We did not detect earthquakes deeper than 40 km during the observations.

We conducted three weeks of seismic observations at the Rodriguez Triple Junction (RTJ) in the Indian Ocean using 18 ocean‐bottom seismometers over an area of 90km × 90km. We identified six teleseismic events and obtained significant anomalies in the relative travel‐time residuals of the teleseismic P‐waves. The residuals are positive at the RTJ, the northern part of the Southeast Indian Ridge (SEIR), and the eastern side of the Central Indian Ridge (CIR), and are negative at and around the Southwest Indian Ridge (SWIR). It is suggested that there is a relatively hotter mantle under the triple junction and the northern part of the SEIR segment, along‐axis variations in mantle temperature along the SEIR segment, and cooler mantle under the SWIR segment. The CIR segment has an asymmetrical distribution of relative residuals.

Volcano-related seismicity associated with the July 13, 1989, submarine eruption off Ito, Izu Peninsula, central Japan, began with a swarm activity on June 30. Two ocean bottom seismometers (OBSs) deployed near the future eruptive vent observed several precursor explosive activities during the two days preceding the surface eruption. OBS records show precursor waveforms, called "spindle packets," which resemble those during the surface eruption. After the eruption, a sonobuoy was repeatedly deployed above the vent and, finally a cable OBS was deployed 1 km NW of the vent. Isolated volcanic explosions show high frequency sound arrivals and low frequency (1.2 Hz) coda on the vertical seismometer; this coda, which has a group velocity of approximately 330 m/s, is identified as a Rayleigh wave. The high frequency arrivals result from the propagation of sound waves through the water column. Hypocenters before the surface eruption trended WNW-ESE and had depths between the ocean bottom and 6 km. After the eruption, earthquakes occurred around the vent with depths a few kilometers below the surface.Sonobuoy records show an X phase. Assuming the X phase is generated by the conversion of an S wave to a P wave at the magma surface, the depth to the top of the magma is estimated as 1 km below the hypocenter. Quiet vent site hydrothermal activity was observed one and half months after the surface eruption. Although rising bubbles were seen, no earthquake was located beneath the vent site.

We conducted a microearthquake survey with ocean bottom seismometers (OBS) in the middle Okinawa trough in 1984. During about one month of operation, very active microseismicity was observed. The microearthquake activity was characterized by the ceaseless occurrence of numerous small earthquakes in the vicinity of OBS stations. This activity was recorded by almost every OBS station but not by land‐based instruments. Swarm activities also occurred at many places. Average activity levels of earthquakes whose durations are longer than 30 s, recorded at individual OBSs, are from 4 to 20 events per day, which seem higher than those of ordinary mid‐oceanic ridge‐fracture zone systems and other back arc basins. Apparently, the number of smaller size earthquakes is fewer than expected. This might indicate the rather high attenuation of seismic waves or low‐Q of the crust around OBS stations, which makes it difficult to detect small size events at a distant range. Attenuation of seismic waves was large beyond the epicentral distance of about 120 km. This seems to indicate low‐Q in the lower crust. Also, a particularly low‐Q area was found at the northern most part of the research area where high heat flow measurements were obtained. This suggests that igneous activity reaches to the surface in that vicinity from the bottom of the crust where magmatic activity has been implied in other studies. At least two types of swarm activities are indicated for this area. Some have no dominant events, show high “b” value properties, and seem to be volcanic. Others have dominant events and appear to be tectonic in nature. Among the swarm activity, two kinds of signals closely resembling volcanic tremors were observed. From these features, we conclude that the crust of this area is highly fractured and subject to intense tectonic activities, as well as some kind of igneous processes.

To determine whether or not the Mariana Trough, the Okinawa Trough, and the Japan Sea are spreading or had spread, three active and passive seismic experiments were conducted using ocean bottom seismometer/hydrophone (OBSH) during 1984 and 1985. This paper focuses on the results of the middle-Okinawa Trough. In the middle-Okinawa Trough, swarm microearthquake activities were observed. The swarm earthquake activities were spotted in three areas within the central region of the middle-Okinawa Trough. Seismic signals were not observed in common by plural stations because their magnitudes were not large enough and there may be high attenuation characteristics in this region. In the middle-Okinawa Trough, spiky seismic waves were observed after S wave arrival in the OBSH records, suggesting reflection phases generated by magma-like structures. In the North Mariana Trough the seismic activity was not so intense as in the Okinawa Trough, but numerous microearthquakes were observed. In the middle-Okinawa Trough, refraction experiments along two lines were performed: one was on the rift area and the other was along the average axis of the central region of the trough. The results suggest presence of continental crustal structure beneath the middle-Okinawa Trough even though high seismic activities similar to those at spreading centers were observed. The velocities obtained are 4.1, 6.0, and 6.8 km/s. The discrepancy between depth variation calculated from the. intercept times and the dip angles along the layer boundaries suggest the existence of a localized low velocity region beneath the OBSH station (No. 6) which is close to the high heat flow area. Based on present results on crustal structure, seismicity, and presence of spiky phase and other geophysical results such as high heat flow values, weak magnetic lineations and graben structures, it seems that the middle-Okinawa Trough is at the stage of incipient back-arc spreading in contrast to the more developed stages of the

Observations of high‐frequency (HF) Pn phases by an oceanbottom seismometer (OBS) array were made at distances between 6° and 18° in the north‐western Pacific. At this distance range, mantle‐refracted P phases arrive earlier than HF‐Pn phases. These two phases are successfully separated for near surface earthquakes by applying appropriate filtering, since the dominant frequencies of HF‐Pn phases are higher than 6 Hz and the frequencies of mantle‐refracted P phases are rather low, about 3 Hz. The apparent velocities of HF‐Pn phases in this distance range are estimated to be from 8.3 to 8.1 km/sec or even lower, while those of mantle‐refracted P waves are from 8.4 to 8.6 km/sec. The reported value of 8.33 km/sec for Hf‐Pn phases appears to be an average of both HF‐Pn and mantle‐refracted P phases. Therefore, these observations suggest that a proposed additional HF‐Pn wave guide for the higher velocity of 8.33 km/sec is questionable.