Katsutada Kaminuma

Long-period tides observed with a superconducting gravimeter at Syowa Station, Antarctica, and their implication to global ocean tide modeling

Abstract The long-period tides (Mf and Mm waves) were analyzed with the 2 years data obtained with a superconducting gravimeter (SG) installed at Syowa Station (69.0° S, 39.6° E), Antarctica. The observed amplitudes, phase lags and amplitude factors (δ factors) were 11.642 ± 0.035 μGal, −0.12° ± 0.17° and 1.1218 ± 0.0034 for the Mf wave, and 6.143 ± 0.058 μGal, 0.33° ± 0.54° and 1.1205 ± 0.0106 for the Mm wave, respectively (1 μGal = 10 −8 m s −2 ). The ocean tide effects (effects of the attraction and loading due to the ocean mass) at the observation site were estimated using the five global ocean tide models: equilibrium ocean tide model, Schwiderski model (Schwiderski, E.W., 1980. On charting global ocean tides. Rev. Geophys. Space Phys. 18, 243–268.), Dickman model (Dickman, S.R., 1989. A complete spherical harmonic approach to luni-solar tides. Geophys. J. Int. 99, 457–468.), CSR model (Eanes, R.J., 1995. Private communication.), and Desai and Wahr model (Desai, S.H., Wahr, J.M., 1995. Empirical ocean tide models estimated from TOPEX/POSEIDON altimetry. J. Geophys. Res. 100, 25205–25228.). The averages of the five estimates are 0.433 μGal and 0.244 μGal in amplitude and 192.9° and 179.5° in phase for the Mf and Mm waves, respectively. The five estimates differ by a maximum of 0.104 μGal in amplitude and 18.8° in phase for the Mf wave, and by 0.033 μGal and 6.4° for the Mm wave. The estimated Mm phases are nearly 180° for the five models, and the variation of their values among the models is relatively small compared with that of the Mf phases. These indicate that the Mm wave is much closer to an equilibrium tide than the Mf wave. Due to the variation of the ocean tide corrections, the corrected δ factors were scattered within the ranges of 1.158 to 1.169 for the Mf wave and of 1.163 to 1.169 for the Mm wave. However, it is noted that the mean δ factors of the five ocean models, i.e. 1.162 ± 0.023 for the Mf wave and 1.165 ± 0.014 for the Mm wave, prefer slightly larger value rather than those estimated from the theory of the elastic tide.

Structure and Evolution of the East Antarctic Lithosphere: Tectonic Implications for the Development and Dispersal of Gondwana

Abstract Lithospheric evolution of the Antarctic shield is one of the keystones for understanding continental growth during the Earths evolution. Architecture of the East Antarctic craton is characterized by comparison with deep structures of the other Precambrian terrains. In this paper, we review the subsurface structure of the Lower Paleozoic metamorphic complex around the Lutzow-Holm area (LHC), East Antarctica, where high-grade metamorphism occurred during the Pan-African orogenic event. LHC is considered to be one of the collision zones in the last stage of the formation of Gondwana. A geoscience program named ‘Structure and Evolution of the East Antarctic Lithosphere (SEAL)’ was carried out since 1996-1997 austral summer season as part of the Japanese Antarctic Research Expedition (JARE). Several geological and geophysical surveys were conducted including a deep seismic refraction/wide-angle reflection survey in the LHC. The main target of the SEAL seismic transect was to obtain lithospheric structure over several geological terrains from the western adjacent Achaean Napier Complex to the eastern Lower Paleozoic Yamato-Belgica Complex. The SEAL program is part of a larger deep seismic profile, LEGENDS (Lithospheric Evolution of Gondwana East iNterdisciplinary Deep Surveys) that will extend across the Pan-African belt in neighboring fragments of Gondwana.

Seismic Activity Associated with Surface Environmental Changes of the Earth System, around Syowa Station, East Antarctica

The Japanese Antarctic Station, Syowa (69° S, 39° E; SYO), is located on Lutzow-Holm Bay of western Enderby Land, East Antarctica. Seismic observations at SYO started in 1959, and the arrivaltimes of the major phases for teleseismic events have been reported from the National Institute of Polar Research every year since 1968. Here, we summarize records from local earthquakes around SYO in the last three decades. In particular, the fifteen years since 1987 divided into three periods are examined in detail, with respect to the location of epicenters and estimation of magnitudes. A three-station seismic array was deployed around SYO in 1987–1989. By using these data, epicenters of local earthquakes were determined for the first time. Many different types of earthquakes, such as a mainshock-aftershock sequence, twin earthquake, and earthquake swarms were detected and clearly identified. The seismic activity during this period was higher than that of the following decade. Earthquake location was concentrated along the coast and central Lutzow-Holm Bay.

Volcanic earthquake swarms at Mt. Erebus, Antarctica

Abstract Mount Erebus is an active volcano in Antarctica located on Ross Island. A convecting lava lake occupies the summit crater of Mt. Erebus. Since December 1980 the seismic activity of Mt. Erebus has been continuously monitored using a radio-telemetered network of six seismic stations. The seismic activity observed by the Ross Island network during the 1982–1983 field season shows that: (1)Strombolian eruptions occur frequently at the Erebus summit lava lake at rates of 2–5 per day; (2)centrally located earthquakes map out a nearly vertical, narrow conduit system beneath the lava lake; (3)there are other source regions of seismicity on Ross Island, well removed from Mt. Erebus proper. An intense earthquake swarm recorded in October 1982 near Abbott Peak, 10 km northwest of the summit of Mt. Erebus, and volcanic tremor accompanying the swarm, may have been associated with new dike emplacement at depth.

Local Seismic Activity around the Lützow-Holm Bay, East Antarctica

The seismic monitoring at Syowa Station (69 o S, 39 o E: SYO), located on the continental margin of the Eastern Dronning Maud Land, East Antarctica, began in 1959. Phase readings of the earthquakes have been reported since 1967 and have been annually published as part of the Data Report Series of the National Institute of Polar Research since 1968. An observation of a tripartite seismic network was carried out at SYO for a period of three years from 1987 to 1990. Epicenters of local earthquakes were determined for the first time by using the array network for the three-year period. Many different types of earthquakes, such as the mainshock-aftershock type, twin earthquake, earthquake swarms, etc., were detected during the period. After this, local events around SYO have been detected empirically from their waveforms recorded on seismograms. The seismic activity for the period of 1987-1990 was higher than that of the following decade. Earthquake epicenters, occurring during that period, were highly localized along the coast and in the central part of the Lutzow-Holm Bay (LHB). Nine local earthquakes, recorded during the period of 1990-1996, showed many different types of events. The seismicity for the period of 1990-1996 was very low and the magnitudes ranged from 0.1 to 1.4. The locations of some events were determined by using the single station method for SYO, i.e., using the particle motions of the initial phase and S-P time. Two local events were detected in 1998 and one event in 2001. It would be estimated that the stress concentration was related to the glacial rebound around the LHB. Afterwards, we will be able to eventually examine the rela- tionship between the seismicity around Antarctica and deglacial phenomena such as crustal uplift, and sea level change within the earth environmental system.

Polar Motion Effect on Gravity Observed with a Superconducting Gravimeter at Syowa Station, Antarctica

Polar motion effect on gravity was analyzed using the 2 years data obtained with a superconducting gravimeter at Syowa Station, Antarctica. From the analysis, we obtained the amplitude factor (δ-factor) of 1.198±0.035 and a time lag about 20 days against to the gravity changes predicted from the IERS (International Earth Rotation Service) polar motion data. Our analysis using simulation data suggests that the results for least-squares fitting are affected by the accuracy of the correction for step-like changes in the observed gravity data.

Gravity anomaly in and around Antarctica and its tectonic implications

Abstract During the nineteen-seventies, the geophysical satellites EOS-3 and SEASAT-1 provided very accurate sea-surface heights, which could be employed as information on the marine geoid. Geoid height can easily be converted to gravity anomalies, and since the tracks of GEOS-3 and SEASAT-1 were extremely dense, the gravity anomaly data thus obtained were the densest and of the widest coverage ever obtained for gravity measurements. The authors completed a self-consistent free-air gravity anomaly map in the Antarctic region, covering from 45°S to the South Pole, using all the gravity data available at present: namely, those obtained by satellites and the ground-truth data obtained by land gravimeters and ship borne gravimeters (Segawa et at. , 1984). The bouguer anomaly was also calculated, from which estimates of crustal and lithospheric structures were made. This has resulted in clarifying the relationships among sub-bottom structures between the mid-oceanic ridges surrounding the Antarctic plate and the Ross Sea.

Geophysical studies of crustal structure of the Ongul Islands and the Northern Mizuho Plateau, East Antarctica

Abstract Explosion seismic experiments, gravity measurements and aeromagnetic surveys were made in the northern Mizuho Plateau including the Ongul Islands, East Antarctica, from 1979 to 1982 by the Japanese Antarctic Research Expeditions. The objective of these field operations was to determine the crustal structure along the 300 km-long oversnow traverse route between Syowa and Mizuho Stations. Three big shots were fired; at sea near Syowa Station, in an ice hole near Mizuho Station and in an ice hole between both stations. Twenty-seven temporal seismic stations were set up along the route. Gravity measurements were carried out at 30 points along this route. Aeromagnetic surveys over the area were made four times. In the seismic experiments, clear refracted waves from the Conrad (estimated depth 30 km) and the Moho (estimated depth 40 km) discontinuities were recorded. No layer with a velocity of less than 6 km/s was found in the Ongul Islands nor beneath the ice sheet in the surveyed area. The P-wave velocity in the upper layer varies with depth from 6.0 km/s on the surface to 6.4 km/s at a depth of 13 km. Comparing the observed record section with synthetic seismograms, it was derived that the Conrad was not associated with a sharp velocity discontinuity, but a linear velocity increase of 0.55 km/s in a transition zone of 2.4 km thick. Velocities of P∗ and Pn were determined as 6.95 km/s and 7.93 km/s assuming a flat layered structure. Bouguer gravity anomalies could not be calculated along the whole profile because of a lack of data on bedrock topography, so reduced gravity anomalies were calculated. These anomalies indicate no abrupt changes of the bedrock topography.

Crustal Uplift around Syowa Station, Antarctica

There is a great deal of evidence concerning crustal uplift, after deglaciation, in the vicinity of Syowa Station (69.0°S, 39.612) from tide gauge data, seismic evidence, elevated beaches, marine terraces, etc. The geomorphological and tide gauge data show that crustal uplift is going on around Syowa Station. Local earthquake activity corroborates the crustal uplift, and it is an intermittent phenomenon. Sea level falling of 4.5 mm/y was found using data in 1975–1992. This falling rate is consistent with geomorphological data. A route for repeat leveling survey was established in East Ongul Island. No appreciable change was observed for the last 14 years. The observations from repeat leveling measurements suggest no significant changes, which further supports the idea that the crustal uplift is not a tilt trend movement but a block movement.

Earthquake swarms on Mount Erebus, Antarctica

Abstract Mount Erebus (3794 m), located on Ross Island in McMurdo Sound, is one of the few active volcanoes in Antartica. A high-sensitivity seismic network has been operated by Japanese and US parties on and around the Volcano since December, 1980. The results of these observations show two kinds of seismic activity on Ross Island: activity concentrated near the summit of Mount Erebus associated with Strombolian eruptions, and micro-earthquake activity spread through Mount Erebus and the surrounding area. Seismicity on Mount Erebus has been quite high, usually exceeding 20 volcanic earthquakes per day. They frequently occur in swarms with daily counts exceeding 100 events. Sixteen earthquake swarms with more than 250 events per day were recorded by the seismic network during the three year period 1982–1984, and three notable earthquake swarms out of the sixteen were recognized, in October, 1982 (named 82-C), March–April, 1984 (84-B) and July, 1984 (84-F). Swarms 84-B and 84-F have a large total number of earthquakes and large Ishimoto-Iidas “m”; hence these two swarms are presumed to constitute on one of the precursor phenomena to the new eruption, which took place on 13 September, 1984, and lasted a few months.